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Chapter 4 Rocks Minerals
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• Minerals
• Naturally occurring (cannot be man-made)
• Solid (not a gas or liquid)
• Inorganic (not living or once-living)
• Definite chemical composition (not a mixture)
• Specific internal crystal structure (molecules
  arent randomly arranged)

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• Building blocks of rocks
• Many form under narrow ranges of physical
  conditions
• Can indicate pressure temperature during
  formation
• Some form from ocean water
• Evidence of former marine conditions
• Some form under arid conditions
• Evidence of former arid/tropical areas
• Magnetic minerals record direction of Earths
  magnetic field at time of their formation
• Radioactive minerals allow age dating of rocks

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• Over 4000 mineral species have been identified
• Only about 400 may be considered common
• If you become a geologist, you need to be
  familiar with about 200
• Only 25 or so form the majority of rocks

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• The common minerals are usually identified on
  basis of easily seen/measured properties
• Color
• Streak
• Luster
• Cleavage
• Hardness
• Density
• Crystal form
• Other special properties
• These properties not covered here those in lab
  will see them up close personal

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• The common rock-forming minerals are broken into
  two groups
• Silicates (contain silicon oxygen)
• Non-silicates (do not contain silicon)

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• Silicates
• Make up the bulk of the crust
• Only 8 elements make up the majority of these
• Note that silicon oxygen constitute about 75
  (by weight) of the crust oxygen is most abundant
• Basis of silicates a silicon atom is surrounded
  by four oxygen atoms, forming a silicate
  tetrahedron
• These can combine somewhat like polymers, by
  sharing one or more oxygens

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Common rock-forming silicates

• These can form from molten rock
• Quartz silicon dioxide (silica)
• Potassium feldspar group
• Orthoclase microcline
• Potassium aluminosilicate
• Plagioclase feldspar group
• Several species
• Sodium-calcium aluminosilicates

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• Muscovite (mica) potassium aluminosilicate
  w/water
• Biotite (mica) magnesium-potassium-iron
  aluminosilicate w/water
• Pyroxene group aluminum-calcium-magnesium-iron
  silicates
• Amphibole group - aluminum-calcium-magnesium-iron
  silicates w/water
• Olivine group magnesium-iron silicates
• Biotite, pyroxene, amphibole, olivine are
  collectively called ferromagnesian minerals

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• Clay minerals
• Silicates of hydrogen, aluminum, magnesium, iron,
  potassium
• Structure is similar to mica
• Formed by the weathering of aluminum-bearing
  silicates
• Cover about 75 of continental surfaces, are
  the most abundant materials deposited in oceans
• clay also refers to a grain size (more on this
  later)

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• Nonsilicate minerals
• Do not contain silicon
• Make up about 8 of crust
• Includes carbonate, sulfide, sulfate, chloride,
  oxide minerals, among others
• As far as rocks, we have two main groups
• Carbonates
• Evaporites

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• Carbonates
• Contain carbon oxygen (carbonate radical)
• Calcite
• Calcium carbonate
• Main constituent of limestone marble
• Shells of some organisms can also precipitate
  directly from seawater or groundwater
• Identified by reaction with hydrochloric acid,
  and by its cleavage (if in reasonably
  crystallized masses)
• Aragonite
• Also calcium carbonate, but different crystal
  structure
• Forms skeletons of corals molluscs (clams,
  snails)
• Makes up mother-of-pearl
• Metastable changes to calcite over time

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• Dolomite
• Calcium magnesium carbonate
• Has cleavage like calcite
• Reacts with hydrochloric acid only if powdered
• Forms from limestone by reaction with
  magnesium-bearing waters (the exact process isnt
  really known)

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• Evaporites
• Form by evaporation of water containing dissolved
  salts
• Indicates arid climate
• Several, the most common are halite and gypsum

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• Halite (rock salt)
• Sodium chloride common table salt
• Cubic cleavage
• Salty taste (!)
• Gypsum
• Calcium sulfate dihydrate
• Used to make Plaster of Paris drywall
• Soft can be scratched by fingernail
• Has two cleavages, but is often fine-grained or
  fibrous

