Biaxial Minerals

All minerals that crystallize in the orthorhombic, monoclinic, or triclinic crystal systems are biaxial.  Biaxial crystals have 2 optic axes, and this distinguishes biaxial crystals from uniaxial crystals.  Like uniaxial crystals, biaxial crystals have refractive indices that vary between two extremes.

Extinction Angle

Extinction angle is the property that involves determining the angle between the a crystallographic direction as exhibited by a crystal face or cleavage and one of the principal vibration directions.  We have already discussed parallel and symmetrical extinction in uniaxial minerals.  The concept is the same for Biaxial minerals, except that the extinction angles could be different from 90o or 0o, as is the case for parallel extinction.  

Three different cases are observed depending on whether the mineral is orthorhombic, monoclinic, or triclinic.

Orthorhombic Minerals

In orthorhombic minerals the principal vibration directions are coincident with the 3 crystallographic axes.  Thus, for most orientations of the mineral on the stage, cleavages that are parallel to a crystallographic axis will show extinction that is parallel to or at 90o to such a cleavage. Monoclinic Minerals

In monoclinic crystals, only one of the principal vibration directions will coincide with a crystallographic axis.  The others will be at some angle to the crystallographic axes.  Triclinic Minerals

In triclinic minerals none of the principal vibration directions are constrained to coincide with crystallographic directions. 

 Pleochroism

As discussed under uniaxial minerals, pleochroism is the property where the mineral shows a different absorption color associated with different vibration directions.  In uniaxial minerals the two main vibration directions could have different absorption colors, and any intermediate direction would show an intermediate color.  In biaxial crystals, three different absorption colors are possible, one associated with each of the principal indices.  Intermediate directions will give intermediate colors.  

The pleochroic formula is usually given in terms of the three principal refractive indices, for example a biaxial mineral could have the pleochroic formula = red, = pink, = clear.

Of course, since only two vibration directions can be observed at any one time, only two of the

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colors will be seen on rotation of a given grain.  Thus, several grains of the same mineral must be observed in order to determine the pleochroic formula of the mineral.Twinning

Since twinning is an intergrowth of two or more crystals, optical properties will change at the boundaries between twins.

Thus, different parts of the crystal will go extinct at different times as a result of twin planes.  Plagioclase polysynthetic twinning is seen as dark and light colored stripes running through the crystal under crossed polars (left-hand illustration).  Cyclical twins and simple contact twins are shown in the other illustrations.   

Summary of Optical Properties 

Determination of Isotropic, Uniaxial, or Biaxial Character

Mineral is

o Isotropic if all grains are extinct under crossed polars during 360o rotation.

o Uniaxial if it gives a uniaxial interference figure.

o Biaxial if it gives a biaxial interference figure.

Estimation of Birefringence - in thin section with thickness of minerals of 0.03 mm, birefringence is estimated using interference color chart. Note that only absolute birefringence is diagnostic:

 || for uniaxial minerals

() for biaxial minerals

Relief - from comparison with surrounding minerals or cement in which the crystals are mounted,  or with oil in immersion method.

Absorption Color or Pleochroism

o Absorption color - if present, may be observable in isotropic, uniaxial, and biaxial minerals with analyzer not inserted.

o Uniaxial minerals may have pleochroic formula: = color1, = color2.  If optic

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axis is perpendicular to the stage, only one color will be observed. 

o Biaxial minerals may have pleochroic formula  = color1, = color2, = color3, but only 2 colors will be observed in any one grain, unless the optic axis is perpendicular to stage - then only one color.

Uniaxial Minerals, Uniaxial Indicatrix, Optic Sign, & Ray Path 

Uniaxial minerals are a class of anisotropic minerals that include all minerals that crystallize in the tetragonal and hexagonal crystal systems.  They are called uniaxial because they have a single optic axis.  Light traveling along the direction of this single optic axis exhibits the same properties as isotropic materials in the sense that the polarization direction of the light is not changed by passage through the crystal.  Similarly, if the optic axis is oriented perpendicular to the microscope stage with the analyzer inserted, the grain will remain extinct throughout a 360o

rotation of the stage.  The single optic axis is coincident with the c-crystallographic axis in tetragonal and hexagonal minerals.  Thus, light traveling parallel to the c-axis will behave as if it were traveling in an isotropic substance because, looking down the c-axis of tetragonal or hexagonal minerals one sees only equal length a-axes, just like in isometric minerals.

Double Refraction

All anisotropic minerals exhibit the phenomenon of double refraction.  Only when the birefringence is very high, however, is it apparent to the human eye.  Such a case exists for the hexagonal (and therefore uniaxial) mineral calcite.  Calcite has rhombohedral cleavage which means it breaks into blocks with parallelogram - shaped faces.  If a clear rhombic cleavage block is placed over a point and observed from the top, two images of the point are seen through the calcite crystal.  This is known as double refraction.

 

 

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Optic Sign

Recall that uniaxial minerals can be divided into 2 classes based on the optic sign of the mineral.

If the optic sign is negative and the uniaxial indicatrix would take the form of an oblate spheroid.  Note that such an indicatrix is elongated in the direction of the stroke of a minus sign.

If the optic sign is positive and the uniaxial indicatrix would take the form of a prolate spheroid.  Note that such an indicatrix is elongated in the direction of the vertical stroke of a plus sign.

PLEOCHROISM IN UNIAXIAL MINERALS

Pleochroism is defined as the change in colour of a mineral, in plane light, on rotating the stage. It occurs when the wavelengths of the ordinary & extraordinary rays are absorbed differently on passing through a mineral, resulting in different wavelengths of light passing the mineral.

Coloured minerals, whether uniaxial or biaxial, are generally pleochroic.

To describe the pleochroism for uniaxial minerals must specify the colour which corresponds to the ordinary and extraordinary rays.

e.g. Tourmaline, Hexagonal mineral o omega = dark green o epsilon = pale green

If the colour change is quite distinct the pleochroism is said to be strong.

If the colour change is minor = weak pleochroism.

For coloured uniaxial minerals, sections cut perpendicular to the c axis will show a single colour, corresponding to ordinary ray.

Sections parallel to the c crystallographic axis will exhibit the widest colour variation as both omega and epsilon are present.

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Nicol prism

A Nicol prism

A Nicol prism is a type of polarizer, an optical device used to generate a beam of polarized light. It was the first type of polarizing prism to be invented, in 1828 by William Nicol (1770-1851) of Edinburgh. It consists of a rhombohedral crystal of calcite (Iceland spar) that has been cut at a 68° angle, split diagonally, and then joined again using Canada balsam (a transparent liquid.)

Unpolarized light enters one end of the crystal and is split into two polarized rays by birefringence. One of these rays (the ordinary or o-ray) experiences a refractive index of no = 1.658 and at the balsam layer (refractive index n = 1.55) undergoes total internal reflection at the interface, and is reflected to the side of the prism. The other ray (the extraordinary or e-ray) experiences a lower refractive index (ne = 1.486), is not reflected at the interface, and leaves through the second half of the prism as plane polarized light.

Nicol prisms were once widely used in microscopy and polarimetry, and the term "crossed Nicols" (abbreviated as XN) is still used to refer to observation of a sample between orthogonally orientated polarizers. In most instruments, however, Nicol prisms have been supplanted by other types of polarizers such as Polaroid sheets and Glan–Thompson prisms.

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CROSSED NICOLS

Mining a. In optical mineralogy, an anisotropic crystal is interposed between the nicol prisms to observe its optical interference effects. The petrographic microscope is normally used with nicol prisms (or equivalent polarizing devices) in the crossed positionb. Nicols is often capitalized (crossed Nicols). Two nicol prisms placed one in front of the other, or one below the other, and so oriented that their transmission planes for plane-polarized light are at right angles with the result that light transmitted by one is stopped by the other unless modified by some intervening bodyc. In polarized-light microscopy, the arrangement where the permitted electric vectors of the two nicol prisms are at right angles.

Polarizing Microscope

The polarizing microscope is a much an optical measuring instrument as it is an instrument for the detailed examination of specimens. In addition to standard microscope optics, there is a polarizer in the condenser and another mounted in a slider in the tube above the objective, both in rotatable, graduated, mounts. The specimen is illuminated with plane polarized light, and its rotation of this light can be analyzed. The polarizing microscope is particularly useful in the study of birefringent materials such as crystals and strained non-crystalline substances. It is widely used for chemical microscopy and optical mineralogy. The current specimen is equipped with a quick change, centering nosepiece and a graduated, rotating stage. The upper slider contains a Bertrand lens, to allow telescopic observation of the rear lens element of the objective. It is described by the manufacturer as suitable for routine work or for student use.

What is a Polarizing Microscope?

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A polarizing microscope is a microscope that is mainly used in geological studies to study geological specimens. For this reason, it is also known as a petrographic microscope. It is used in other scientific fields such as medicine and biology as well.

Polarizing microscopes are built like regular optical microscope, but are fitted with some extra features. Unlike regular microscopes which use normal light, a polarizing microscope uses polarized light to study specimens. In polarized light, the light waves vibrate in one direction; in normal light, the light waves vibrate in random directions.

Polarized light cannot be seen by human eyes in normal circumstances. It can, however, be used in polarized light microscopy to highlight features of minerals and other materials. A polarizing microscope uses the birefringent optical properties of anisotropic materials to study them.

Anisotropic materials are solid substances that have several refractive indices; isotropic materials, which includes gases and liquids, have only one refractive index. Birefringence or double refraction occurs when a light wave passing through an anisotropic material is split into two rays of differing velocities.

Extinction angle

Extinction is a term used in optical mineralogy and petrology, which describes when cross-polarized light dims, as viewed through a thin section of a mineral in a petrographic microscope. An isotropic mineral or an opaque (metallic) mineral shows no light (i.e. constant extinction). Anisotropic minerals will show one extinction for each 90 degrees of stage rotation.

The Extinction angle is the measure between the cleavage direction or habit of a mineral and the extinction. To find this, simply line up the cleavage lines/long direction with one of the cross hairs in the microscope, and turn the mineral until the extinction occurs. The number of degrees the stage was rotated is the extinction angle, between 0-89 degrees. 90 degrees would be considered zero degrees, and is known as parallel extinction. Inclined extinction is a measured angle between 1-89 degrees. Minerals with two cleavages can have two extinction angles, and minerals in which the multiple angles are the same are called symmetrical extinction. Minerals that have no cleavage or elongation can not have an extinction angle.

EXTINCTION

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Anisotropic minerals go extinct between crossed polars every 90° of rotation. Extinction occurs when one vibration direction of a mineral is parallel with the lower polarizer. As a result no component of the incident light can be resolved into the vibration direction of the upper polarizer, so all the light which passes through the mineral is absorbed at the upper polarizer, and the mineral is black.

Upon rotating the stage to the 45° position, a maximum component of both the slow and fast ray is available to be resolved into the vibration direction of the upper polarizer. Allowing a maximum amount of light to pass and the mineral appears brightest.

Types of Extinction1. Parallel Extinction

The mineral grain is extinct when the cleavage or length is aligned with one of the crosshairs.The extinction angle (EA) = 0°

e.g.

o orthopyroxene o biotite

 

2. Inclined ExtinctionThe mineral is extinct when the cleavage is at an angle to the crosshairs.EA > 0°

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e.g. o clinopyroxene o hornblende  

3. Symmetrical ExtinctionThe mineral grain displays two cleavages or two distinct crystal faces. It is possible to measure two extinction angles between each cleavage or face and the vibration directions. If the two angles are equal then Symmetrical extinction exists.EA1 = EA2

e.g.

o amphibole o calcite

 

4. No CleavageMinerals which are not elongated or do not exhibit a prominent cleavage will still go extinct every 90° of rotation, but there is no cleavage or elongation direction

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from which to measure the extinction angle.

e.g.

o quartz o olivine

Extinction angle

This is the angle formed by one line of the crystal with the extinction position. Either the longest dimension of the mineral or the system of cleavage lines are generally used as this line of reference.

In order to determine the angle, at first only the polariser is incorporated and the line is made to coincide with the direction of the polariser (E-W). The analyser is introduced and turned slowly until extinction occurs and the angle turned will be the extinction angle. If the angle is greater than 45°, the analyser should be turned the other way to find the real angle. If when the analyser is introduced, the crystal is already dark, the extinction angle is 0° and the mineral is said to have a straight, or parallel extinction.

The measurement of the extinction angle is of more importance if it is measured in relation to a certain direction of vibration, e.g. the slow component of the mineral.

Pleochroism

 What is?

Pleochroism is the ability of a mineral to absorb different wavelengths of transmitted light depending upon its crystallographic orientations.

This is the property which some minerals present by which they appear to be a different colour depending on the direction in which they are observed, i.e. according to the direction of vibration of the waves that cross them.

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Why does it appear?

Just as the refractive index changes with direction, the selective absorption of light by some anisotropic minerals also does so, and as this is responsible for colour, this changes with direction.

Interference coloursinterference colours (polarization colours) In mineral optics, the colours produced when the analyser is inserted on a thin-section microscope. They are produced as a result of birefringence (double refraction) whereby one ray of light is retarded relative to the other. The different degrees of retardation give different interference colours. These colours are used in a number of ways as an aid to identification.

In order to distinguish the colours produced by different multiples of wavelengths, the interference colours are grouped in orders, as shown in the figure, which is a reproduction of the Michel-Levy chromatic scale.

Black-grey, yellow, orange and red are grouped together in the first order. These colours represent retardations equivalent to: less than landa of any colour, landa of violet, blue and green respectively.

The second order is made up of the six basic colours: violet, blue, green, yellow, orange and red. The first three correspond to the retardations of one landa of their

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complementary colour, whilst the other three (yellow, orange and red) represent the retardations of two landa of their complementary colours.

The next orders are also made up of these six colours, but logically they represent retardations of increasingly higher multiples of landas.

How can we determine the order of the colour?

Figure

CIPW-norm

An igneous rock of given chemical composition (of elements) usually contains crystals of the same minerals (crystalline molecules) as other rocks of that composition. Figure shows the minerals that are associated with the various silica contents of igneous rocks. This led to the use of the CIPW-norm in the classification of rocks. The CIPW-norm was developed in 1902 by four eminent petrologists who also left the initial letters of their surnames (Cross, Iddings, Pirrson, and Washington) to posterity. Using their knowledge of coarse grained rocks and the results of chemical analysis, they were able to reconstitute their samples theoretically as if they were coarse-grained. This operation allowed rocks of different grain size to be compared in terms of their bulk chemistry with recourse to only a few simple rules and some elementary, but tedious, arithmetic. Comparison of the normative composition of basalt, which is very fine grained, with gabbro, which is coarse grained, shows that the two are identical in CIPW-norm terms. This implies that, apart from their cooling history, they have a similar petrogenesis. Modem techniques for the determination of bulk chemistry have made analyses widely available for comparison with one another, and the CIPW-norm as an intermediary has become largely redundant. However, as many rock analyses in the literature have been classified in terms of the CIPW-norm, it is still a standard method of classification.

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Fig.. Proportions of minerals in igneous rocks classified according to where they are located on the ‘ultrabasic’.

Plutonic rocks

A pluton in geology is an intrusive igneous rock (called a plutonic rock) body that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies. In practice, "pluton" usually refers to a distinctive mass of igneous rock, typically kilometers in dimension, without a tabular shape like those of dikes and sills. Batholiths commonly are aggregations of plutons. The most common rock types in plutons are granite, granodiorite, tonalite, monzonite, and quartz diorite. The term granitoid is used for a general, light colored, coarse-grained igneous rock in which a proper, or more specific name, is not known. Use of granitoid should be restricted to the field wherever possible.

The term originated from Pluto, the ancient Roman god of the underworld. The use of the name and concept goes back to the beginnings of the science of geology in the late 1700s and the then hotly debated theories of Neptunism, Vulcanism and Plutonism regarding the origin of basalt.

Hypabyssal rock Or (Subvolcanic rock)

A subvolcanic rock is an igneous rock that originates at medium to shallow depths within the crust and contain intermediate grain size and often porphyritic texture. They have textures between volcanic and plutonic rocks. Subvolcanic rocks include diabase and porphyry.

