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Igneous rocks in thin section

Contents Image scales (frame widths) Quartz

Plagioclase Potassium feldspars

Myrmeckite Micas

Amphiboles Pyroxenes

Other primary minerals and minor minerals

Secondary (subsolidus) minerals

20x = 6 mm 40x = 3 mm

100x = 1 mm 200x = 0.5 mm

400x = 0.25 mm 1000x = 0.1 mm

Quartz

Quartz crystal in alkali granite

Plane polarized light, 20x

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Quartz crystal in alkali granite

Cross polarized light, 20x

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Strained quartz crystal in a metaluminous granite. Strain has caused the quartz crystal to deform into domains with slightly different extinction angles.


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Fluid inclusions in quartz in alkali granite. The inclusion in the center has an irregular outer boundary, inside of which is a layer of liquid water, a layer of liquid CO2, and a central bubble of vapor (mostly CO2). At the time of trapping of this secondary inclusion, the fluid was a binary H2O-CO2 fluid.


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Fluid inclusions in quartz in alkali granite. Several inclusions containing (probably) water and a central vapor bubble.


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Plagioclase

Plagioclase, unzoned, in a hornblende diorite


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Plagioclase, unzoned, in a hornblende diorite. Note the strong, parallel sets of albite twins, and the less visible set of pericline twins inclined almost at right angles to the albite twins.


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Plagioclase, zoned, in a dacite porphyry. This plagioclase appears quite homogeneous in plane light.


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Plagioclase, zoned, in a dacite porphyry. This plagioclase has fine oscillatory zoning, in which the composition varies between more and less anorthite-rich compositions. Internal unconformities followed by euhedral overgrowths are also visible.


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Plagioclase, zoned, in a dacite porphyry. Notice the concentric layers (zones) of inclusions in this crystal.


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Plagioclase, zoned, in a dacite porphyry. Notice that the concentric inclusion zones also have different birefringence, indicating these zones have different anorthite content. The interior of this crystal has patchy zoning rather than concentric, indicating skeletal early growth. Albite, carlsbad, and pericline twins cut across the composition zones.


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Potassium feldspars

Orthoclase in a dacite hypobyssal intrusive.


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Orthoclase in a dacite hypobyssal intrusive. Notice the two carlsbad twin domains, separated by the carlsbad composition plane.


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Microcline from a peraluminous granite.


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Microcline from a peraluminous granite. Note the "grid" or "tartan plaid" twinning pattern that results from crossing albite and pericline twin domains that form during inversion of monoclinic orthoclase to triclinic microcline during cooling. This inversion and change in crystal system is associated with increasing ordering or aluminum in crystallographic sites.

Cross polarized light, 20x.

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Microcline from a peraluminous granite. Closeup of the plaid twinning. Notice how the twin domains are spindly and somewhat wispy. This is in contrast to the straight and generally continuous twin domains in plagioclase.


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Perthite from a metaluminous biotite granite. Note the faint, irregular stripes that run from the upper right to lower left.

Plane polarized light, 20x.

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Perthite from a metaluminous biotite granite. Note the lighter irregular stripes and patches of albite that separate gray stripes and patches of microcline. The color difference is mostly due to the different optical orientations of the two different minerals. The microcline and albite both exsolved (unmixed) from an originally homogeneous high temperature of intermediate composition. The sample is oriented to obscure the twinning.


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Perthite from a metaluminous biotite granite. The somewhat less altered and narrower albite exsolution lamellae are in sharp contact with a much larger microcline domains. You can see a lamella closeup here, and then see a Becke line test from focused to a lowered stage position at the lamella-microcline contact. The Becke line goes into the higher index phase, albite.


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Perthite from a metaluminous biotite granite. Same as the image above in cross polarized light.


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Perthite from a metaluminous biotite granite. Closeup showing the characteristic grid twinning in the microcline host (upper center and upper left) and albite twinning in the lamellae (lower center to center right).


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Myrmeckite

Myrmekite patch that appears to be replacing microcline.


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Myrmekite patch that appears to be replacing microcline. Faint twins in the myrmekite clearly shows that the probably quartz "worms" are in a plagioclase matrix.


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Micas

Muscovite, peraluminous granite. Igneous muscovite is generally colorless with good cleavage. Radiation halos can be visible around radioactive inclusions (but not visible here).


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Muscovite, peraluminous granite.


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Biotite, metaluminous granite, showing several grains in different orientations. Grain on the far right is oriented with the cleavages N-S, and so is almost opaque. The largest grain is inclined and is lighter in color. Grains with the cleavages E-W have the least absorption (not shown in this image).


