minerals and rocks chapter 2. earth materials – minerals some minerals, –such as gold, –have...
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Minerals and Rocks
Chapter 2
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Earth Materials – Minerals
• Some minerals, – such as gold, – have fascinated people for
thousands of years – and have been supposed – to have mystical or curative
powers• Minerals have many essential
uses – in industrial societies
• Minerals are the basic units – that make up most of Earth’s
materials
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Earth Materials – Rocks
• Rocks also have many uses:– rocks crushed for aggregate in cement and for
roadbeds– sawed and polished rocks for tombstones,
monuments, mantle pieces and countertops– Even the soils we depend on
• for most of our food
• are formed by alteration of rocks
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Rocks
• Mountains around Tenaya Lake in Yosemite National Park– Made of granite– Granite is
composed of quartz and feldspar minerals
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Minerals
Minerals on display – at the
California Academy of Sciences in San Francisco
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Earth Materials
• Some materials formed by the Earth – are interesting and
attractive
– such as this metamorphic rock
• from the shoreline of Lake Superior at Marquette, Michigan
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Matter and Its Composition
• Matter– anything that has mass and occupies space– exists as solids, liquids, gases, and plasma– consists of elements and atoms
• Element– a chemical substance – composed of tiny particles called atoms
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Atoms
• Atoms are the smallest units of matter – that retain the characteristics of the element
• Atoms have– a compact nucleus containing
• protons – particles with a positive electrical charge
• neutrons – electrically neutral particles
– particles outside the nucleus • electrons – negatively charged particles
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Atoms
• Atomic number = the number of protons
• Atomic mass number = number of protons + number of neutrons
• The number of neutrons in nucleus of an element may vary
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Carbon Isotopes
• Three isotopes of carbon (all with 6 protons) – 6 neutrons = Carbon 12 (12C)
– 7 neutrons = Carbon 13 (13C)– 8 neutrons = Carbon 14 (14C)
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Electrons and Shells
• Electrons lie outside the nucleus in one or more shells• The outermost shells are involved
– in chemical bonding
– and contain up to 8 electrons
• Noble gas configuration of 8 electrons • or 2 for helium
– have complete outer shells
– and are stable
• Other atoms attain – a noble gas configuration
– through the process of bonding
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Bonding and Compounds
• Bonding – the process whereby atoms join to other atoms
• Compound – a substance resulting from the bonding – of two or more elements
• Oxygen gas (O2) is an element• Ice (H2O) is a compound
– made up of hydrogen and oxygen atoms
• Most minerals are compounds
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Ionic Bonding
• One way for atoms to attain the noble gas configuration – is by transferring electrons, producing ions
• Ion – an atom that has gained or lost one or more
electrons – and thus has a negative or positive charge
• Ionic bonding – attraction between two ions of opposite charge
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Covalent Bonding
• Another way for atoms – to attain the noble gas configuration – is by sharing electrons
• Covalent bonding– results from
sharing electrons
shared electrons
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Minerals
• Geological definition of a mineral:– naturally occurring– crystalline solid
• crystalline means that minerals • have atoms arranged in specific 3-dimensional
frameworks– inorganic– minerals have a narrowly
defined chemical composition– and characteristic physical
properties such as• density• hardness
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Minerals—The Building Blocks of Rocks
• A mineral’s composition is shown by a chemical formula– a shorthand way of indicating how many atoms
of different kinds it contains
Quartz: SiO2
Ratio: 1: 2
– Quartz molecules consist of 1 silicon atom and 2 oxygen atoms
– Orthoclase molecules consists of 1 potassium, 1 aluminum, 3 silicon, and 8 oxygen atoms
KAlSi3O8
1: 1: 3: 8
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Native Elements
• A few minerals consist of only one element.
• They are not compounds.
• They are known as native elements.
• Examples: – Gold: Au– Diamond: C
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Crystalline Solids
• By definition, minerals are crystalline solids– with atoms arranged in a specific 3D framework
• If given enough room to grow freely, – minerals form perfect crystals with – planar surfaces, called crystal faces– sharp corners– straight edges
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Narrowly Defined Chemical Composition
• Some minerals have very specific compositions– Examples: halite (NaCl), quartz (SiO2)
• Other minerals have a range of compositions – because one element can substitute for another – if the atoms of the two elements have
• the same electrical charge• and are about the same size
– Example: olivine • (Mg,Fe)2SiO4
• iron and magnesium substitution in any proportion
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Mineral Properties
• Mineral properties are controlled by– Chemical composition
– Crystalline structure
• Mineral properties are particularly useful – for mineral identification and include:
• cleavage • fracture• hardness• specific gravity
• color• streak• luster• crystal form
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How Many Minerals Are There?