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Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals are called phenocrysts. In its non-geologic, traditional use, the term "porphyry" refers to the purple-red form of this stone, valued for its appearance.

Volcanic rock

Ignimbrite is a deposit of a pyroclastic flow.

Volcanic rock is an igneous rock of volcanic origin.

Texture

Volcanic rocks are usually fine-grained or aphanitic to glass in texture. They often contain clasts of other rocks and phenocrysts. Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye. Rhomb porphyry is an example with large rhomb shaped phenocrysts embedded in a very fine grained matrix.

Volcanic rocks often have a vesicular texture caused by voids left by volatiles escaping from the molten lava. Pumice is an example of explosive volcanic eruption. It is so vesicular that it floats in water.

Naming

Vesicular olivine basalt from La Palma (green phenocrysts are olivine).

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Volcanic rocks are named according to both their chemical composition and texture. Basalt is a very common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has silica content similar to that of granite while basalt is compositionally equal to gabbro. Intermediate volcanic rocks include andesite, dacite, trachyte, and latite.

Pyroclastic rocks are the product of explosive volcanism. They are often felsic (high in silica). Pyroclastic rocks are often the result of volcanic debris, such as ash, bombs and tephra, and other volcanic ejecta. Examples of pyroclastic rocks are tuff and ignimbrite.

Shallow intrusions, which possess structure similar to volcanic rather than plutonic rocks are also considered to be volcanic.

Composition of volcanic rocks

Aā next to pāhoehoe lava at the Craters of the Moon National Monument and Preserve, Idaho, United States.

The sub-family of rocks that form from volcanic lava are called igneous volcanic rocks (to differentiate them from igneous rocks that form from magma below the surface, called igneous plutonic rocks).

The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a rhyolite lava-stream cools quickly, it can quickly freeze into a black glassy substance called obsidian. When filled with bubbles of gas, the same lava may form the spongy mineral pumice. Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite.

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Batholith

Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevada batholith.

A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of igneous intrusive (also called plutonic) rock that forms from cooled magma deep in the earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite.

Laccolith

A laccolith is an igneous intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.

A laccolith intruding into and deforming strata

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Laccolith exposed by erosion of overlying strata in Montana

Laccoliths tend to form at relatively shallow depths and are typically formed by relatively viscous magmas, such as those that crystallize to diorite, granodiorite, and granite. Cooling underground takes place slowly, giving time for larger crystals to form in the cooling magma. The surface rock above laccoliths often erodes away completely, leaving the core mound of igneous rock.

Phacolith

A phacolith is a pluton parallel to the bedding plane or foliation of folded country rock. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an anticline or the trough of a syncline. In rare cases the body may extend as a sill from the crest of an anticline through the trough of an adjacent syncline, such that in cross section it has an S shape. In intensely folded terrain the hinge of folds would be areas of reduced pressure and thus potential sites for magma migration and emplacement.

Lopolith

A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body. The term was first defined and used by Frank Fitch Grout during the early 1900s in describing the Duluth gabbro complex in northern Minnesota and adjacent Ontario.

Sill

In geology, a sill is a tabular pluton that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means that the sill does not cut across preexisting rocks, in contrast to dikes which do cut across older rocks.

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Illustration showing the difference between a dike and a sill.

Sills are always parallel to beds (layers) of the surrounding country rock. Usually they are in a horizontal orientation, although tectonic processes can cause rotation of sills into near vertical orientations. They can be confused with solidified lava flows; however, there are several differences between them. Intruded sills will show partial melting and incorporation of the surrounding country rock. On both the "upper" and "lower" contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed (contact metamorphism). Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will typically show evidence of vesicles (bubbles) where gases escaped into the atmosphere. Because sills generally form at depth (up to many kilometers), the pressure of overlying rock prevents this from happening much, if at all. Lava flows will also typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock, typically do not.

Dike

Banded gneiss with dike of granite orthogneiss.

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A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across

planar wall rock structures, such as bedding or foliation massive rock formations, like igneous/magmatic intrusions and salt diapirs.

Dikes can therefore be either intrusive or sedimentary in origin.

Vesicular texture

Vesicular olivine basalt.

Vesicular texture is a volcanic rock texture characterised by, or containing, many vesicles. The texture is often found in extrusive aphanitic, or glassy, igneous rock. The vesicles are small cavities formed by the expansion of bubbles of gas or steam during the solidification of the rock.

A related texture is amygdaloidal in which the volcanic rock, usually basalt or andesite, has cavities, or vesicles, that are filled with secondary minerals, such as zeolites, calcite, quartz, or chalcedony. Individual cavity fillings are termed amygdules (American usage), or amygdales (British usage). Sometimes these can be sources of semi-precious stones such as agate.

Rock types that display a vesicular texture include pumice and scoria.

Amygdaloidal Structure

Amygdules or amygdales form when the gas bubbles or vesicles in volcanic lava (or other extrusive igneous rocks) are infilled with a secondary mineral such as calcite, quartz, chlorite or one of the zeolites. Amygules usually form after the rock has been emplaced, and are often associated with low-temperature alteration. Amydules may often be concentrically zoned. Rocks containing amygdules can be described as amygdaloidal.

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Block Lava

These lavas, which have surfaces covered by angular fragments, differ from aa in that the fragments have more regular forms and smoother faces. The surface blocks often approach cubes in form. Blocky lava flows form from more viscous lavas than aa flows, with the angular blocks formed by breaking up of the partly to wholly congealed upper part of the flow as still-mobile magma moves beneath the thick crust.

Pillow Lava

These flows are subaqueously extruded lava marked by bulbous forms. Pillow lavas may form by the discharge of lavas into rivers, lakes, ponds or under glaciers, as well into oceans. The pillow structures result from the protrusion of elongate lava lobes, which detach from and fall down the moving flow front. Lava pillows are often confused with pahoehoe toes, but the former have several distinguishing characteristics.

Phenocryst

A phenocryst is a relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of a porphyritic igneous rock. Phenocrysts often have euhedral forms either due to early growth within a magma or by post-emplacement recrystallization.

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Vitrophyre  

A vitrophyre is another name for a phenocryst-bearing obsidian. The phenocrysts in the above photomicrograph are mostly plagioclase. The groundmass is obsidian glass. Can you think of some possible explanations to account for the extremely large difference in grain size in this rock?

vitrophyre(from the article `igneous rock`) ...rapidly, and congealed to form a finer-grained or glassy groundmass. A porphyritic volcanic rock with a glassy groundmass.

Granulitic texture Granulite - At the highest grades of metamorphism most of the

hydrous minerals and sheet silicates become unstable and thus there are few minerals present that would show a preferred orientation.  The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks.

Formation

Granulites form at high temperature conditions at a range of pressure conditions, typically during regional metamorphism. In some cases, the high temperatures are difficult to account for at the inferred depths at typical geothermal gradients. In extreme cases, granulites may form at temperatures in excess of 1000 degrees Celsius.

AllotriomorphicOf minerals in igneous rock not bounded by their own crystal faces but having their outlines impressed on them by the adjacent minerals. Also known as anhedral; xenomorphic.

Ophitic

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A variety of this, known as ophitic is very characteristic of many diabases, in which large plates of augite enclose smaller laths of plagioclase feldspar. Biotite and hornblende frequently enclose feldspar ophitically; less commonly iron oxides and sphene do so. In peridotites the "lustre-mottled" structure arises from pyroxene or hornblende enveloping olivine in the same manner.

TrachyticA light-colored igneous rock consisting essentially of alkali feldspar.

Graphic texture

Graphic texture. Photo: Eurico ZimbresThe feldspar is white and roughly 10 x 10 centimeters. Quartz are the little cuneiform and gray ones

Graphic texture is commonly created by exsolution and devitrification and immiscibility processes in igneous rocks. It is called 'graphic' because the exsolved or devitrified minerals form wriggly lines and shapes which are reminiscent of writing.

Micrographic texture

In petrology, micrographic texture is a fine-grained intergrowth of quartz and alkali feldspar, interpreted as the last product of consolidation in some igneous rocks which contain high or moderately high percentages of silica. Micropegmatite is an outmoded terminology for micrographic texture.

This fine-grained texture is similar to the coarser intergrowths in certain pegmatites and coarse granitic veins; the quartz forms angular patches scattered through a matrix of feldspar. In polarized light the separate areas of each mineral extinguish at the same time, and this proves that, even though apparently discontinuous, they have the same crystalline orientation.

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Other textures that may be evident on microscopic examination of igneous rocks are as follows:

Myrmekitic texture - an intergrowth of quartz and plagioclase that shows small wormlike bodies of quartz enclosed in plagioclase. This texture is found in granites.

Ophitic texture - laths of plagioclase in a coarse grained matrix of pyroxene crystals, wherein the plagioclase is totally surrounded by pyroxene grains. This texture is common in diabases and gabbros.

Subophitic texture - similar to ophitic texture wherein the plagioclase grains are not completely enclosed in a matrix of pyroxene grains.

Poikilitic texture - smaller grains of one mineral are completely enclosed in large, optically continuous grains of another mineral.

Intergranular texture - a texture in which the angular interstices between plagioclase grains are occupied by grains of ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides.

Intersertal texture - a texture similar to intergranular texture except that the interstices between plagioclase grains are occupied by glass or cryptocrystalline material.

Hyaloophitic texture - a texture similar to ophitic texture except that glass completely surrounds the plagioclase laths.

Hyalopilitic texture - a texture wherein microlites of plagioclase are more abundant than groundmass, and the groundmass consists of glass which occupies the tiny interstices between plagioclase grains.

Trachytic texture - a texture wherein plagioclase grains show a preferred orientation due to flowage, and the interstices between plagioclase grains are occupied by glass or cryptocrystalline material.

Coronas or reaction rims - often times reaction rims or coronas surround individual crystals as a result of the crystal becoming unstable and reacting with its surrounding crystals or melt. If such rims are present on crystals they should be noted in the textural description.

Patchy zoning - This sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different compositions as evidenced by going extinct at angles different from other zones in the crystal.

Oscillatory zoning - This sometimes occurs in plagioclase grains wherein concentric zones around the grain show thin zones of different composition as evidenced by extinction phenomena.

Moth eaten texture (also called sieve texture)- This sometimes occurs in plagioclase wherein individual plagioclase grains show an abundance of glassy inclusions.

Perthitic texture - Exsolution lamellae of albite occurring in orthoclase or microcline.

IGNEOUS ROCK TEXTURES  Phaneritic Texture

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  Examples of Phaneritic Rocks (the three images below show a hand sample, low magnification of a hand sample and a thin section of phaneritic textured rocks)

Phaneritic textured rocks are comprised of large crystals that are clearly visible to the eye with or without a hand lens or binocular microscope. The entire rock is made up of large crystals, which are generally 1/2 mm to several centimeters in size; no fine matrix material is present. This texture forms by slow cooling of magma deep underground in the plutonic environment.

The cartoon sketch above, though highly idealized, attempts to make the point that in order to be truly phaneritic all of the mineral grains must be visible. The beginner often makes the mistake of identifying porphyritic textured (see discussion below) aphanitic rocks as phaneritic. For the more felsic rocks like granite, phaneritic texture is rarely misidentified. But dark rocks like gabrro are more problematic. A good rule of thumb is that fine grained or aphanitic rocks are dull appearing, while phaneritic rocks are brighter or shinier (of course be careful of a glassy rock like obsidian).

  Aphanitic Texture

 

Examples of Aphanitic Rocks (the two images below show a hand sample and a thin section of aphanitic textured rocks)

Aphanitic texture consists of small crystals that cannot be seen by the eye with or hand lens. The entire rock is made up of small crystals, which are generally less than 1/2 mm in size. This texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments.

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Porphyritic Texture

  Porphyritic Rocks (the two images below show a hand sample and a thin section of porphyritic aphanitic textured rocks)

Porphyritic rocks are composed of at least two minerals having a conspicuous (large) difference in grain size. The larger grains are termed phenocrysts and the finer grains either matrix or groundmass (see the drawing below and image to the left). Porphyritic rocks are thought to have undergone two stages of cooling; one at depth where the larger phenocrysts formed and a second at or near the surface where the matrix grains crystallized.

Both aphanitic and phaneritic rocks can be porphyritic, but the former are far more common. Most often the

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porphritic term is utilized as a modifier. For instance, an andesite with visible phenocrysts of plagioclase feldspar would be termed an andesite porphyry or porphyritic andesite (see photo above).

  Glassy Texture

Glassy textured igneous rocks are non-crystalline meaning the rock contains no mineral grains. Glass results from cooling that is so fast that minerals do not have a chance to crystallize. This may happen when magma or lava comes into quick contact with much cooler materials near the Earth's surface. Pure volcanic glass is known as obsidian (see photo).

  Vesicular Texture

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This term refers to vesicles (holes, pores, or cavities) within the igneous rock. Vesicles are the result of gas expansion (bubbles), which often occurs during volcanic eruptions. Pumice and scoria are common types of vesicular rocks. The image to the left shows a basalt with vesicles, hence the name "vesicular basalt".

  Fragmental Texture

The last textural term is reserved for pyroclastic rocks, those blown out into the atmosphere during violent volcanic eruptiions. These rocks are collectively termed fragmental. If you examine a fragmental volcanic rock closely you can see why. You will note that it is comprised of numerous grains or fragments that have been welded together by the heat of volcanic eruption. The terminology for fragmental rocks is voluminous, but most are

simply identified as "tuff".

 

IGNEOUS TEXTURES AND STRUCTURES

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I. SHAPE OF INDIVIDUAL CRYSTALS (indicates sequence of crystallization)

A. Euhedral. Crystal completely bounded by its own crystal faces. Indicates earlycrystallization from the magma, i.e. before enough other crystals were present to causeinterference for space.B. Anhedral. Crystal not bounded by any of its own crystal faces; rather, its form is imposedon it by the adjacent crystals. Indicates late crystallization from the magma, i.e. after most ofthe available space was already occupied by earlier-formed crystals.C. Subhedral. Intermediate between euhedral and anhedral.

II. GENERAL ROCK TEXTURES

A. Based on Degree of Crystallinity1. Holohyaline. Rock composed entirely of glass; no crystals visible even withmagnification. Indicates cooling so rapid that no crystal growth could occur, i.e.quenching.2. Holocrystalline. Rock composed entirely of crystals (which may or may not bevisible without magnification). Indicates cooling that was sufficiently slow to allowcomplete crystallization to occur.3. Hypocrystalline (or Hyalocrystalline). Rock composed of both crystals and glass.Indicates a period of relatively slow cooling (sufficiently slow to allow crystallization)followed by quenching of the remaining magma.B. Based on Crystal Size (obviously does not apply to holohyaline rocks)1. Equigranular. All of the crystals are approximately the same size. Indicates that theentire rock crystallized under a single set of P-T (depth) conditions.a. Aphanitic. Uniformly fine-grained texture in which the individual crystals are toosmall to be seen easily without magnification. Indicates rapid cooling (but notquenching), i.e. volcanic extrusion.i. Microcrystalline. Individual crystals large enough to be seen easily with apetrographic microscope.ii. Cryptocrystalline. Individual crystals to small to be seen easily even with apetrographic microscope; but sufficiently crystalline to give a strong x-raydiffraction pattern.b. Phaneritic. Uniformly coarse-grained texture in which all the individual crystalsare easily visible without magnification. Indicates slow cooling, i.e. intrusion.i. Fine phaneritic. Average crystal size < 1 mm. Indicates shallow intrusivecooling, i.e. hypabyssal intrusion (dikes and sills).ii. Medium phaneritic. Average crystal size 1-5 mm. The most common texturefor common plutonic rocks (i.e. granites).iii. Coarse phaneritic. Average crystal size 5-10 mm. Indicates deep intrusivecooling, i.e. plutonic intrusion (batholiths).iv. Pegmatitic. Average crystal size > 10 mm. Indicates intrusive cooling of an

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abnormally gas-rich, silicic magma.2. Porphyritic. Texture consisting of crystals of two distinctly different sizes. Indicatestwo distinctly different modes (episodes) of cooling. The larger crystals (usually of atmost two or three different minerals) are called phenocrysts and are usually nearlyeuhedral. The material surrounding the phenocrysts is called the matrix orgroundmass.a. Porphyritic-hyaline (or Vitrophyric). Phenocrysts surrounded by glassygroundmass. Indicates an initial period of slow (intrusive) crystallization followed byquenching of the remaining magma.b. Porphyritic-aphanitic. Phenocrysts surrounded by aphanitic groundmass.Indicates an initial period of slow (intrusive) crystallization followed by rapid(extrusive) crystallization of the remaining magma. The most common texture forcommon volcanic rocks (i.e. basalts).c. Porphyritic-phaneritic. Phenocrysts surrounded by phaneritic groundmass.Indicates two stages of slow crystallization, i.e. intrusion at two different levels.C. Based on Crystal Shape (used almost exclusively for holocrystalline, phaneritic rocks)1. Allotriomorphic (or Xenomorphic). Rock composed almost entirely of anhedralcrystals. Indicates simultaneous growth of all the various minerals present. A specialtype of allotriomorphic texture formed by the interpenetration of very large,crystallographically continuous crystals of quartz and alkali feldspar is known asGraphic Intergrowth.2. Hypidiomorphic. Rock composed of intergrown euhedral and anhedral crystals.Indicates sequential growth of the various minerals present.3. Idiomorphic (or Panidiomorphic). Rock composed almost entirely of euhedralcrystals. This is the hypothetical opposite of allotriomorphic; it almost never occurs innature.