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Biotite, metaluminous granite. This is the same area as the image above in cross polarized light. The birefringent colors of the biotite are muted because of the color of the biotite itself.


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Biotite, metaluminous granite, showing a closeup of one of the same biotite crystals above at extinction, occupying the entire center of the image. Damage produced to this soft mineral during thin section grinding causes speckles of light on the biotite, where the crystal lattice has been deformed. This means that biotite in standard thin sections rarely goes completely extinct. This is called "incomplete extinction" or sometimes "birds eye maple extinction".


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Biotite, peraluminous granite. This graphite- and garnet-bearing granite has high-Ti, low-Fe3+ red-brown biotite. This patch of biotite rimming garnet (mineral occupying the lower part of the image) shows the range of pleochroic colors because of different crystals being in different orientations.


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Amphiboles

Green hornblende in a diorite.


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Green hornblende in a diorite.


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Green hornblende in a diorite. The ~120° and ~60° cleavage intersections are clearly visible in this end section of a crystal.


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Brown hornblende, hornblende gabbro. The extensive dark and light brown areas are several hornblende crystals. Note the numerous inclusions of opaques and plagioclase.


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Brown hornblende, hornblende gabbro, in the image above. The two hornblende orientations do not happen to yield the maximum interference colors, which happen to be second order blue in this section.. Note how plagioclase inclusions have more closely spaced twins than does the hornblende.


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Pyroxenes

Enstatite (orthopyroxene, OPX) in norite. The large OPX in the center is oriented with its c crystallographic axis oriented N-S.


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Enstatite (orthopyroxene, OPX) in norite. The large N-S oriented enstatite grain near the center of the image (see image above) is extinct, in keeping with the orthorhombic symmetry of this mineral. Birefringence ranges to upper first order.


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Enstatite (orthopyroxene, OPX) in norite. Here the section has been rotated clockwise ~45° to show the birefringence of the large grain.


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Augite (clinopyroxene, CPX), gabbro. This augite is slightly brownish, and has tiny exsolved rods and plates of Fe-Ti oxides. These form the darkish, cloudy regions along with numerous cracks.


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Augite (clinopyroxene, CPX), gabbro. This augite has irregular birefringence up to second order blue.


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Other primary minerals

Olivine, phenocryst in an Iceland basalt. Notice the fractures concentric with the crystal margin. Smaller "microphenocrysts" of olivine and plagioclase also occur, and olivine and brownish augite also occur in the matrix.


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Olivine, phenocryst in an Iceland basalt. Smaller "microphenocrysts" of olivine have birefringence generally in the second and third order.


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Garnet in a peraluminous granite. The high refractive index of this mineral cause fractures and the grain margin to stand out as dark lines.


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Garnet in a peraluminous granite. The lack of birefringence distinguishes garnet from all other common high refractive index minerals.


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Epidote in a calc-alkaline granodiorite. In general, epidote in igneous rocks has rather high Fe3+ content, and is therefore pistacite. It may be pale yellow-green, but more commonly it is essentially colorless. It has high relief and is commonly associated with other Ca-rich minerals such as hornblende, plagioclase, and titanite.


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Epidote in a calc-alkaline granodiorite. The higher the Fe3+ content, the higher the birefringence. Typically, the birefringence is 2nd and 3rd order and so epidote, with its high relief and typical lack of color, can resemble olivine. Epidote, however, can occur with quartz and various hydrous minerals, including secondary sericite, with which olivine is unlikely to occur.


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Apatite crystals in a norite. This view has several apatite crystals mostly arranged in plagioclase with surrounding OPX. Apatite is colorless, commonly elongate, and typically has hexagonal end sections. Its relief is less than the pyroxenes but higher than any feldspar.


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Apatite crystal in a norite. The very low 1st order birefringence is obvious. Apatite has negative elongation. Apatite is extremely common in small amounts in igneous rocks. Indeed, it is rare to find a terrestrial igneous rock without at least some apatite.


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Titanite in a metaluminous granite. Titanite (formerly sphene) has a typical double wedge or diamond shape, is typically light brown, and has very high relief, higher than the pyroxenes or garnet.


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Titanite in a metaluminous granite. Titanite birefringence is very high, making it difficult to determine interference color order from the high-order pastel interference colors normally seen.


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Titanite in a metaluminous granite. At high magnification, the magenta bands can be counted up a thin edge to get a better idea of the interference order. This example is approximately 6th order.


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Zircon in a metaluminous granite. This is a rather blocky zircon fully enclosed within biotite. Small apatite crystals surround it. Zircon has relief considerably higher than garnet or pyroxenes. The moderate uranium and thorium content of zircon cause the development of pleochroic radiation halos around it.