• More than 3500 minerals are known• Only about 2 dozen are particularly common• Many others are important resources• Mineral groups:
– minerals with the same negatively charged ion or ion group
– belong to the same mineral group
• Most minerals in the crust – belong to the group called silicates
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Silicates
• Silicates are minerals containing silica – Si and O
• These minerals make up almost 95% of Earth’s crust– and account for about 1/3 of all known minerals
• The basic building block of silicates – is the silica tetrahedron
• which consists of one silicon atom• surrounded by four oxygen atoms
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Types of Silicates
• Silica tetrahedra can be – isolated units bonded to
other elements– arranged in chains (single or
double)– arranged in sheets– arranged in complex
3D networks
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Types of Silicates
• Ferromagnesian silicates – contain iron (Fe), magnesium (Mg), or both
• Nonferromagnesian silicates – do not contain iron or magnesium
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Ferromagnesian Silicates• Common ferromagnesian silicates include
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Nonferromagnesian Silicates
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Other Mineral Groups
• Carbonates – minerals with carbonate ion (CO3)-2
• calcite (CaCO3), – constituent of limestone
• dolomite [CaMg(CO3)2], – constituent of dolostone
• Other mineral groups are important, – but more as resources – than as constituents of rocks
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Rock-Forming Minerals
• Most rocks are solid aggregates – of one or more minerals
• Hundreds of minerals occur in rocks, – but only a few are common – and called rock-forming minerals
• Most rock-forming minerals are silicates, – but carbonates are also important
• Accessory minerals are present in small amounts – and are ignored in classifying rocks
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Rock Cycle
• The rock cycle is a pictorial representation – of events leading to – the origin, destruction, change – and reformation of rocks
• Rocks belong to 3 major families– igneous– sedimentary– metamorphic
• The rock cycle shows – how these rock families are interrelated – and can be derived from one another
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Rock Cycle
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Igneous Rocks
• All igneous rocks – cool and crystallize from magma, – solidify from lava, – or consolidate from pyroclastic materials
• Magma is molten material – below the surface
• Lava is molten material on the surface
• Pyroclastic materials – are particles such as volcanic ash
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Pyroclastic material
Lava
Igneous Part of the Rock Cycle
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Categories of Igneous Rocks
• Extrusive or volcanic rocks – formed at the surface – from lava or pyroclastic materials
• Intrusive or plutonic rocks – formed from magma injected into the crust– or formed in place in the crust– Plutons are intrusive bodies
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Plutons
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Igneous Rock Textures
• Texture – is the size, shape, and arrangement– of crystals, grains, and other constituents of a
rock
• Igneous rocks have several textures – that relate to cooling rate of magma or lava
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Igneous Rock Textures
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Cooling-Rate Textures• phaneritic,
– with visible grains • cooled slowly
• aphanitic, – with grains too small to see without magnification
• cooled quickly
• porphyritic, – with larger grains (phenocrysts) surrounded by a
finer-grained groundmass • cooled slowly intrusively, then expelled onto the surface
• glassy, – with no grains
• cooled too quickly for minerals to grow
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Igneous Rock Textures• Other textures reveal further details
– of the formation of the rock• Vesicular texture, with holes (vesicles),
– indicates the rock formed– as water vapor and other gases– became trapped during cooling of lava
• Pyroclastic or fragmental texture, – containing fragments,– formed by consolidation of volcanic ash– or other pyroclastic material
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Igneous Rock Textures
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Classifying Igneous Rocks
• Texture and composition are the criteria – used to classify most igneous rocks
• Composition categories are based on mineral composition – FELSIC, light colored, >65% silica– INTERMEDIATE, 53-65% silica– MAFIC, dark colored, 45-52% silica– ULTRAMAFIC, <45% silica
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Classifying Igneous Rocks
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Common Igneous Rocks
Basalt Gabbro
Andesite Diorite
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Common Igneous Rocks
Rhyolite Granite
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Classifying Igneous Rocks with Special Textures
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Igneous Rocks with Special Textures
Outcrop with basalt underlain by tuff
Pumice is glassy and extremely vesicular.
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Sedimentary Rocks
• Sedimentary rocks form – by the lithification of sediment
• In the rock cycle, sediment originates when– mechanical and chemical weathering
• disintegrate and decompose rocks at the surface
– Transport removes sediment • from its source area and carries it elsewhere
– Sediments accumulate in deposits, • or as minerals that precipitate from solution
• or that organisms extract from solution.