1. Xenoliths. Rock fragment inclusions.2. Xenocrysts. Inclusions of individual crystals of wall-rock material. Indicate that thewall rock soft enough to be easily disaggregated (i.e. still hot). Often difficult todistinguish from phenocrysts unless of "anomalous" composition.D. Flow Textures and Structures (Flow Banding)1. Aligned Crystals (Trachytoid or Trachytic Texture).2. Broken Phenocrysts.3. Elongated Vesicles.4. Elongated Xenoliths (Schlieren Structure).E. Volatile-escape Textures. Gas bubble holes (vesicles) indicate that gas was being liberatedfrom the magma at the time that solidification was being completed.1. Vesicular Texture. Aphanitic rock containing isolated vesicles (constituting less than50% of the total volume).2. Scoriaceous Texture. Aphanitic rock containing abundant vesicles (constituting morethan 50% of the total volume).3. Pumiceous Texture. Glassy material containing vesicles.4. Miarolitic Texture. Phaneritic rock containing angular gas holes..F. Post-solidification Textures and Structures

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1. Amygdules. Filling of vesicles by precipitation from aqueous solutions.

Granite

Granite (pronounced) is a common and widely occurring type of intrusive, felsic, igneous rock. Granites usually have a medium to coarse grained texture. Occasionally some individual crystals (phenocrysts) are larger than the groundmass in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is sometimes known as a porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to form tors, and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels.

Granite is nearly always massive (lacking internal structures), hard and tough, and therefore it has gained widespread use as a construction stone. The average density of granite is 2.75 g/cm3 and its viscosity at standard temperature and pressure is ~4.5 • 1019

Pa·s.[1]

The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.

Granitoid is used as a descriptive field term for general, light colored, coarse-grained igneous rocks for which a more specific name requires petrographic examination.[2]

Mineralogy

Orbicular granite near the town of Caldera, northern Chile

Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is named according to the percentage of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar on the A-Q-P half of the diagram.

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Chemical composition

A worldwide average of the chemical composition of granite, by weight percent: [3]

The Stawamus Chief is a granite monolith in British Columbia SiO2 — 72.04% Al2O3 — 14.42% K2O — 4.12% Na2O — 3.69% CaO — 1.82% FeO — 1.68% Fe2O3 — 1.22% MgO — 0.71% TiO2 — 0.30% P2O5 — 0.12% MnO — 0.05%

Based on 2485 analyses

Occurrence

Granite is currently known only on Earth where it forms a major part of continental crust. Granite often occurs as relatively small, less than 100 km² stock masses (stocks) and in batholiths that are often associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of granitic intrusions. In some locations very coarse-grained pegmatite masses occur with granite.

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Origin

Close-up of granite exposed in Chennai, India.

Granite is an igneous rock and is formed from magma. Granitic magma has many potential origins but it must intrude other rocks. Most granite intrusions are emplaced at depth within the crust, usually greater than 1.5 kilometres and up to 50 km depth within thick continental crust.

Granodiorite

A sample of granodiorite rock from Slovakia

Photomicrograph of thin section of granodiorite from Slovakia (in crossed polarised light)

Granodiorite is an intrusive igneous rock similar to granite, but contains more plagioclase than potassium feldspar. It usually contains abundant biotite mica and hornblende, giving it a darker appearance than true granite. Mica may be present in well-formed hexagonal crystals, and hornblende may appear as needle-like crystals.

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On average the upper continental crust has the same composition as granodiorite.

Granodiorite is a plutonic igneous rock, formed by an intrusion of quartz-rich magma, which cools in batholiths or stocks below the Earth's surface. It is usually only exposed at the surface after erosion and uplift have occurred.

Syenite

Syenite

Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts (<5%). The feldspar component of syenite is predominantly alkaline in character (usually orthoclase) . Plagioclase feldspars may be present in small quantities, less than 10%. When present, ferromagnesian minerals are usually hornblende amphibole, rarely pyroxene or biotite. Biotite is rare, because in a syenite magma most aluminium is used in producing feldspar. Syenites are usually peralkaline and peraluminous, with high proportions of alkali elements and aluminium.

Nepheline syenite

Nepheline syenite from Sweden

Nephelene syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusive equivalent.

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Petrology

Nepheline syenites are silica-undersaturated and some are peralkaline (terms discussed in igneous rock). Nepheline is a feldspathoid, a solid-solution mineral, that does not coexist with quartz; rather, nepheline would react with quartz to produce alkali feldspar.

.Chemical composition

The chemical peculiarities of the nepheline-syenites are well marked. They are exceedingly rich in alkalis and in alumina (hence the abundance of felspathoids and alkali feldspars) with silica varying from 50 to 56%, while lime, magnesia[disambiguation needed] and iron are never present in great quantity, though somewhat more variable than the other components.

Diorite

Diorite

Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory minerals.[1] It can also be black or bluish-grey, and frequently has a greenish cast. Varieties deficient in hornblende and other dark minerals are called leucodiorite. When olivine and more iron-rich augite are present, the rock grades into ferrodiorite, which is transitional to gabbro. The presence of significant quartz makes the rock type quartz-diorite (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassium feldspar) is present at greater than ten percent the rock type grades into monzodiorite or granodiorite. Diorite has a medium grain size texture, occasionally with porphyry.

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Diorite classification on QAPF diagram

Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from partial melting of a mafic rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building such as in the Andes Mountains as large batholiths. The extrusive volcanic equivalent rock type is andesite.

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Pegmatite

Pegmatite with blue corundum crystals

A pegmatite is a very coarse-grained, intrusive igneous rock composed of interlocking grains usually larger than 2.5 cm in size;[1] such rocks are referred to as pegmatitic.

Most pegmatites are composed of quartz, feldspar and mica; in essence a granite. Rarer intermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene and other minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions.

Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm in size. Individual crystals over 10 meters across have been found, and the world's largest

Low rates of nucleation of crystals coupled with high diffusivity to force growth of a few large crystals instead of many smaller crystals

High vapor and water pressure, to assist in the enhancement of conditions of diffusivity

High concentrations of fluxing elements such as boron and lithium which lower the temperature of solidification within the magma or vapor

Low thermal gradients coupled with a high wall rock temperature, explaining the preponderance for pegmatite to occur only within greenschist metamorphic terranes

Mineralogy

Pegmatitic granite, Rock Creek Canyon, eastern Sierra Nevada, California. Note pink potassium feldspars and cumulate-filled chamber.

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The mineralogy of a pegmatite is in all cases dominated by some form of feldspar, often with mica and usually with quartz, being altogether "granitic" in character. Beyond that, pegmatite may include most minerals associated with granite and granite-associated hydrothermal systems, granite-associated mineralisation styles, for example greisens, and somewhat with skarn associated mineralisation.

Geochemistry

Pegmatite is difficult to sample representatively due to the large size of the constituent mineral crystals. Often, bulk samples of some 50–60 kg of rock must be crushed to obtain a meaningful and repeatable result. Hence, pegmatite is often characterised by sampling the individual minerals which comprise the pegmatite, and comparisons are made according to mineral chemistry.

Economic importance

Pegmatites are important because they often contain rare earth minerals and gemstones, such as aquamarine, tourmaline, topaz, fluorite, apatite and corundum, often along with tin and tungsten minerals, among others. For example, beautiful crystals of aquamarines and topaz can be found in pegmatites in the mountains of Colorado and Idaho.

Occurrence

Pegmatite is essentially restricted to Barrovian Facies Sequence metamorphic rocks of at least middle greenschist facies, and often also intimately associated with granites intruding into such terranes.

Worldwide, notable pegmatite occurrences are within the major cratons, and within greenschist-facies metamorphic belts. However, pegmatite localities are only well recorded when economic mineralisation is found.

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Gabbro

Gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.

Photomicrograph of a thin section of gabbro.

Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are Intrusive/volcanic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass.

The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced by basalt magmatism at mid-ocean ridges.

Petrology

A gabbro landscape on the main ridge of the Cuillin, Isle of Skye, Scotland.

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Gabbro as a xenolith in a granite, eastern Sierra Nevada, Rock Creek Canyon, California.

Gabbro is dense, greenish or dark-colored and contains pyroxene, plagioclase, amphibole, and olivine (olivine gabbro when olivine is present in a large amount).

The pyroxene is mostly clinopyroxene; small amounts of orthopyroxene may be present. If the amount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock is then a norite. Quartz gabbros are also known to occur and are probably derived from magma that was over-saturated with silica. Essexites represent gabbros whose parent magma was under-saturated with silica, resulting in the formation of the feldspathoid mineral nepheline. (Silica saturation of a rock can be evaluated by normative mineralogy). Gabbros contain minor amounts, typically a few percent, of iron-titanium oxides such as magnetite, ilmenite, and ulvospinel.

Distribution

Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene and plagioclase, or as part of a layered intrusion as a cumulate formed by settling of pyroxene and plagioclase. Cumulate gabbros are more properly termed pyroxene-plagioclase orthocumulate.

Uses

Gabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides.

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Anorthosite

A coarse-grained basic igneous rock made largely of plagioclase feldspar (95%), with small amounts of pyroxene (4%), olivine, and iron oxides. Usually light in color, it may have bands of darker minerals. Anorthosite forms deep underground, and in dikes and intrusions; it makes up about 60% of the Earth's crust. Anorthosite was found in all the rocks returned from the Moon, including the oldest (dating back 4.4 to 4.5 billion years), and is believed to make up a significant fraction of the lunar crust.

Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar (90–100%), and a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

Anorthosite on Earth can be divided into two types: Proterozoic anorthosite (also known as massif or massif-type anorthosite) and Archean anorthosite. These two types of anorthosite have different modes of occurrence, appear to be restricted to different periods in Earth's history, and are thought to have had different origins.

Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.[1]

Lunar anorthosite, Apollo 16 Image credit: NASA

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Pyroxenite

A sample of the orthopyroxenite

Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified (see diagram below) into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes. Closely allied to this group are the hornblendites, consisting essentially of hornblende and other amphiboles.

They are essentially of igneous origin, though some pyroxenites are included in the metamorphic complex of the Lewisian of Scotland. The pyroxene-rich rocks which result from the contact metamorphism of impure limestones are described as pyroxene hornfelses (calc-silicate hornfelses).

.

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Classification diagram for peridotite and pyroxenite, based on proportions of olivine and pyroxene. The pale green area encompasses the most common compositions of peridotite in the upper part of the Earth's mantle

The pyroxenites are often subject serpentinization under low temperature retrograde metanorphism and weathering. The rocks are often completely replaced by serpentines, which sometimes preserve the original structures of the primary minerals, such as the lamination of hypersthene and the rectangular cleavage of augite. Under pressure-metamorphism hornblende is developed and various types of amphibolite and hornblende-schist are produced.

Peridotite

 —  Igneous Rock  —

Peridotite xenolith from San Carlos, southwestern United

States. The rock is typical olivine-rich peridotite, cut by a

centimeter-thick layer of greenish-black pyroxenite.

Composition

olivine, pyroxene

A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from the Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

Types of peridotite

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Dunite: more than 90% olivine, typically with Mg/Fe ratio of about 9:1. Wehrlite: mostly composed of olivine plus clinopyroxene. Harzburgite: mostly composed of olivine plus orthopyroxene, and relatively low

proportions of basaltic ingredients (because garnet and clinopyroxene are minor). Lherzolite: mostly composed of olivine, orthopyroxene (commonly enstatite), and

clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients (garnet and clinopyroxene).

Classification diagram for peridotite and pyroxenite, based on proportions of olivine and pyroxene. The pale green area encompasses the most common compositions of peridotite in the upper part of the Earth's mantle (partly adapted from Bodinier and Godard (2004)).

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Distribution and location

Peridotite is the dominant rock of the Earth's mantle above a depth of about 400 km; below that depth, olivine is converted to the higher-pressure mineral wadsleyite. Oceanic plates consist of up to about 100 km of peridotite covered by a thin crust; the crust, commonly about 6 km thick, consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, "abyssal peridotite," is found on the walls of rifts in the deep sea floor. Oceanic plates are usually subducted back into the mantle in subduction zones.

Morphology and texture

Some peridotites are layered or are themselves layers; others are massive. Many layered peridotites occur near the base of bodies of stratified gabbroic complexes. Other layered peridotites occur isolated, but possibly once composed part of major gabbroic complexes. Both layered and massive peridotites can have any of three principal textures: (1) rather well formed crystals of olivine separated by other minerals.

Origin

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Peridotites have two primary modes of origin, as mantle rocks formed during the accretion and differentiation of the Earth, or as cumulate rocks formed by precipitation of olivine ± pyroxenes from basaltic or ultramafic magmas; these magmas are ultimately derived from the upper mantle by partial melting of mantle peridotites.

Mantle peridotites are sampled as alpine-type massifs in collisional mountain ranges or as xenoliths in basalt or kimberlite. In all cases these rocks are pyrometamorphic (that is, metamorphosed in the presence of molten rock) and represent either fertile mantle (lherzolite) or partially depleted mantle (harzburgite, dunite).

Rhyolite

Rhyolite

 —  Igneous Rock  —

Composition

Felsic: igneous quartz and alkali feldspar (orthoclase, sanidine

and sodic plagioclase), biotite and hornblende.

This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.

Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (silica-rich) composition (typically > 69% SiO2 — see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, alkali feldspar and plagioclase (in a ratio > 1:2 — see the QAPF diagram). Biotite and hornblende are common accessory minerals.

Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, and consequently, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolite melts are highly polymerized and form highly viscous lavas. They can also occur as breccias or in volcanic plugs and dikes. Rhyolites that cool too quickly to grow crystals form a natural glass or vitrophyre, also called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic, nodular, and lithophysal

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structures. Some rhyolite is highly vesicular pumice. Many eruptions of rhyolite are highly explosive and the deposits may consist of fallout tephra or of ignimbrites.

During the second millennium BC, rhyolite was quarried extensively in what is now eastern Pennsylvania in the United States. Among the leading quarries was the Carbaugh Run Rhyolite Quarry Site in Adams County, where as many as fifty small quarry pits are known.[1]

The name rhyolite has been introduced into science by the German traveler and geologist Ferdinand von Richthofen after his explorations in the Rocky Mountains in the 1860s.