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Zircon in a metaluminous granite. Birefringence is moderately high. This example has birefringence zoning that is probably caused originally by igneous compositional zoning. High uranium layers accumulate more radiation damage and become metamict. A metamict state is basically a glass induced by extreme radiation damage, in which the crystalline structure has been destroyed.


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Zircon compared to monazite.

Zircon oriented N-S. Zircon is usually colorless in thin section.


Monazite oriented N-S. Monazite can be very pale yellow-green in thin section.


In the same orientation zircon is extinct, and so it has parallel extinction.


In the same orientation Monazite is not extinct, and so it must have inclined extinction.

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Allanite in a metaluminous granite. This allanite has concentric zoning probably resulting from changing mineral composition during successive stages of growth. Allanite contains substantial amounts of Th and U, and sustains considerable radiation damage over time. Swelling of the crystal as damage accumulates, and absorption of water, causes swelling of the grain that result in the many radial cracks extending out into the surrounding minerals.


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Allanite in a metaluminous granite. The allanite grain mostly has low birefringence, a result of radiation damage that essentially turns the crystal lattice into a glass.


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Secondary minerals

Chlorite after biotite in a muscovite granite. Small residual brownish patches of biotite still occur, as to dark bits of rutile.


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Chlorite after biotite in a muscovite granite. The low first order, anomalous Berlin blue interference color indicates that this is an Fe-rich chlorite, as are the high birefringence patches of residual biotite.


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Chlorite after biotite in a metaluminous granite. The green chlorite has only replaced the biotite on the top left of side of the biotite grain. Lenses of colorless material fill much of the contact region between the two sheet silicates.


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Chlorite after biotite in a metaluminous granite. The chlorite mostly has an low first order interference color, indicative of Mg-rich chlorite. The purple regions are of intermediate composition. The adjacent biotite has third order birefringence.


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Sericite replacing plagioclase in a metaluminous granite. Sericite is grungy-looking fine-grained stuff that commonly replaces feldspars.


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Sericite replacing plagioclase in a metaluminous granite. Because it is generally made up of very small crystals, its birefringence is irregular and generally low. Some of the larger crystals can be seen here to have first order birefringence. Albite twinning in the plagioclase is clearly visible.


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Sericite replacing plagioclase in a metaluminous granite. At higher magnification, some of the grungy-looking clearly resolves into little, colorless, platy crystals.


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Sericite replacing plagioclase in a metaluminous granite. The crystals now clearly have up to middle first order birefringence. They have a positive sign of elongation and are probably small white micas that grew during subsolidus hydrothermal alteration.


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Serpentine in an altered harzbergite nodule in a kimberlite from Pennsylvania. Much of this sample is made up of calcite (colorless) and talc (darker browns). The serpentine is the yellowish material that occupies the thin veins.


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Serpentine in an altered harzbergite nodule in a kimberlite from Pennsylvania. The calcite has very high birefringence, and the talc has irregular second and third order interference colors that are modified by the brown staining. The serpentine is most easily seen in the veins, and veins have lower first order birefringence and fibers perpendicular to the vein walls.


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Calcite in an alkaline granite. In this image, the calcite grain is oriented so that a N-S line bisects the obtuse angle between the cleavages. This means the c axis of the calcite is approximately N-S, so the N-S polarized light is parallel to the low calcite refractive index. The grain has rather low relief.


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Calcite in an alkaline granite. In this image, the calcite grain, above, has been rotated so that a N-S line bisects the acute angle between the cleavages. This means the c axis of the calcite is approximately E-W, and the N-S polarized light is now parallel to the high calcite refractive index.. The grain now has high relief. Calcite and dolomite are the only two common minerals that can vary from low to high relief in thin section.


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Calcite in an alkaline granite. Calcite has very high birefringence and interference order is difficult to judge from the high-order pastel colors. It is more reliable to look at a thin edge and count the number of magenta bands. The calcite crystal in the lower center of this image has ~8th order birefringence.


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Calcite in an alkaline granite. Calcite is quite soft and undergoes substantial surface deformation during normal thin section grinding. The distorted surface of the crystal, combined with its high birefringence, results in incomplete, speckled extinction..


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Fluorite in a metaluminous granite. The fluorite here is doubtless secondary, since it occurs along the biotite cleavage along with some chlorite. It has been colored purple by radiation damage, probably by nearby allanite and other inclusions in the biotite.


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Fluorite in a metaluminous granite. Fluorite, of course, is isotropic.


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