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Sedimentary Part of the Rock Cycle
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Lithification
• Lithification means – converting sediment into sedimentary rock
• Lithification occurs by – compaction
• Pressure exerted by overlying sediments
• reduction of the amount of pore space between particles
– cementation • precipitation of minerals within pores • effectively binds sediment together
– calcium carbonate (CaCO3) cement is common– silica (SiO2) cement is common– iron oxide or iron hydroxide (Ex: Fe2O3) cement is less common
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Categories of Sedimentary Rocks
• Detrital sedimentary rocks – consist of solid particles – derived from preexisting rocks (detritus)
• Chemical sedimentary rocks – consist of minerals derived from materials in
solution and – extracted by either
• inorganic chemical processes • or by the activities of organisms
– subcategory biochemical sedimentary rocks, in which
• the activities of organisms are important
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Detrital Sedimentary Rocks• are composed of fragments or particles
– known as clasts = Clastic texture• These rocks are defined primarily by size of
clasts• conglomerate
– composed of gravel (>2mm)– with rounded clasts
• sedimentary breccia– also composed of gravel (>2mm)– but clasts are angular
• sandstone– composed of sand
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Detrital Sedimentary Rocks
• Mudrocks consist of particles < 1/16 mm– mudstone
• composed of particles less than 1/16 mm particles
• consists of both silt- and clay-size particles
– siltstone• composed of silt-sized particles 1/16-1/256 mm
– claystone• composed of clay-sized particles <1/256 mm
– shale• mudstone or claystone that
• breaks along closely spaced parallel planes (fissile)
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Chemical Sedimentary Rocks
• Recall that these rocks result – when inorganic chemical processes – or organisms extract minerals from solution
• This can result in different textures– Crystalline texture
• has an interlocking mosaic of mineral crystals
– Clastic texture • has an accumulation of broken pieces of shells
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Chemical Sedimentary Rocks• Limestone – carbonate rock made of calcite
precipitated chemically or (most commonly) by organisms
• Dolostone – carbonate rock made of dolomite usually altered from limestone
• Evaporites formed by – inorganic chemical precipitation during evaporation– Rock salt and rock gypsum – evaporites made of
sodium chloride and gypsum– Chert – compact, hard, fine grained silica, formed by
chemical or biological precipitation
– Coal – made of partially altered, compressed remains of land plants accumulated in swamps
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Common Sedimentary Rocks
Conglomerate Sedimentary breccia
Quartz sandstone Shale
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Common Sedimentary Rocks
Fossiliferous limestone
Rock salt
Chert Coal
Rock gypsum
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Metamorphic Rocks
• Metamorphic rocks – result from transformation of other rocks – in the solid state, without melting
• Changes from metamorphism include– compositional
• new minerals form
– textural• minerals become aligned
– or both
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Metamorphic Part of the Rock Cycle
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Agents of Metamorphism• Heat
– Increases the rate of chemical reactions– Yields different minerals from parent rock
• Pressure – Lithostatic pressure
• Weight of overlying rocks• Forms smaller, denser minerals
– Differential pressure • exerts force more intensely from one direction
• Fluid activity is an important metamorphic agent as well
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Types of Metamorphism• Contact metamorphism
– heat and chemical fluids– from an igneous body– alter adjacent rocks
• Regional metamorphism– most common– large, elongated areas– tremendous pressure, elevated temperatures, and fluid
activity– occurs at convergent and divergent plate boundaries
• Dynamic metamorphism– Confined to zones adjacent to faults– Differential pressure
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Metamorphic Textures
• Foliated texture– platy and elongate minerals aligned parallel to
one another– caused by differential pressure
• Nonfoliated texture– mosaic of roughly equidimensional minerals
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Formation of Foliation
• When rocks are subjected to differential pressure the minerals typically rearrange in a parallel fashion
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Formation of Foliation
• Microscopic view of a metamorphic rock with foliation showing the parallel arrangement of minerals
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Foliated Metamorphic Rocks
• Slate – very fine-grained, low-grade metamorphism
• Phyllite – fine-grained (coarser than slate but grains are
still too small to see without magnification)
• Schist – clearly visible platy and/or elongate minerals
• Gneiss – alternating dark and light bands of minerals
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Nonfoliated Metamorphic Rocks
• Marble – Composed of calcite or dolomite metamorphosed from
limestone or dolostone
• Quartzite – Composed of quartz metamorphosed from quartz
sandstone
• Greenstone– Green, altered mafic igneous rock
• Hornfels– Clay-rich, results from contact metamorphism
• Anthracite– Black, lustrous, hard coal
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Common Metamorphic Rocks
Slate Schist
Gneiss Marble Quartzite
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Plate Tectonics and the Rock Cycle
• The atmosphere, hydrosphere and biosphere – act on earth materials
– and cause weathering, erosion, and deposition
• Earth’s internal heat – aids melting and metamorphism
• Plate tectonics recycles Earth materials– heat and pressure at convergent plate boundaries
• lead to metamorphism and igneous activity
– Some rocks in a subducted plate are deformed and incorporated into an evolving mountain system
• that in turn weather and erode to form sediment