Andesite

Andesite

 —  Igneous Rock  —

Photomicrograph of andesite in thin section (between crossed

polars)

Composition

Intemediate

Major minerals: plagioclase (often andesine) and pyroxene

and/or hornblende

Accessory minerals: magnetites, biotite, sphene, quartz

Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and granite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals.[1] Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams. Relative alkali and silica contents are illustrated in TAS diagrams.

Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.

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Andesite can be considered as the extrusive equivalent of plutonic diorite. Andesites are characteristic of subduction zones, such as the western margin of South America. The name andesite is derived from the Andes mountain range.

Basalt

Basalt

 —  Igneous Rock  —

Composition

Mafic: igneous amphibole and pyroxene, sometimes

plagioclase, feldspathoids, and/or olivine.

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey.

On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt has also formed on Earth's Moon, Mars, Venus, and even on the asteroid Vesta. Source rocks for the partial melts probably include both peridotite and pyroxenite. The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.

The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarse) groundmass are generally referred to as diabase (also called dolerite) or gabbro.

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Columnar basalt at Sheepeater Cliff in Yellowstone

Vesicular basalt at Sunset Crater, Arizona. US quarter for scale.

Uses

It's used in construction (e.g. as building blocks or in the groundwork), making cobblestone (from cloumnal basalt) and in making of statues. If heated and extruded, you get stone wool which is a great thermal insulator.

Types

Large masses must cool slowly to form a polygonal joint pattern. Tholeiitic basalt is relatively poor in silica and poor in sodium. Included in this

category are most basalts of the ocean floor, most large oceanic islands, and continental flood basalts such as the Columbia River Plateau.

MORB (Mid Ocean Ridge Basalt), is characteristically low in incompatible elements. MORB is commonly erupted only at ocean ridges. MORB itself has been subdivided into varieties such as NMORB and EMORB (slightly more enriched in incompatible elements).[3][4]

High alumina basalt may be silica-undersaturated or -oversaturated (see normative mineralogy). It has greater than 17% alumina (Al2O3) and is intermediate in composition between tholeiite and alkali basalt; the relatively alumina-rich composition is based on rocks without phenocrysts of plagioclase.

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Alkali basalt is relatively poor in silica and rich in sodium. It is silica-undersaturated and may contain feldspathoids, alkali feldspar and phlogopite.

Boninite is a high-magnesium form of basalt that is erupted generally in back-arc basins, distinguished by its low titanium content and trace element composition.

Petrology

Photomicrograph of a volcanic (basaltic) sand grain; upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter. Note white plagioclase 'microlites' in cross-polarized light picture, surrounded by very fine grained volcanic glass.

The mineralogy of basalt is characterized by a preponderance of calcic plagioclase feldspar and pyroxene. Olivine can also be a significant constituent. Accessory minerals present in relatively minor amounts include iron oxides and iron-titanium oxides, such as magnetite, ulvospinel, and ilmenite. Because of the presence of such oxide minerals, basalt can acquire strong magnetic signatures as it cools, and paleomagnetic studies have made extensive use of basalt.

Geochemistry

Relative to most common igneous rocks basalt compositions are rich in MgO and CaO and low in SiO2 and the alkali oxides, i.e., Na2O + K2O, consistent with the TAS classification.

Basalt generally has a composition of 45–55 wt% SiO2, 2–6 wt% total alkalis, 0.5–2.0 wt% TiO2, 5–14 wt% FeO and 14 wt% or more Al2O3. Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%.

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Morphology and textures

An active basalt lava flow

The shape, structure and texture of a basalt is diagnostic of how and where it erupted — whether into the sea, in an explosive cinder eruption or as creeping pahoehoe lava flows, the classic image of Hawaiian basalt eruptions.

Pillow basalts

When basalt erupts underwater or flows into the sea, contact with the water quenches the surface and the lava forms a distinctive pillow shape, through which the hot lava breaks to form another pillow. This pillow texture is very common in underwater basaltic flows and is diagnostic of an underwater eruption environment when found in ancient rocks.

When pahoehoe lava enters the sea it usually forms pillow basalts. However when a'a enters the ocean it forms a littoral cone, a small cone-shaped accumulation of tuffaceous debris formed when the blocky a'a lava enters the water and explodes from built-up steam.

Sediment

Sediment in the thalweg of Campbell creek in Alaska.

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Sediment is naturally-occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself.

Sediments are most often transported by water (fluvial processes) transported by wind (aeolian processes) and glaciers. Beach sands and river channel deposits are examples of fluvial transport and deposition, though sediment also often settles out of slow-moving or standing water in lakes and oceans. Desert sand dunes and loess are examples of aeolian transport and deposition. Glacial moraine deposits and till are ice transported sediments.

Grain size

Sediment size is measured on a log base 2 scale, called the "Phi" scale, which classifies particles by size from "colloid" to "boulder".

Weathering

It is the breaking down of Earth's rocks, soils and minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind and gravity.

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Weathering also is the effect of atmospheric exposure to man-made structures and materials.

Two important classifications of weathering processes exist — physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals (also known as biological weathering) in the breakdown of rocks, soils and minerals.[1]

The materials left over after the rock breaks down combined with organic material creates soil. The mineral content of the soil is determined by the parent material, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types (as in glacial, aeolian or alluvial sediments) often makes more fertile soil.

Physical weathering

Physical weathering is the only process that causes the disintegration of rocks without chemically changing it. The primary process in physical weathering is abrasion (the process by which clasts and other particles are reduced in size). However, chemical and physical weathering often go hand in hand. For example, cracks exploited by physical weathering will increase the surface area exposed to chemical action. Furthermore, the chemical action at minerals in cracks can aid the disintegration process.

Thermal expansion

Thermal expansion, also known as exfoliation, insolation weathering or thermal shock, often occurs in areas, like deserts, where there is a large diurnal temperature range. The temperatures soar high in the day, while dipping greatly at night. As the rock heats up and expands by day, and cools and contracts by night, stress is often exerted on the outer layers. The stress causes the peeling off of the outer layers of rocks in thin sheets. Though this is caused mainly by temperature changes, thermal expansion is enhanced by the presence of moisture. Forest fires and range fires are also known to cause significant weathering of rocks and boulders exposed along the ground surface. Intense, localized heat can rapidly expand a boulder, causing its surface to exfoliate or spall.

Pressure release

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Pressure Release of granite.

In pressure release, also known as unloading, overlying materials (not necessarily rocks) are removed (by erosion, or other processes), which causes underlying rocks to expand and fracture parallel to the surface. Often the overlying material is heavy, and the underlying rocks experience high pressure under them, for example, a moving glacier. Pressure release may also cause exfoliation to occur.

Hydraulic action

This is when water (generally from powerful waves) rushes into cracks in the rockface rapidly. This traps a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly released with explosive force. The explosive release of highly pressurized air cracks away fragments at the rockface and widens the crack itself.

Biological Weathering

Living organisms may contribute to mechanical weathering (as well as chemical weathering, see 'biological' weathering below). Lichens and mosses grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger scale seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration. Burrowing animals and insects disturb the soil layer adjacent to the bedrock surface thus further increasing water and acid infiltration and exposure to oxidation processes.

Chemical weathering

Comparison of unweathered (left) and weathered (right) limestone.

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Chemical weathering involves the change in the composition of rocks, often leading to a 'break down' in its form. This is done through a combination of water and various chemicals to create an acid which directly breaks down the material.

Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near surface environment. New or secondary minerals develop from the original minerals of the rock. In this the processes of oxidation and hydrolysis are most important.

Dissolution

Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulphur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide, SO2, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to the rocks on which it falls.

One of the most well-known solution weathering processes is carbonation, the process in which atmospheric carbon dioxide leads to solution weathering. Carbonation occurs on rocks which contain calcium carbonate, such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate (the limestone) and forms calcium bicarbonate.

The reactions as follows:

CO2 + H2O -> H2CO3

carbon dioxide + water -> carbonic acidH2CO3 + CaCO3 -> Ca(HCO3)2

carbonic acid + calcium carbonate -> calcium bicarbonate

Carbonation on the surface of well-jointed limestone produces a dissected limestone pavement which is most effective along the joints, widening and deepening them.

Hydration

Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral.

When rock minerals take up water, the increased volume creates physical stresses within the rock. For example iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum.

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A freshly broken rock shows differential chemical weathering (probably mostly oxidation) progressing inward. This piece of sandstone was found in glacial drift near Angelica, New York

Biological

A number of plants and animals may create chemical weathering through release of acidic compounds, i.e. moss on roofs is classed as weathering.

Biological weathering of lava by lichen, La Palma.

The most common form of biological weathering is the release of chelating compounds, i.e. acids, by plants so as to break down aluminium and iron containing compounds in the soils beneath them. Decaying remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering. Extreme release of chelating compounds can easily affect surrounding rocks and soils, and may lead to podsolisation of soils.

Sediment transport

Sediment transport is the movement of solid particles (sediment), typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. An understanding of sediment transport is typically used in natural systems, where the particles are clastic rocks (sand, gravel, boulders, etc.), mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles due to the sloping surface on which they are resting. Sediment transport due to fluid motion occurs in rivers, the oceans, lakes, seas, and other bodies of water, due to currents and tides; in glaciers as they flow, and on terrestrial surfaces under

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the influence wind. Sediment transport due only to gravity can occur on sloping surfaces in general, including hillslopes, scarps, cliffs, and the continental shelf—continental slope boundary.

Aeolian

Aeolian or eolian (depending on the parsing of æ) is the term for sediment transport by wind. This process results in the formation of ripples and sand dunes. Typically, the size of the transported sediment is fine sand (<1 mm) and smaller, because air is a fluid with low density and viscosity, and can therefore not exert very much shear on its bed.

Aeolian sediment transport is common on beaches and in the arid regions of the world, because it is in these environments that vegetation does not prevent the presence and motion of fields of sand.

Fluvial

In geology, physical geography, and sediment transport, fluvial processes relate to flowing water in natural systems. This encompasses rivers, streams, periglacial flows, flash floods and glacial lake outburst floods. Sediment moved by water can be larger than sediment moved by air because water has both a higher density and viscosity. In typical rivers the largest carried sediment is of sand and gravel size, but larger floods can carry cobbles and even boulders.

Fluvial sediment transport can result in the formation of ripples and dunes, in fractal-shaped patterns of erosion, in complex patterns of natural river systems, and in the development of floodplains.

Sand ripples, Laysan Beach, Hawaii. Coastal sediment transport results in these evenly-spaced ripples along the shore. Monk seal for scale.

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Coastal

Coastal sediment transport takes place in near-shore environments due to the motions of waves and currents. At the mouths of rivers, coastal sediment and fluvial sediment transport processes mesh to create river deltas.

Coastal sediment transport results in the formation of characteristic coastal landforms such as beaches and barrier islands. In coastal-fluvial systems, river deltas form.

A glacier joining the Gorner Glacier, Zermatt, Switzerland. These glaciers transport sediment and leave behind lateral moraines.

Glacial

Glaciers can carry the largest sediment, and areas of glacial deposition often contain a large number of glacial erratics, many of which are several meters in diameter.

Hillslope

In hillslope sediment transport, a variety of processes move regolith downslope. These include:

Soil creep Tree throw Movement of soil by burrowing animals Slumping and landsliding of the hillslope

These processes generally combine to give the hillslope a profile that looks like a solution to the diffusion equation, where the diffusivity is a parameter that relates to the ease of sediment transport on the particular hillslope. For this reason, the tops of hills generally have a parabolic concave-down profile, which grades into a concave-up profile around valleys.

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Sedimentary depositional environment

In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more likely that direct modern analogues are not available.

Types of depositional environment

Continental

Alluvial Aeolian Fluvial Lacustrine

Transitional

Deltaic Tidal Lagoonal Beach

Marine

Shallow water marine Deepwater marine Reef

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Others

Evaporite Glacial

Alluvium"Alluvial" redirects here. For the American racehorse, see Alluvial (horse).

Section of alluvium at the Blue Ribbon Mine in Alaska

Alluvium (from the Latin, alluvius, from alluere, "to wash against") is loose, unconsolidated (not cemented together into a solid rock), soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel. When this loose alluvial material is deposited or cemented into a lithological unit, or lithified, it would be called an alluvial deposit.

The term "alluvium" is not typically used in situations where the formation of the sediment can clearly be attributed to another geologic process that is well described. This includes (but is not limited to): lake sediments (lacustrine), river sediments (fluvial), or glacially-derived sediments (glacial till). Sediments that are formed and/or deposited in a perennial stream or river are typically not referred to as alluvial.

Alluvium can contain valuable ores such as gold and platinum and a wide variety of gemstones. Such concentrations of valuable ores is termed a placer deposit.

Aeolian processes

Aeolian (or Eolian or Æolian) processes pertain to the activity of the winds and more specifically, to the winds' ability to shape the surface of the Earth and other planets. Winds may erode, transport, and deposit materials, and are effective agents in regions with sparse vegetation and a large supply of unconsolidated sediments. Although water is

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a much more powerful eroding force than wind, aeolian processes are important in arid environments such as deserts.

The term is derived from the name of the Greek god, Æolus, the keeper of the winds.

Fluvial

Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used.

Lake

The Caspian Sea is considered either the world's largest lake or smallest sea.[1]

A lake (from Latin lacus) is a terrain feature (or physical feature), a body of liquid on the surface of a world that is localized to the bottom of basin (another type of landform or terrain feature; that is, it is not global) and moves slowly if it moves at all. Another definition is, a body of fresh or salt water of considerable size that is surrounded by land. On Earth a body of water is considered a lake when it is inland, not part of the ocean, is larger and deeper than a pond, and is not fed by a river.

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River delta

Nile River delta, as seen from Earth orbit. The Nile is an example of a wave-dominated delta that has the classic Greek delta (Δ) shape after which River deltas were named. Photo courtesy of NASA.

A delta is a landform that is created at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river. Over long periods of time, this deposition builds the characteristic geographic pattern of a river delta.

Tide"Tidal" redirects here. For other uses, see Tidal (disambiguation).This article is about tides in the Earth's oceans. For other uses, see Tide (disambiguation).See also: Theory of tides and Tide-predicting machine

Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth. The tides occur with a period of approximately 12 and a half hours and are influenced by the shape of the near-shore bottom.

The Bay of Fundy at high tideThe Bay of Fundy at low tide60

LagoonFor other meanings, see Lagoon (disambiguation)

Garabogaz-Göl lagoon in Turkmenistan.

A lagoon is a body of comparatively shallow salt or brackish water separated from the deeper sea by a shallow or exposed barrier beach, sandbank of marine origin, coral reef, or similar feature.[1] Thus, the enclosed body of water behind a barrier reef or barrier islands or enclosed by an atoll reef is called a lagoon. When used within this context of a distinctive portion of coral reef ecosystems, the term "lagoon" is synonomous with the term "back reef" or "backreef", which is more commonly used by coral reef scientists to refer to the same area.[2]

Reef.

Pamalican island with surrounding reef, Sulu Sea, Philippines.

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A reef surrounding an islet.

In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water (six fathoms or less at low water).

Many reefs result from abiotic processes—deposition of sand, wave erosion planning down rock outcrops, and other natural processes—but the best-known reefs are the coral reefs of tropical waters developed through biotic processes dominated by corals and calcareous algae. Artificial reefs such as shipwrecks are sometimes created to enhance physical complexity on generally featureless sand bottoms in order to attract a diverse assemblage of organisms, especially fish.

Glacier.

The Baltoro Glacier in the Karakoram Mountains, Pakistan-administered Kashmir. At 62 kilometres (39 mi) in length, it is one of the longest alpine glaciers on earth

A glacier is a perennial mass of ice which moves over land. A glacier forms in locations where the mass accumulation of snow and ice exceeds ablation over many years. The word glacier comes from French via the Vulgar Latin glacia, and ultimately from Latin glacies meaning ice.

Glacier ice is the largest reservoir of fresh water on Earth, and is second only to oceans as the largest reservoir of total water.

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Lithification

Lithification (from the Greek word lithos meaning 'rock' and the Latin-derived suffix -ific) is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithification is a process of porosity destruction through compaction and cementation. Lithification includes all the processes which convert unconsolidated sediments into sedimentary rocks. Petrification, though often used as a synonym, is more specifically used to describe the replacement of organic material by silica in the formation of fossils. In geology consolidation is a synonym for lithification.

Cementation

Cementation is the process of deposition of dissolved mineral components in the interstices of sediments. It is the sticking together of sediment to form a new rock and is an important factor in the consolidation of coarse-grained clastic sedimentary rocks such as sandstones, conglomerates, or breccias during diagenesis or lithification. Cementing materials may include carbonates, quartz, iron oxides, or clay minerals.

Diagenesis

diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. The boundary between diagenesis and metamorphism, which occurs under conditions of higher temperature and pressure, is gradational.

After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis. Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization.

Diagenesis

Pressure solution at work in a clastic rock. While material dissolves at places where grains are in contact, material crystallizes from the solution (as cement) in open pore spaces. This means there is a net flow of material from areas under high stress to those

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under low stress. As a result, the rock becomes more compact and harder. Loose sand can become sandstone in this way.Main article: diagenesis

The role of diagenesis in hydrocarbon generation

When animal or plant matter is buried during sedimentation, the constituent organic molecules (lipids, proteins, carbohydrates and lignin-humic compounds) break down due to the increase in temperature and pressure. This transformation occurs in the first few hundred meters of burial and results in the creation of two primary products: kerogens and bitumens.

It is generally accepted that hydrocarbons are formed by the thermal alteration of these kerogens (the biogenic theory). In this way, given certain conditions (which are largely temperature-dependent) kerogens will break down to form hydrocarbons through a chemical process known as cracking, or catagenesis.

A kinetic model based on experimental data can capture most of the essential transformation in diagenesis[7], and a mathematical model in a compacting porous medium to model the dissolution-precipitation mechanism. [8] These models have been intensively studies and applied in real geological applications.

StratificationStratification is the building up of layers. Stratified is an adjective referring to the arranging of layers, and is also the past form of the verb stratify, to separate or become separated into layers.

Sedimentary processes

Sediments are formed by the breakdown (both physical and chemical) of pre-existing rocks, which may be of igneous, metamorphic or sedimentary origin.

The main factors that control the breakdown of rocks into sediments are:

climate topography vegetation properties (physical and chemical) of the rock.

Sediments can then be transported from their source, often to great distances. The main factors that control the transportation of sediments are:

water wind (particularly in arid regions)

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Sedimentary layers - some terms Laminations are thin discrete layers of rock. Formations are groups of sedimentary rocks which have formed at the same time

and contain similar sedimentary rocks. They are mappable units that formed under distinctive environmental conditions.

Unconformities are major time-gaps between layers.

Primary sedimentary structures

Cross-bedding in a fluviatile sandstone, Middle Old Red Sandstone (Devonian) on Bressay, Shetland Islands.

Ripple marks formed by a current in a sandstone that was later tilted. Location: Haßberge, Bavaria.

Structures in sedimentary rocks can be divided in 'primary' structures (formed during deposition) and 'secondary' structures (formed after deposition). Unlike textures, structures are always large-scale features that can easily be studied in the field. Sedimentary structures can tell something about the sedimentary environment or can serve to tell which side was originally facing up in case sedimentary layers have been tilted or overturned by tectonics.

Sedimentary rocks are laid down in layers called beds or strata. A bed is defined as a layer of rock that has a uniform lithology and texture. Beds form by the deposition of layers of sediment on top of each other. The sequence of beds that characterizes sedimentary rocks is called bedding.[21] Single beds can be a couple of centimetres to several meters thick. Finer, less pronounced layers are called laminae and the structure it forms in a rock is called lamination. Laminae are usually less than a few centimetres thick.[22] Though bedding and lamination are often originally horizontal in nature, this is

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not always the case. In some environments, beds are deposited at a (usually small) angle. Sometimes multiple sets of layers with different orientations exist in the same rock, a structure called cross-bedding.[23] Cross-bedding forms when small-scale erosion occurs during deposition, cutting off part of the beds. Newer beds will then form at an angle with older ones.

The opposite of cross-bedding is parallel lamination, where all sedimentary layering is parallel.[24] With laminations, differences are generally caused by cyclic changes in the sediment supply, caused for example by seasonal changes in rainfall, temperature or biochemical activity. Laminae which represent seasonal changes (like tree rings) are called varves. Some rocks have no lamination at all, their structural character is called massive bedding.

Graded bedding is a structure in which beds with a smaller grain size occur on top of beds with larger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clasts in suspension will settle first; smaller clasts follow later. Though graded bedding can form in many different environments, it is characteristic for turbidity currents.

The bedform (the surface of a particular bed) can be indicative for a particular sedimentary environment too. Examples of bed forms are scour marks, tool marks and ripple marks. Scour marks are hollow traces in the surface where sediment particles were taken into suspension by the flow. Tool marks are tracks of larger clasts rolling over the sedimentary surface in the direction of the flow. Both are often elongated structures and can be used to establish the direction of the flow during deposition.[26]

Ripple marks also form in flowing water. There are two types: asymmetric wave ripples and symmetric current ripples. Environments where the current is in one direction, such as rivers, produce asymmetric ripples. The longer flank of such ripples is oriented opposite to the direction of the current.[27] Wave ripples occur in environments where currents occur in all directions, such as tidal flats.

Another type of bed form are mud cracks, caused by the dehydration of sediment that occasionally comes above the water surface. Such structures are commonly found at tidal flats or point bars along rivers.

Classification sedimentary

A Basic Classification Derived From the

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Classification

Sedimentary rocks are classified into three groups. These groups are clastic, chemical precipitate and biochemical (or biogenic).

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ClasticMain article: Clastic rock

Claystone deposited in Glacial Lake Missoula, Montana, USA. Note very fine and flat bedding, common for distal lacustrine deposition.

Clastic sedimentary rocks are composed of discrete fragments or clasts of materials derived from other minerals. They are composed largely of quartz with other common minerals including feldspar, amphiboles, clay minerals, and sometimes more exotic igneous and metamorphic minerals.

Clastic sedimentary rocks, such as limestone or sandstone, were formed from rocks that have been broken down into fragments by weathering, which then have been transported and deposited elsewhere.

The classification of clastic sedimentary rocks is complex because there are many variables involved. Particle size (both the average size and range of sizes of the particles), composition of the particles (in sandstones, this includes quartz arenites, arkoses, and lithic sandstones), the cement, and the matrix (the name given to the smaller particles present in the spaces between larger grains) must all be taken into consideration.

All rocks disintegrate when exposed to mechanical and chemical weathering at the Earth's surface.

Lower Antelope Canyon was carved out of the surrounding sandstone by both mechanical weathering and chemical weathering. Wind, sand, and water from flash flooding are the primary weathering agents.

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Clastic rock

Clastic rocks are composed of fragments, or clasts, of pre-existing rock. Geologists most commonly, but not always, use the term with reference to sedimentary rocks.

Clastic sediments

Claystone from Montana

Clastic sedimentary rocks are rocks composed predominantly of broken pieces or clasts of older weathered and eroded rocks. Clastic sediments or sedimentary rocks are classified based on grain size, clast and cementing material (matrix) composition, and texture. The classification factors are often useful in determining a sample's environment of deposition.

Grain size determines the basic name of a clastic sedimentary rock. Grain size varies from clay in shales and claystones; through silt in siltstones; sand in sandstones; and gravel, cobble, to boulder sized fragments in conglomerates and breccias. The Krumbein phi (φ) scale numerically orders these terms in a logarithmic size scale.

Sedimentary breccias

Sedimentary breccias are a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. They may form either

1. in submarine debris flows, avalanches, mud flow or mass flow in an aqueous medium. Technically, turbidites are a form of debris flow deposit and are a fine-grained peripheral deposit to a sedimentary breccia flow.

2. as angular, poorly sorted, very immature fragments of rocks in a finer grained groundmass which are produced by mass wasting. These are, in essence, lithified colluvium. Thick sequences of sedimentary (colluvial) breccias are generally formed next to fault scarps in grabens.

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Rudaceous rocksRocks with larger grain sizes, including breccias and conglomerates are termed rudaceous sediments.

Arenaceous rockArenite (Latin Arena, sand) is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00246 in) and 2 mm (0.08 in) and contain less than 15% matrix.[1] The related adjective is arenaceous.

Since it refers to grain size rather than chemical composition, the term is used for example in the classification of clastic carbonatic limestones, as the granulometrically equivalent term sandstone is not appropriate for limestone. Other arenites include sandstones, arkoses, greensands and greywackes.

Arenites mainly form by erosion of other rocks or turbiditic re-deposition of sands. Some arenites contain a varying amount of carbonatic components and thus belong to the rock-category of carbonatic sandstones or silicatic limestones. Arenites often appear as massive or bedded medium-grained rocks with a medium- to wide-spaced preferred foliation and often develop a pronounced cleavage.

Argillaceous minerals

Argillaceous components are fine-grained (less than 2 µm) aluminosilicates, and more particularly clay minerals such as kaolinite, montmorillonite-smectite, illite, and chlorite. Clays and shales are thus predominantly argillaceous.

The adjective "argillaceous" is also used to define rocks in which clay minerals are a minor but significant component. For example, argillaceous limestones are limestones consisting predominantly of calcium carbonate, but including 10-40% of clay minerals: such limestones, when soft, are often called marls. Similarly, argillaceous sandstones are sandstones consisting primarily of quartz grains, with the interstitial spaces filled with clay minerals.

Argillaceous Rock

A sedimentary rock formed from clay deposits.

A sedimentary rock composed of clay-grade particles; i.e., composed of minute mineral fragments and crystals less than 0.002 mm in diameter; containing much colloidal-size material. In addition to finely divided detrital matter, argillaceous rocks consist essentially of illite, montmorillonite, kaolinite, gibbsite, and diaspore.

Non-Clastic Rocks

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Chemical sedimentary rocks have a non-clastic texture, consisting entirely of crystals

Organic Sedimentary Rocks

The sediment in an organic sedimentary rock is made of fossils!

The hard parts of animals, such as bones and shells, can become

cemented together over time to make rock. Usually the bones and shells are made of calcite, or similar minerals, and the organic rock that is made from them is called limestone. Some types of microorganisms that live in the ocean or lakes have tiny skeletons made of silica. The organic rock made from their skeletons is called chert.

Plant remains squashed deep underground over millions of years make an organic sedimentary rock called coal. Coal doesn’t look like it is made of sediment. It is also often difficult to see the plant fossils within it because they have become so compacted over time, the less stable materials have left, and all that remains is carbon.

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This image of Australia's Great Barrier Reef shows organic sedimentary rocks in production! Click on image for full size (54K JPG)Courtesy of Abi Howe, American Geological Institute

Chemical Sedimentary Rocks

Unlike most other sedimentary rocks, chemical rocks are not made of pieces of sediment. Instead, they have mineral crystals made from elements that are dissolved in water.

The water in the oceans, lakes, and ground is often full of dissolved elements. All sorts of things can dissolve into water. If you put a spoonful of salt into water, the salt will eventually dissolve. Seawater tastes salty mainly because there are salty minerals such as halite dissolved in it.

Sometimes water becomes so full of dissolved elements that they will not all fit. Some are not able to remain dissolved and form solid mineral crystals. This usually happens when some of the water has evaporated away, leaving less room for the dissolved elements. If enough water evaporates, they do not all fit and some form crystals of minerals such as halite, gypsum, and calcite. In the picture to the left, minerals are forming out of shallow water that has flooded the bottom of Death Valley in California. The valley is so hot and dry that water evaporates very quickly, leaving behind the minerals that were once dissolved in it.

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Mineral crystals are made as the shallow water that has flooded the bottom of Death Valley evaporates. Click on image for full size (66K JPG)Courtesy of Martin Miller, University of Oregon

Clastic Rocks

Clastic sedimentary rocks are made up of little pieces of other rocks called sediment. Mineral crystals called cement hold the sediment together.

There are many different types of clastic sedimentary rocks. To figure out which type of rock you have, you will need to figure out the answers to these three questions:

How big are the sediments? Are all of the sediments about the same

size? Are the sediments rounded or angular in

shape?

.

Different types of sedimentary rocks form in different environments. For instance, sandstone, a sedimentary rock made of sand grains, may form in a beach or desert sand dunes. Shale, a sedimentary rock made of mud and clay, may form in a swamp, the bottom of a lake, or some other muddy environment. Conglomerate, a sedimentary rock make of gravel and sand, may

form from the sediments at the bottom of a stream.

Making a clastic sedimentary rock is a four-step process.

Calcareous

Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.

Cathedral Rock in Arizona is made of a clastic rock called sandstone. . These rocks are more than 250 million years old and are now being weathered and eroded away by the creek. Click on image for full size (37K JPG)Courtesy of Larry Fellows and the Arizona Geological Survey

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The term calcareous can be applied to a sediment, sedimentary rock, or soil type which is formed from, or contains a high proportion of, calcium carbonate in the form of calcite or aragonite.

Marine sediments

Calcareous sediments are usually deposited in shallow water near land, since the carbonate is precipitated by marine organisms that need land-derived nutrients. Generally speaking, the farther from land sediments fall, the less calcareous they are. Some areas can have interbedded calcareous sediments due to storms, or changes in ocean currents.

Calcareous ooze is a form of calcium carbonate derived from planktonic organisms that accumulates on the sea floor.

Sedimentary RocksThe four classes of sedimentary rocks are:

1. Shales 2. Sandstones 3. Carbonates 4. Other sedimentary rocks

o Conglomerates o Evaporites o Chert o Iron rocks o Phosphorites -- and o Carbonaceous sedimentary rocks

Shales

Shales are a study all to themselves. Studying them reveals ancient environments, often times in breathtaking detail. In the mineralogy of the silt and clay sized particles, the rocks are called by their particle size first, so "silt-stone" and "silty-shale" define the particle size as does "claystone" and "clayshale."

Sandstones

Sandstones are composed primarily of silicate minerals derived from

1. Ferromagnesian minerals: Olivine, pyroxene, amphibole and biotite 2. Feldspar minerals: Ca and Na feldspars are collectively known as plagioclase

feldspars; while Na and K feldspars are known as the alkali group. Globally, K and Na are greater than Ca and Na, but in volcanic sediments Ca and Na outnumber K. Feldspars weather to micas and clays.

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3. Quartz minerals form from silica tetrahedrons loosened by physical weathering and trasported. If by water, they may be rounded, have chatter marks and show conchoidal fracture. If by air, grains are usually frosted.

4. Accessory minerals like zircon and rutile which can be used for age dating.

Carbonates

Whole books can and have been written on carbonates, compounds containing (C03) in their structure. Carbonates include:

1. CaCO3 -- Calcite // Aragonite // Limestone. Living organisms build aragonite and deep sea oozes contain calcite. Whether this relationship is stable over all of geologic time is an open question. Limestones older than the Cretaceous show little aragonite.

2. (Mg,Ca)CO3 -- Dolomite // Dolostone. Modern examples are forming on the Bahama Banks.

3. (Fe,Ca)CO3 -- Siderite // Ironstone. Include the Mazon Creek fossils and are discussed under "other sedimentary rocks" below.

Limestone Textures

1. Carbonate Grains Allochems, broken pieces of shells or forams Ooids (Illinois' Neda Oolite is a metal oolite.) Sponge spicules and other SiO2 particles in a limestone matrix.

Types of Limestone

o Coquina - made of broken shells o Chalk - dead forams o Diatomaceous earth or "fullers' earth" - dead diatoms o Marl - gooey lime mud - lime ooze o Fossiliferous limestone - a catch-all phrase to denote dead stuff, can be

divided into subcategories "crinoidal limestone," "bryozoal limestone," and so on if one organism out numbers the rest substantially.

o Lithographic limestone - former marl of a shallow lake or basin, very good for preserving fossils, ex. Archeopteryx and other dinosaur era beasts.

o Lightly metamorphosed limestone may preserve fossils (ex. Pittsfield Building, Chicago) or they may be chemically or pressure changed beyond all recognition (Connemarah Marble, Ireland).

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Iron rocks form in oxic and anoxic environments o Bog-iron, found in fresh water lakes probably as a result of hydrogen

sulfide bacterial anaerobic decomposition o Laterites form below rain forest soils. They need an iron source to form an

iron caliche (iron calcite) or a siderite layer. Some other laterites include aluminum ore "bauxite" which forms in areas with a high aluminum source.

o Placer iron forms with other placer deposits on the point bars of streams and in the intertidal areas of beaches as seen in the "black sand" of Lake Michigan.

Phosphorites: Phosphates are forming now in areas of upwelling of rich, bottom currents along the Pacific margin of North and South America, near California, Chile and Peru.We use modern phosphates as fertilizer. When preserved as rocks, they occur as

o nodules o layers, either massive or bedded o as bioclastics including guano from birds or bats o

Carbonaceous sedimentary rocks All carbonaceous sediments and sedimentary rocks contain carbon and are sometimes called "organic" rocks in other books.

phosphate rockAny of various rocks composed largely of phosphate minerals, especially apatite, used as fertilizer and as a source of phosphorous compounds.

Limestone

Limestone

 —  Sedimentary Rock  —

Limestone cropping at São Pedro de Moel beach, Marinha

Grande, Portugal

Composition

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Calcium carbonate: inorganic crystalline calcite and/or

organic calcareous material.

Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3). Like most other sedimentary rocks, limestones are composed of grains, however, around 80-90% of limestone grains are skeletal fragments of marine organisms such as coral or foraminifera. Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and extraclasts. Some limestones do not consist of grains at all and are formed completely by the chemical precipitation of calcite or aragonite. i.e. travertine.

Description

Limestone often contains variable amounts of silica in the form of chert (aka chalcedony, flint, jasper, etc) or siliceous skeletal fragment (sponge spicules, diatoms, radiolarians), as well as varying amounts of clay, silt and sand sized terrestrial detritus carried in by rivers. The primary source of the calcite in limestone is most commonly marine organisms. These organisms secrete shells made of aragonite or calcite and leave these shells behind after the organism dies. Some of these organisms can construct mounds of rock known as reefs, building upon past generations. Below about 3,000 meters, water pressure and temperature causes the dissolution of calcite to increase non-linearly so that limestone typically does not form in deeper waters (see lysocline).

Uses

Limestone is very common in architecture, especially in North America and Europe. Many landmarks across the world, including the Great Pyramid and its associated Complex in Giza, Egypt, are made of limestone. So many buildings in Kingston, Canada were constructed from it that it is nicknamed the 'Limestone City'. [7] On the island of Malta, a variety of limestone called Globigerina limestone was for a long time the only building material available, and is still very frequently used on all types of buildings and sculptures. Limestone is readily available and relatively easy to cut into blocks or more elaborate carving. It is also long-lasting and stands up well to exposure. However, it is a very heavy material, making it impractical for tall buildings, and relatively expensive as a building material.

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The Great Pyramid of Giza. One of the Seven Wonders of the Ancient World, the structure is made entirely from limestone.

Evaporite

Cobble encrusted with halite evaporated from the Dead Sea, Israel.

Evaporites are water-soluble mineral sediments that result from the evaporation of bodies of surficial water. Evaporites are considered sedimentary rocks.

Formation of evaporite rocks

Although all water bodies on the surface and in aquifers contain dissolved salts, the water must evaporate into the atmosphere for the minerals to precipitate. For this to happen the water body must enter a restricted environment where water input into this environment remains below the net rate of evaporation. This is usually an arid environment with a small basin fed by a limited input of water. When evaporation occurs, the remaining water is enriched in salts, and they precipitate when the water becomes oversaturated.

Evaporite depositional environments

Evaporite depositional environments which meet the above conditions include;

Graben areas and half-grabens within continental rift environments fed by limited riverine drainage, usually in subtropical or tropical environments

o Example environments at the present which match this is the Denakil Depression, Ethiopia; Death Valley, California

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Use:

Halite formations are famous for their ability to form diapirs which produce ideal locations for trapping petroleum deposits.

Iron Ore

Earth's most important iron ore deposits are found in sedimentary rocks. They formed from chemical reactions that combined iron and oxygen in marine and fresh waters. The two most important minerals in these deposits are iron oxides: hematite (Fe2O3) and magnetite (Fe3O4). These iron ores have been mined to produce almost every iron and steel object that we use today - from paper clips to automobiles to the steel beams in skyscrapers.

Iron Ore: A specimen of oolitic hematite iron ore. The specimen shown is about two inches (five centimeters) across.

How Does Iron Ore Form?

The iron ore deposits began forming when the first organisms capable of photosynthesis began releasing oxygen into the waters. This oxygen immediately combined with the abundant dissolved iron to produce hematite or magnetite. These minerals deposited on the sea floor in great abundance, forming what are now known as the "banded iron formations." The rocks are "banded" because the iron minerals deposited in alternating bands with silica and sometimes shale. The banding might have resulted from seasonal changes in organism activity.

Banded Iron Formation: Close-up of a banded iron formation. In this specimen

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bands of hematite (silver) alternate with bands of jasper (red). This photo spans an area of rock about one foot wide. Photo taken by André Karwath, GNU Free Documentation License.

Metamorphism

Schematic representation of a metamorphic reaction. Abbreviations of minerals: act = actinolite; chl = chlorite; ep = epidote; gt = garnet; hbl = hornblende; plag = plagioclase. Two minerals represented in the figure do not participate in the reaction, they can be quartz and K-feldspar. This reaction takes place in nature when a rock goes from amphibolite facies to greenschist facies.

Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process.

Three types of metamorphism exist:contact, dislocation and regional. Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.

Kinds of metamorphism

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Regional metamorphism

Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges, particularly subduction zones or the roots of previously eroded mountains. Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism. These orogenic mountains are later eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects. The techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane.

Metamorphic facies

Metamorphic facies are recognizable terranes or zones with an assemblage of key minerals that were in equilibrium under specific range of temperature and pressure during a metamorphic event. The facies are named after the metamorphic rock formed under those facies conditions from basalt. Facies relationships were first described by Pentti Eskola in 1921.

Facies:

Low T - Low P : Zeolite Mod - High T - Low P : Prehnite-Pumpellyite Low T - High P : Blueschist Mod to High T - Mod P : Greenschist - Amphibolite - Granulite Mod - High T - High P : Eclogite

Metamorphic grades

In the Barrovian sequence (described by George Barrow in zones of progressive metamorphism in Scotland), metamorphic grades are also classified by mineral assemblage based on the appearance of key minerals in rocks of pelitic (shaly, aluminous) origin:

Low grade ------------------- Intermediate --------------------- High grade

Greenschist ------------- Amphibolite ----------------------- GranuliteSlate --- Phyllite ---- Schist --------- Gneiss -----------------------Migmatite(partial melting) >>>meltChlorite zone Biotite zone Garnet zone

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Staurolite zone Kyanite zone Sillimanite zone

Contact (thermal) metamorphism

Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock. The area surrounding the intrusion (called aureoles) where the contact metamorphism effects are present is called the metamorphic aureole. Contact metamorphic rocks are usually known as hornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are often fine-grained.

Contact metamorphism is greater adjacent to the intrusion and dissipates with distance from the contact. The size of the aureole depends on the heat of the intrusive, its size, and the temperature difference with the wall rocks. Dikes generally have small aureoles with minimal metamorphism whereas large ultramafic intrusions can have significantly thick and well-developed contact metamorphism.

The metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole. This is usually related to the metamorphic temperatures of pelitic or alumonisilicate rocks and the minerals they form. The metamorphic grades of aureoles are andalusite hornfels, sillimanite hornfels, pyroxene hornfels.

Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. Extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatism. If the intruded rock is rich in carbonate the result is a skarn. Fluorine-rich magmatic waters which leave a cooling granite may often form greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic ore minerals and thus are of economic interest.

Hydrothermal metamorphism

Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of variable composition. The difference in composition between existing rock and the invading fluid triggers a set of metamorphic and metasomatic reactions. The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating groundwater, or ocean water. Convective circulation of water in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. The patterns of this hydrothermal alteration is used as a guide in the search for deposits of valuable metal ores.

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Shock metamorphism

This kind of metamorphism occurs when either an extraterrestrial object (a meteorite for instance) collides with the Earth's surface or during an extremely violent volcanic eruption. Impact metamorphism is, therefore, characterized by ultrahigh pressure conditions and low temperature. The resulting minerals (such as SiO2 polymorphs coesite and stishovite) and textures are characteristic of these conditions.

Dynamic metamorphism

Dynamic metamorphism is associated with zones of high to moderate strain such as fault zones. Cataclasis, crushing and grinding of rocks into angular fragments, occurs in dynamic metamorphic zones, giving cataclastic texture.

The textures of dynamic metamorphic zones are dependent on the depth at which they were formed, as the temperature and confining pressure determine the deformation mechanisms which predominate. Within depths less than 5 km, dynamic metamorphism is not often produced because the confining pressure is too low to produce frictional heat. Instead, a zone of breccia or cataclasite is formed, with the rock milled and broken into random fragments. This generally forms a mélange. At depth, the angular breccias transit into a ductile shear texture and into mylonite zones.

Prograde and retrograde metamorphism

Metamorphism is further divided into prograde and retrograde metamorphism. Prograde metamorphism involves the change of mineral assemblages (paragenesis) with increasing temperature and (usually) pressure conditions. These are solid state dehydration reactions, and involve the loss of volatiles such as water or carbon dioxide. Prograde metamorphism results in rock characteristic of the maximum pressure and temperature experienced. Metamorphic rocks usually do not undergo further change when they are brought back to the surface.

Retrograde metamorphism involves the reconstitution of a rock via revolatisation under decreasing temperatures (and usually pressures), allowing the mineral assemblages formed in prograde metamorphism to revert to those more stable at less extreme conditions. This is a relatively uncommon process, because volatiles must be present.

Cataclastic rock

A cataclastic rock is a type of metamorphic rock that has been wholly or partly formed by the progressive fracturing and comminution of existing rock, a process known as cataclasis, and is mainly found associated with fault zones. Mylonite was originally defined as a cataclastic rock but is now understood to have formed mainly by crystal-plastic processes.[1]

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Classification

Various classification schemes have been proposed for the cataclastic rocks, but changes in understanding of the processes involved in their formation and better knowledge of the variety of such rocks has made a simple classification difficult, particularly where distinctions cannot be made in hand specimens. Sibson's 1977 classification of fault rocks was the first to include an understanding of the deformation mechanisms involved and all subsequent schemes have been based on this. Fault breccias have been further classified in terms of their origins; attrition, distributed crush and implosion brecciation, and, borrowing from the cave-collapse literature, crack, mosaic and chaotic from their clast concentration.

Types

Cataclasite

Cataclasite is a fault rock that consists of angular clasts in a finer-grained matrix.[1] It is normally non-foliated but some varieties have been described with a well-developed planar fabric that are known as foliated cataclasites.[3] Cataclasite grades into fault breccia as the percentage of visible clasts increases to more than 30%.

Fault breccia

Fault breccia is a fault rock that consists of large fragments of rock in a fine-grained matrix. It may be either cohesive or incohesive. The matrix may also include mineral veins formed in voids between the clasts, which may themselves become fractured by later movements on the fault.

Fault gouge

Fault gouge is an unconsolidated and incohesive type of fault rock consisting almost entirely of finely crushed material. Varieties that have a large clay mineral content are known as clay gouges.

Pseudotachylite

Pseudotachylite is a fault rock that has the appearance of the basaltic glass, tachylyte. It is dark in color and has a glassy appearance. It is generally found either along a fault surface or as veins injected into the fault walls. Most pseudotachylites have clearly formed by frictional melting, associated with either seismic faulting, some large landslides or meteorite impacts.

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Formation

Cataclastic rocks form by brittle processes in the upper part of the crust in areas of moderate to high strain, particularly in fault zones. The two main mechanisms involved are microfracturing (breaking the original rock into fragments) and frictional sliding/rolling of the fragments, combined with further fracturing.

Granulose

Having a surface covered with granules.

1. A small grain or pellet; a particle.2. Geology A rock or mineral fragment larger than a sand grain and smaller than a pebble, between 2 and 4 millimeters in diameter.

Schistose

Any of various medium-grained to coarse-grained metamorphic rocks composed of laminated, often flaky parallel layers of chiefly micaceous minerals.

Schist

The schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. By definition, schist contains more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. Schist is often garnetiferous.

The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen by the naked eye. Schist is characteristically foliated, meaning the individual mineral grains split off easily into flakes or slabs. The word schist is derived from the

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Greek word σχίζειν meaning "to split", which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie.

Most schists have been derived from clays and muds which have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists have been derived from fine-grained igneous rocks such as basalts and tuffs. Most schists are mica schists, but graphite and chlorite schists are also common.

Schists are named for their prominent or perhaps unusual mineral constituents, such as garnet schist, tourmaline schist, glaucophane schist, etc.

Schists are frequently used as dimension stone.

Formation

During metamorphism, rocks which were originally sedimentary or igneous are converted into schists and gneisses. If the composition of the rocks was originally similar, they may be very difficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry, for example, and a fine grained feldspathic sandstone, may both be converted into a grey or pink mica-schist. Usually, however, it is possible to distinguish between sedimentary and igneous schists and gneisses. If the whole district, for example, occupied by these rocks have traces of bedding, clastic structure, or unconformability then it may be a sign that the original rock was sedimentary. In other cases intrusive junctions, chilled edges, contact alteration or porphyritic structure may prove that in its original condition a metamorphic gneiss was an igneous rock. The last appeal is often to the chemistry, for there are certain rock types which occur only as sediments, while others are found only among igneous masses, and however advanced the metamorphism may be, it rarely modifies the chemical composition of the mass very greatly. Such rocks, for example, as limestones, dolomites, quartzites and aluminous shales have very definite chemical characters which distinguish them even when completely recrystallized.

The schists are classified principally according to the minerals they consist of and on their chemical composition. For example, many metamorphic limestones, marbles, and calc-schists, with crystalline dolomites, contain silicate minerals such as mica, tremolite, diopside, scapolite, quartz and feldspar. They are derived from calcareous sediments of different degrees of purity. Another group is rich in quartz (quartzites, quartz schists and quartzose gneisses), with variable amounts of white and black mica, garnet, feldspar, zoisite and hornblende. The majority of mica-schists, however, are altered claystones and shales, and pass into the normal sedimentary rocks through various types of phyllite and mica-slates. They are among the most common metamorphic rocks; some of them are graphitic and others calcareous. The diversity in appearance and composition is very

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great, but they form a well-defined group not difficult to recognize, from the abundance of black and white micas and their thin, foliated, schistose character. As a special subgroup we have the andalusite, staurolite, kyanite and sillimanite-schists which usually make their appearance in the vicinity of gneissose granites, and have presumably been affected by contact metamorphism. The schist rock can be found worldwide and is popular for collectors.

Gneissose

Having properties similar to gneiss.

Gneiss

Gneiss rock

Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.

Composition

Gneissic rocks are usually medium- to coarse-foliated and largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. Depending on their composition, they may also be called garnet gneiss, biotite gneiss, albite gneiss, etc.

Gneiss resembles schist, except that the minerals are arranged into bands. Sometimes it is difficult to tell the difference between gneiss and a schist because some gneiss appears to have more mica than it really does. This is especially true with mica-rich parting planes.

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Types of gneiss

Orthogneiss designates a gneiss derived from an igneous rock, and paragneiss is one from a sedimentary rock. Gneissose is used to describe rocks with properties similar to gneiss.

Augen gneiss

Augen gneiss, from the German Augen, meaning "eyes", is a coarse-grained gneiss, interpreted as resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear bound feldspar porphyroclasts, normally microcline, within the layering of the quartz, biotite and magnetite bands.

Crystalloblastic texture

A crystalline texture resulting from metamorphic recrystallization under conditions of high viscosity and directed pressure.

Xenoblastic texture

Xenoblastic A textural term applied to metamorphic rocks which contain anhedral porphyroblasts.

Schist

Schist

The schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. By definition, schist contains more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. Schist is often garnetiferous.

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The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen by the naked eye. Schist is characteristically foliated, meaning the individual mineral grains split off easily into flakes or slabs.

Schists are named for their prominent or perhaps unusual mineral constituents, such as garnet schist, tourmaline schist, glaucophane schist, etc.

Slate

Slate

 —  Metamorphic Rock  —

Slate

Composition

Primary quartz, muscovite/illite

Secondary biotite, chlorite, hematite, pyrite

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering. Slate is frequently grey in color, especially when seen in masse covering roofs. However, slate occurs in a variety of colors even from a single locality. Slate is not to be confused with shale, from which it may be formed, or schist.

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Phyllite

Phyllite

Photomicrograph of thin section of phyllite (in cross polarised light)

Phyllite is a type of foliated metamorphic rock(foliation= the arrangement of leaflike layers in a rock) primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the surfaces of cleavage (or schistosity). Phyllite is formed from the continued metamorphism of slate.

The protolith (or parent rock) for a phyllite is a shale or pelite. Its constituent platy minerals are larger than those in slate but are not visible with the naked eye. Phyllites are said to have a "phyllitic texture" and are usually classified as having a low grade in the regional metamorphic facies.

Phyllite has a good fissility (a tendency to split into sheets) and will form under low grade metamorphic conditions. Phyllites are usually black or gray. The foliation is commonly crinkled or wavy in appearance.

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Quartzite

Quartzite

Quartzite (from German Quarzit[1]) is a hard metamorphic rock which was originally sandstone.[2] Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of iron oxide (Fe2O3). Other colors, such as yellow and orange, are due to other mineral impurities.

When sandstone is metamorphosed to quartzite, the individual quartz grains recrystallize along with the former cementing material to form an interlocking mosaic of quartz crystals. Most or all of the original texture and sedimentary structures of the sandstone are erased by the metamorphism. Minor amounts of former cementing materials, iron oxide, carbonate and clay, often migrate during recrystallization and metamorphosis. This causes streaks and lenses to form within the quartzite.

Quartzite is very resistant to chemical weathering and often forms ridges and resistant hilltops. The nearly pure silica content of the rock provides little to form soil from and therefore the quartzite ridges are often bare or covered only with a very thin soil and little vegetation.

Marble

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Marble.

The Taj Mahal is made of marble.

Marble is a non foliated metamorphic rock resulting from the metamorphism of limestone, composed mostly of calcite (a crystalline form of calcium carbonate, Ca C O 3). It is extensively used for sculpture, as a building material, and in many other applications

Granulite

A sample of granulite-facies metamorphic rock of felsic composition, with garnet porphyroblasts

Granulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure. [1] They are of particular interest to geologists because many granulites represent samples of the deep continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth.

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Eclogite

Eclogite piece with a garnet (red) and omphacite (greyish-green) groundmass. The sky-blue crystals are kyanite. Some white quartz is seen too, it was probably once coesite. A few gold-white phengite mica minerals can be seen at the top. Coin of 1 euro (23 mm) for scale.

Eclogite is a coarse-grained mafic (basaltic in composition) metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth. Second, being unusually dense rock, eclogite can play an important role in driving convection within the solid Earth.

The fresh rock can be striking in appearance, with red to pink garnet (almandine-pyrope) in a green matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dolomite, corundum, and, rarely, diamond. Plagioclase is not stable in eclogites. Glaucophane and titanite (sphene) form in eclogite as pressures decrease during exhumation of the rocks, or may be earlier formed minerals that did not entirely react away.

Amphibolite

Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of

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hornblende amphibole is a hornblendite, which are usually crystal cumulates. Rocks with >90% amphibole which have a feldspar groundmass may be a lamprophyre.

Amphibolite is a grouping of rocks composed mainly of amphibole (as hornblende) and plagioclase feldspars, with little or no quartz. It is typically dark-colored and heavy, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.

Migmatite

Ptygmatic folding in migmatite

Migmatite is a rock at the frontier between igneous and metamorphic rocks. They can also be known as diatexite.

Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally molten material, and are not generally the result of solid-state reactions. Migmatites are composed of a leucosome, new material crystallized from incipient melting, and a mesosome, old material that resisted melting. Commonly, migmatites occur within extremely deformed rocks that represent the base of eroded mountain chains, typically within Precambrian cratonic blocks.

khondalite   nom féminin singulier  (minéralogie) roche métamorphique à grenats et graphite Khondalites are quartz- feldspar- sillimanite gneisses, with graphite, garnet and biotite, ± cordierite.

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Charnockite

Charnockite is applied to any orthopyroxene-bearing granite, composed mainly of quartz, perthite or antiperthite and orthopyroxene (usually hypersthene), as an end-member of the charnockite series

Textures / Structures of MetamorphicRocksTextures are the relationships of crystals and glass at the smallest scale; structures are larger-scale features, at times requiring awhole outcrop to fully describe. (Note: much of this document comes from the SCMR at http://www.bgs.ac.uk/SCMR/)

Grain Size• aphanitic (not often used in metamorphic rocks): grains too small to see without a microscope, but rock isn’t glassy• phaneritic (not often used in metamorphic rocks): grains visible with the unaided eye• fine grained: < 1 mm (average long dimension of grains)• medium grained: 1 mm: 5 mm• coarse grained: 5 mm: 3 cm• very coarse grained: > 3 cm• microcrystalline: individual crystals require a hand lens to discern• cryptocrystalline: even in the microscope, individual crystals cannot be discerned

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• porphyroblastic: (cf. porphyritic) some grains (the “porphyroblasts”) are markedly larger than others (the “matrix”)

Inclusion textures• poikiloblastic: (cf. poikilitic) grains of one mineral (the “poikiloblasts”) completely enclose others (the “inclusions”)• sieve texture: poikiloblastic in which the inclusions are abundant and fairly closely spaced• helicitic = snowball: S-shaped trails of inclusions in a poikiloblast

Fabric terms• foliation: a planar rock fabric• lineation: a linear rock fabric• mineral lineation = nematoblastic texture: containing a lineationdefined by aligned elongate / acicular / fibrous minerals• schistosity = lepidoblastic texture: containing a foliation defined byaligned platy / micaceous / tabular minerals• schistose structure: well developed schistosity, either uniformly orclosely spaced so the rock will split on < 1 cm scale• gneissose structure = gneissosity: poorly developed, uniformly distributedschistosity, or well-developed but spaced so the rock splitson > 1 cm scale. Mineralogical layering is common.• layered = banded structure: parallel, planar regions of varying mineralogoccurrence and/or abundance, often with mica-rich regions distinctfrom quartz+feldspar-rich regions.• cleavage: property of the rock to split along parallel closely spaced surfaces• slaty cleavage: well developed schistosity and cleavage in a rock where matrix grains are too small to observe unaidedand schistosity is uniformly present• spaced schistosity: foliation with regularly spaced zones of schistose structure distinct from other regions without(or with less developed) schistose structure.• crenulation: small-scale (< 1 cm wavelength), regular folds• crenulation cleavage: cleavage parallel to crenulations

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• S-tectonite, L-tectonite, and LS-tectonite: Rock names used mainly by structural geologists that imply only fabricdevelopment. L means a lineation is present; S means a foliation is present; LS means both are present. Better touse a more mineralogically descriptive rock name and prefix with “foliated”, “lineated”, or “foliated and lineated”.

Lack-of-fabric terms• granofels structure: no schistosity present and grains are generally equant, or if inequant, then randomly oriented• hornfelsic: having a granofels structure and microcrystalline or cryptocrystalline grain size in the matrix• granoblastic texture: coarse-grained granofelsic rock with polygonal grains having generally planar boundaries• polygonal = annealed texture: consisting of grains with polygonal shapes; boundaries of three equant grains willintersect at 120°

Reaction / intergrowth texturesThese terms refer to the texture itself, not the rock or the mineral grains.• symplectitic: an intimate often vermicular (wormy) intergrowth of two minerals on the microscopic scale• myrmekitic: a symplectite of quartz in plagioclase (often oligoclase); typically forms at the contact of K-feldsparand plagioclase• perthitic: sodic plagioclase lamellae within K-feldspar resulting from exsolution during cooling• coronal: a concentric ring of one mineral around another

Crystal PerfectionThese describe individual minerals, or relationships between two specific minerals• euhedral = idioblastic: grains bounded by their own perfect to near-perfect crystal growth faces• subhedral = subidioblastic: partly bound by its own growth faces, or growth faces only moderately well developed• anhedral = xenoblastic: irregular; little or no evidence for its own growth faces

Crystalloblastic SeriesMinerals higher in the series tend to form idioblastic surfaces against those lower in the series.• magnetite, rutile, titanite• andalusite, kyanite, garnet, staurolite, tourmaline• epidote, zoisite, forsterite, lawsonite• amphibole, pyroxene, wollastonite• chlorite, talc, mica, prehnite, stilpnomelane• calcite, dolomite• cordierite, feldspar, scapolite• quartz

Relationship between mineral growth and deformationYou can also describe the relationship between a mineral’s growth and a particular deformational feature or event (say, thethird schistosity to form in the rock) by referring to the structural code for that feature / event (e.g., “pre-S3”).• prekinematic: mineral growth was completed prior to deformation

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• synkinematic: the mineral growth occurred during deformation (most helicitic minerals form synkinematically)• postkinematic: the mineral growth began following deformation

Other textural terms• augen structure: containing eye-shaped strained relict feldspar phenocrysts• relict: any texture or mineral inherited from the protolith (e.g., “relict K-feldspar phenocrysts”, or “relict crossbedding”)• blastoporphyritic: relict porphyritic texture• migmatite: mixture of igneous and metamorphic rocks due to partial meltingMetamorphic Textures

Types of Metamorphism

Metamorphism is defined as follows: 

The mineralogical and structural adjustment of solid rocks to physical and chemical conditions that have been imposed at depths below the near surface zones of weathering and diagenesis and which differ from conditions under which the rocks in question originated.

The word "Metamorphism" comes from the Greek:  meta = change, morph = form, so metamorphism means to change form.  In geology this refers to the changes in mineral assemblage and texture that result from subjecting a rock to conditions such pressures, temperatures, and chemical environments different from those under which the rock originally

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formed.

Note that Diagenesis is also a change in form that occurs in sedimentary rocks.  In geology, however, we restrict diagenetic processes to those which occur at temperatures below 200oC and pressures below about 300 MPa (MPa stands for Mega Pascals), this is equivalent to about 3 kilobars of pressure (1kb = 100 MPa).

Metamorphism, therefore occurs at temperatures and pressures higher than 200oC and 300 MPa.  Rocks can be subjected to these higher temperatures and pressures as they are  buried deeper in the Earth.  Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction.

The upper limit of metamorphism occurs at the pressure and temperature where melting of the rock in question begins.  Once melting begins, the process changes to an igneous process rather than a metamorphic process.

Grade of Metamorphism

As the temperature and/or  pressure increases on a body of rock we say the rock undergoes prograde metamorphism or that the grade of metamorphism increases.   Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.

Low-grade metamorphism takes place at temperatures between about 200 to 320oC, and relatively low pressure.  Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals.  With increasing grade of metamorphism, the hydrous minerals begin to react with other minerals and/or break down to less hydrous minerals.

High-grade metamorphism takes place at temperatures greater than 320oC and relatively high pressure.  As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O, and non-hydrous minerals become more common.

Types of Metamorphism

Contact MetamorphismContact metamorphism occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion.Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to the zone surrounding the intrusion, called a metamorphic or contact aureole. 

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Outside of the contact aureole, the rocks are not affected by the intrusive event.  The grade of metamorphism increases in all directions toward the intrusion.  Because the temperature contrast between the surrounding rock and the intruded magma is larger at shallow levels in the crust where pressure is low, contact  metamorphism is often referred to as high temperature, low pressure metamorphism.  The rock produced is often a fine-grained rock that shows no foliation, called a hornfels. 

Regional MetamorphismRegional metamorphism occurs over large areas and generally does not show any relationship to igneous bodies.  Most regional metamorphism is accompanied by deformation under non-hydrostatic or differential stress conditions.  Thus, regional metamorphism usually results in forming metamorphic rocks that are strongly foliated, such as slates, schists, and gniesses.  The differential stress usually results from tectonic forces that produce compressional stresses in the rocks, such as when two continental masses collide. Thus, regionally metamorphosed rocks occur in the cores of fold/thrust mountain belts or in eroded mountain ranges.  Compressive stresses result in folding of  rock and thickening of the crust, which tends to push rocks to deeper levels where they are subjected to higher temperatures and pressures. 

Cataclastic MetamorphismCataclastic metamorphism occurs as a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone.  Heat is generated by the friction of sliding along such a shear zone, and the rocks tend to be mechanically deformed, being crushed and pulverized, due to the shearing.  Cataclastic metamorphism is not very common and is restricted to a narrow zone along which the shearing occurred.  

Hydrothermal MetamorphismRocks that are altered at high temperatures and moderate pressures by hydrothermal fluids are hydrothermally metamorphosed.  This is common in basaltic rocks that generally lack hydrous minerals.  The hydrothermal metamorphism results in alteration to such Mg-Fe rich hydrous minerals as talc, chlorite, serpentine, actinolite, tremolite, zeolites, and clay minerals. Rich ore deposits are often formed as a result of hydrothermal metamorphism. 

Burial MetamorphismWhen sedimentary rocks are buried to depths of several hundred meters, temperatures greater

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than 300oC may develop in the absence of differential stress.  New minerals grow, but the rock does not appear to be metamorphosed.  The main minerals produced are often the Zeolites.  Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase.

Shock Metamorphism (Impact Metamorphism)When an extraterrestrial body, such as a meteorite or comet impacts with the Earth or if there is a very large volcanic explosion, ultrahigh pressures can be generated in the impacted rock.  These ultrahigh pressures can produce minerals that are only stable at very high pressure, such as the SiO2 polymorphs coesite and stishovite.  In addition they can produce textures known as shock lamellae in mineral grains, and such textures as shatter cones in the impacted rock. 

 Classification of Metamorphic Rocks

Classification of metamorphic rocks is based on mineral assemblage, texture, protolith, and bulk chemical composition of the rock. Each of these will be discussed in turn, then we will summarize how metamorphic rocks are classified.

TextureIn metamorphic rocks individual minerals may or may not be bounded by crystal faces. Those that are bounded by their own crystal faces are termed idioblastic. Those that show none of their own crystal faces are termed xenoblastic. From examination of metamorphic rocks, it has been found that metamorphic minerals can be listed in a generalized sequence, known as the crystalloblastic series, listing minerals in order of their tendency to be idioblastic. In the series, each mineral tends to develop idioblastic surfaces against any mineral that occurs lower in the series. This series is listed below:

rutile, sphene, magnetite tourmaline kyanite, staurolite, garnet, andalusite epidote, zoisite, lawsonite, forsterite pyroxenes, amphiboles, wollastonite micas, chlorites, talc, stilpnomelane, prehnite dolomite, calcite scapolite, cordierite, feldspars quartz

This series can, in a rather general way, enable us to determine the origin of a given rock. For example a rock that shows euhedral plagioclase crystals in contact with anhedral amphibole, likely had an igneous protolith, since a metamorphic rock with the same minerals would be expected to show euhedral amphibole in contact with anhedral plagioclase.

Another aspect of the crystalloblastic series is that minerals high on the list tend to form porphyroblasts (the metamorphic equivalent of phenocrysts), although K-feldspar (a mineral that occurs lower in the list) may also form porphyroblasts. Porphyroblasts are often riddled

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with inclusions of other minerals that were enveloped during growth of the porphyroblast. These are said to have a poikioblastic texture.

Most metamorphic textures involve foliation. Foliation is generally caused by a preferred orientation of sheet silicates. If a rock has a slatey cleavage as its foliation, it is termed a slate, if it has a phyllitic foliation, it is termed a phyllite, if it has a shistose foliation, it is termed a schist. A rock that shows a banded texture without a distinct foliation is termed a gneiss. All of these could be porphyroblastic (i.e. could contain porhyroblasts).

A rock that shows no foliation is called a hornfels if the grain size is small, and a granulite, if the grain size is large and individual minerals can be easily distinguished with a hand lens.

ProtolithProtolith refers to the original rock, prior to metamorphism.  In low grade metamorphic rocks,  original textures are often preserved allowing one to determine the likely protolith.  As the grade of metamorphism increases, original textures are replaced with metamorphic textures and other clues, such as bulk chemical composition of the rock, are used to determine the protolith.

Bulk Chemical CompositionThe mineral assemblage that develops in a metamorphic rock is dependent on

The pressure and temperature reached during metamorphism

The composition of any fluid phase present during metamorphism, and The bulk chemical composition of the rock.

Just like in igneous rocks, minerals can only form if the necessary chemical constituents are present in the rock (i.e. the concept of silica saturation and alumina saturation applies to metamorphic rocks as well).  Based on the mineral assemblage present in the rock one can often estimate the approximate bulk chemical composition of the rock.  Some terms that describe this general bulk chemical composition are as follows:

Pelitic.  These rocks are derivatives of aluminous sedimentary rocks like shales and mudrocks.  Because of their high concentrations of alumina they are recognized by an abundance of aluminous minerals, like clay minerals, micas, kyanite, sillimanite, andalusite, and garnet.

Quartzo-Feldspathic.  Rocks that originally contained mostly quartz and feldspar like granitic rocks and arkosic sandstones will also contain an abundance of quartz and feldspar as metamorphic rocks, since these minerals are stable over a wide range of temperature and pressure.  Those that exhibit mostly quartz and feldspar with only minor amounts of aluminous minerals are termed quartzo-feldspathic.

Calcareous.  Calcareous rocks are calcium rich.   They are usually derivatives of carbonate rocks, although they contain other minerals that result from reaction of the carbonates with associated siliceous detrital minerals that were present in the rock.  At

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low grades of metamorphism calcareous rocks are recognized by their abundance of carbonate minerals like calcite and dolomite.   With increasing grade of metamorphism these are replaced by minerals like brucite, phlogopite (Mg-rich biotite), chlorite, and tremolite.  At even higher grades anhydrous minerals like diopside, forsterite, wollastonite, grossularite, and calcic plagioclase.

Basic.  Just like in igneous rocks, the general term basic refers to low silica content.  Basic metamorphic rocks are generally derivatives of basic igneous rocks like basalts and gabbros.  They have an abundance of Fe-Mg minerals like biotite, chlorite, and hornblende, as well as calcic minerals like plagioclase and epidote.

Magnesian. Rocks that are rich in Mg with relatively less Fe, are termed magnesian.  Such rocks would contain Mg-rich minerals like serpentine, brucite, talc, dolomite, and tremolite.  In general, such rocks usually have an ultrabasic protolith, like peridotite, dunite, or pyroxenite.

Ferriginous. Rocks that are rich in Fe with little Mg are termed ferriginous.  Such rocks could be derivatives of Fe-rich cherts or ironstones. They are characterized by an abundance of Fe-rich minerals like greenalite (Fe-rich serpentine), minnesotaite (Fe-rich talc), ferroactinolite, ferrocummingtonite, hematite, and magnetite at low grades, and ferrosilite, fayalite, ferrohedenbergite, and almandine garnet at higher grades.

Manganiferrous. Rocks that are characterized by the presence of Mn-rich minerals are termed manganiferrous.  They are characterized by such minerals as Stilpnomelane and spessartine.

ClassificationClassification of metamorphic rocks depends on what is visible in the rock and its degree of metamorphism. Note that classification is generally loose and practical such that names can be adapted to describe the rock in the most satisfactory way that conveys the important characteristics. Three kinds of criteria are normally employed. These are:

1. Mineralogical - The most distinguishing minerals are used as a prefix to a textural term. Thus, a schist containing biotite, garnet, quartz, and feldspar, would be called a biotite-garnet schist. A gneiss containing hornblende, pyroxene, quartz, and feldspar would be called a hornblende-pyroxene gneiss. A schist containing porphyroblasts of K-feldspar would be called a K-spar porphyroblastic schist.

2. Chemical - If the general chemical composition can be determined from the mineral assemblage, then a chemical name can be employed. For example a schist with a lot of quartz and feldspar and some garnet and muscovite would be called a garnet-muscovite quartzo-feldspathic schist. A schist consisting mostly of talc would be called a talc-magnesian schist.

3. Protolithic -  If a rock has undergone only slight metamorphism such that its original texture can still be observed then the rock is given a name based on its original name, with the prefix meta- applied. For example: metabasalt, metagraywacke, meta-andesite, metagranite.

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In addition to these conventions, certain non-foliated rocks with specific chemical compositions and/or mineral assemblages are given specific names. These are as follows:

Amphibolites: These are medium to coarse grained, dark colored rocks whose principal minerals are hornblende and plagioclase. They result from metamorphism of basic igneous rocks.  Foliation is highly variable, but when present the term schist can be appended to the name (i.e. amphibolite schist).

Marbles: These are rocks composed mostly of calcite, and less commonly of dolomite. They result from metamorphism of limestones and dolostones.  Some foliation may be present if the marble contains micas.

Eclogites: These are medium to coarse grained consisting mostly of garnet and green clinopyroxene called omphacite, that result from high grade metamorphism of basic igneous rocks. Eclogites usually do not show foliation.

Quartzites: Quartz arenites and chert both are composed mostly of SiO2.  Since quartz is stable over a wide range of pressures and temperatures, metamorphism of quartz arenites and cherts will result only in the recrystallization of quartz forming a hard rock with interlocking crystals of quartz.   Such a rock is called a quartzite.

Serpentinites:  Serpentinites are rocks that consist mostly of serpentine.  These form by hydrothermal metamorphism of ultrabasic igneous rocks.

Soapstones: Soapstones are rocks that contain an abundance of talc, which gives the rock a greasy feel, similar to that of soap.   Talc is an Mg-rich mineral, and thus soapstones from ultrabasic igneous protoliths, like peridotites, dunites, and pyroxenites, usually by hydrothermal alteration.

Skarns: Skarns are rocks that originate from contact metamorphism of limestones or dolostones, and show evidence of having exchanged constituents with the intruding magma.  Thus, skarns are generally composed of minerals like calcite and dolomite, from the original carbonate rock, but contain abundant calcium and magnesium silicate minerals like andradite, grossularite, epidote, vesuvianite, diopside, and wollastonite that form by reaction of the original carbonate minerals with silica from the magma.  The chemical exchange is that takes place   is called metasomatism.

Mylonites: Mylonites are cataclastic metamorphic rocks that are produced along shear zones deep in the crust.  They are usually fine-grained, sometimes glassy, that are streaky or layered, with the layers and streaks having been drawn out by ductile shear.

 

Metamorphic Facies

In general, metamorphic rocks do not drastically change chemical composition during metamorphism, except in the special case where metasomatism is involved (such as in the production of skarns, as discussed above).  The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism.  Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure

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environment that the rock was subjected to.  This pressure and temperature environment is referred to as Metamorphic Facies.  (This is similar to the concept of  sedimentary facies, in that a sedimentary facies is also a set of environmental conditions present during deposition).  The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism.   A high geothermal gradient such as the one labeled "A" , might be present around an igneous intrusion, and would result in metamorphic rocks belonging to the hornfels facies.  Under a normal to high geothermal gradient, such as "B", rocks would progress from zeolite facies to greenschist, amphibolite, and eclogite facies as the grade of metamorphism (or depth of burial) increased.  If a low geothermal gradient was present, such the one labeled "C" in the diagram, then rocks would progress from zeolite facies to blueschist facies to eclogite facies.   Thus, if we know the facies of metamorphic rocks in the region, we can determine what the geothermal gradient must have been like at the time the metamorphism occurred.  This relationship between geothermal gradient and metamorphism will be the central theme of our discussion of metamorphism.

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Metamorphic Rock Textures

Metamorphic rocks exhibit a variety of textures.  These can range from textures similar to the original protolith at low grades of metamorphism, to textures that are purely produced during metamorphism and leave the rock with little resemblance to the original protolith.  Textural features of metamorphic rocks have been discussed in the previous lecture.  Here, we concentrate on the development of foliation, one of the most common  purely metamorphic textures, and on the processes involved in forming compositional layering commonly observed in metamorphic rocks.

Foliation

Foliation is defined as a pervasive planar structure that results from the nearly parallel alignment of sheet silicate minerals and/or compositional and mineralogical layering in the rock. Most foliation is caused by the preferred orientation of phylosilicates, like clay minerals, micas, and chlorite.  Preferred orientation develops as a result of non-hydrostatic or differential stress acting on the rock (also called deviatoric stress).  We here review the differences between hydrostatic and differential stress. Stress and Preferred OrientationPressure increases with depth of burial, thus, both pressure and temperature will vary with depth in the Earth.  Pressure is defined as a force acting equally from all directions.  It is a type of stress, called hydrostatic stress or uniform stress.  If the stress is not equal from all directions, then the stress is called a differential stress.  Normally geologists talk about stress as compressional stress.  Thus, if a differential stress is acting on the rock, the direction along which the maximum principal stress acts is called 1, the minimum principal stress is called 3, and the intermediate principal stress direction is called 2.   Note that extensional stress would act along the direction of minimum principal stress. 

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If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock. 

Rounded grains can become flattened in the direction of maximum compressional stress.

 

Minerals that crystallize or grow in the differential stress field may develop a preferred orientation. Sheet silicates and minerals that have an elongated habit will grow with their sheets or direction of elongation orientated perpendicular to the direction of maximum stress.   This is because growth of such minerals is easier along directions parallel to sheets, or along the direction of elongation and thus will grow along 3 or 2, perpendicular to 1.

Since most phyllosilicates are aluminous minerals, aluminous (pelitic) rocks  like shales, generally develop a foliation as the result of metamorphism in a differential stress field.

  Example - metamorphism of a shale (made up initially of clay minerals and quartz) Shales have fissility that is caused by the preferred orientation of clay minerals with their {001} planes orientated parallel to bedding.   Metamorphic petrologists and structural geologists refer to the original bedding surface as S0.

Slate  Slates form at low metamorphic grade by the growth of fine grained chlorite and clay minerals. The preferred orientation of these sheet silicates causes the rock to easily break planes parallel to the sheet silicates, causing a slatey cleavage.Note that in the case shown here, the maximum principle stress is oriented at an angle to the original bedding planes so that the slatey cleavage develops at an angle to the original bedding. The foliation or surface produced by this deformation is referred to S1.

 

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Schist - The size of the mineral grains tends to enlarge with increasing grade of metamorphism.  Eventually the rock develops a near planar foliation caused by the preferred orientation of sheet silicates (mainly biotite and muscovite).  Quartz and feldspar grains, however show no preferred orientation.  The irregular planar foliation at this stage is called schistosity

Gneiss  As metamorphic grade increases, the sheet silicates become unstable and dark colored minerals like hornblende and pyroxene start to grow.These dark colored minerals tend to become segregated into distinct bands through the rock (this process is called metamorphic differentiation), giving the rock a gneissic banding.  Because the dark colored minerals tend to form elongated crystals,  rather than sheet- like crystals, they still have a preferred orientation with their long directions perpendicular to the maximum differential stress.

 

Granulite - At the highest grades of metamorphism most of the hydrous minerals and sheet silicates become unstable and thus there are few minerals present that would show a preferred orientation.  The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks.

 In general, the grain size of metamorphic rocks tends to increase with increasing grade of metamorphism, as seen in the progression form fine grained shales to coarser (but still fine) grained slates, to coarser grained schists and gneisses.

 

Metamorphism and Deformation

Most regionally metamorphosed rocks (at least those that eventually get exposed at the Earth's surface) are metamorphosed during deformational events.  Since deformation involves the

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application of differential stress, the textures that develop in metamorphic rocks reflect the mode of deformation, and foliations or slatey cleavage that develop during metamorphism reflect the deformational mode and are part of the deformational structures.  The deformation involved in the formation of fold-thrust mountain belts generally involves compressional stresses. The result of compressional stress acting on rocks that behave in a ductile manner (ductile behavior is favored by higher temperature, higher confining stress [pressure] and low strain rates) is the folding of rocks. Original bedding is folded into a series of anticlines and synclines with fold axes perpendicular to the direction of maximum compressional stress.  These folds can vary in their scale from centimeters to several kilometers between hinges.  Note that since the axial planes are oriented perpendicular to the maximum compressional stress direction, slatey cleavage or foliation should also develop along these directions.   Thus, slatey cleavage or foliation is often seen to be parallel to the axial planes of folds, and is sometimes referred to axial plane cleavage or foliation.

 

Metamorphic Differentiation

As discussed above, gneisses, and to some extent schists, show compositional banding or layering, usually evident as alternating somewhat discontinuous bands or layers of dark colored ferromagnesian minerals and lighter colored quartzo-feldspathic layers.  The development of such compositional layering or banding is referred to as metamorphic differentiation.  Throughout the history of metamorphic petrology, several mechanisms have been proposed to explain metamorphic differentiation.

1. Preservation of Original Compositional Layering.  In some rocks the compositional layering may not represent metamorphic differentiation at all, but instead could simply be the result of original bedding.  For example, during the early stages of metamorphism and deformation of interbedded sandstones and shales the compositional layering could be preserved even if the maximum compressional stress direction were at an angle to the original bedding.In such a case, a foliation might develop in the shale layers due to the recrystallization of clay minerals or the crystallization of other sheet silicates with a preferred orientation controlled by the maximum stress direction.

Here, it would be easy to determine that the compositional layers represented original

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bedding because the foliation would cut across the compositional layering.

 

In highly deformed rocks that have undergone both folding and shearing, it may be more difficult to determine that the compositional layering represents original bedding. As shearing stretches the bedding, individual folded beds may be stretched out and broken to that the original folds are not easily seen.

Similarly, if the rock had been injected by dikes or sills prior to metamorphism, these contrasting compositional bands, not necessarily parallel to the original bedding, could be preserved in the metamorphic rock. 

2. Transposition of Original Bedding.  Original compositional layering a rock could also become transposed to a new orientation during metamorphism.  The diagram below shows how this could occur.  In the initial stages a new foliation begins to develop in the rock as a result of compressional stress at some angle to the original bedding.  As the minerals that form this foliation grow, they begin to break up the original beds into small pods.  As the pods are compressed and extended, partly by recrystallization, they could eventually intersect again to form new compositional bands parallel to the new foliation.

3. Solution and Re-precipitation. In fine grained metamorphic rocks small scale folds, called kink bands, often develop in the rock as the result of application of compressional stress. A new foliation begins to develop along the axial planes of the folds.  Quartz and feldspar may dissolve as a result of pressure solution and be reprecipitated at the hinges of the folds where the pressure is lower.  As the new foliation begins to align itself perpendicular to 1, the end result would be alternating bands of micas or sheet silicates and quartz or feldspar, with layering parallel to the new foliation.

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4. Preferential Nucleation.  Fluids present during metamorphism have the ability to dissolve minerals and transport ions from one place in the rock to another.Thus felsic minerals could be dissolved from one part of the rock and preferentially nucleate and grow in another part of the rock to produce discontinuous layers of alternating mafic and felsic compositions. 

5. Migmatization.  As discussed previously, migmatites are small pods and lenses that occur in high grade metamorphic terranes that may represent melts of the surrounding metamorphic rocks.  Injection of the these melts into pods and layers in the rock could also produce the discontinuous banding often seen in high grade metamorphic rocks.  The process would be similar to that described in 4, above, except that it would involve partially melting the original rock to produce a felsic melt, which would then migrate and crystallize in pods and layers in the metamorphic rock.  Further deformation of the rock could then stretch and fold such layers so that they may no longer by recognizable as migmatites. 

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