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• Rocks
• Areally extensive aggregates of one or more
  minerals
• Three groups
• Igneous crystallized from lava or magma
• Sedimentary from sediments that were compressed
  and/or cemented
• Metamorphic rocks that were changed by the
  action of heat, pressure, and/or chemical activity

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• Once formed, a rock is rarely left alone
• On the Earth, anyway
• Environmental conditions change
• It can be weathered (mechanically broken down,
  chemically changed)
• It can be heated/compressed
• It can be remelted
• The rock cycle
• Not a Flintstone-era exercise machine made from
  rock

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Assumed start point molten rock
The rock cycle
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A simpler version
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• Rock groups classified according to their origin
• Specific rock types within each group identified
  based on two characteristics
• Texture the size, shape, arrangement of the
  grains/particles
• Mineral composition
• We now go through the rock groups.

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Igneous rocks

• Fire-formed
• Crystallized from hot, molten lava or magma as it
  cooled
• Magma molten rock beneath the surface of the
  Earth
• Lava molten rock which has flowed out onto the
  surface of the Earth

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• Cooling history texture
• Magma cools crystallizes before it reaches the
  surface
• Termed intrusive (intruded other rocks) or
  plutonic (from Pluto, Roman god of the
  underworld)
• Magma reaches surface, erupts as lava
• Called extrusive (wonder why?) or volcanic (from
  Vulcan, Roman god of fire not from Mr. Spock)

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• Texture of igneous rocks
• Mainly, the grain size how big are the
  individual crystals
• Related to the cooling history of the (igneous)
  rock
• Coarse grains (basically, you can see the grains
  without need of magnification) indicate slow
  cooling
• Fine grains indicate fast cooling
• Large crystals in fine groundmass
• Porphyritic
• Some crystals grew deep in the Earth, then were
  incorporated in the lava as it erupted at the
  surface
• Glass very rapid cooling

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Erupting volcano forming extrusive igneous rock
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Coarse grained igneous rock
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Fined grained igneous rock
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Mixed texture - porphyritic
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Komatiite
Composition/texture ( other properties)
relations of igneous rocks (this is the simple
version)
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• Why the different compositions?
• Studied by geologist Norman Bowen in early to mid
  1900s
• Melted rock samples, let them cool over time,
  quenched samples to see what minerals had formed
• Started with basaltic rock
• Found a sequence of mineral crystallization
• Certain minerals crystallized at higher
  temperatures, others at lower temperatures
• Some minerals seemed to react with early-formed
  minerals as the temperature lowered
• Evolved to Bowens Reaction Series

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• Distribution of the major igneous rocks
• Igneous rocks comprise more than 90 of the
  crust, by volume
• Continental crust is dominated by granodiorite
  (similar to granite, but with more dark minerals)
• Oceanic crust is dominated by basalt (think
  Hawaiian Islands)

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• Characteristics of volcanic activity
• Type of volcanic activity depends on the lavas
  viscosity (how thick) and its water content
• Viscosity also related to the silica (Si O)
  content temperature
• runny, low-viscosity lavas (low silica, high
  temperature) give gentle eruptions (Hawaii)
• Thick, high-viscosity (high silica, high water
  content, lower temperature) lavas give violent
  explosive eruptions (Mt. St. Helens)

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A gentle eruption
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A violent eruption (Mt. St. Helens)
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Sedimentary rocks

• Provide more info on Earths past than igneous or
  metamorphic rocks
• Form by compressing and/or cementing loose
  sediments
• This process is called lithification (making
  rock)
• Common cements
• Calcium carbonate (calcite)
• Silica (quartz)
• Iron oxides (hematite)

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• 3 most abundant sed. rocks (there are others)
• Sandstones (sand-sized grains that are
  compressed/cemented together)
• Shales (similar, but much smaller grain sizes,
  also often contain clay minerals)
• Carbonate rocks (limestones, mainly)
• grain sizeswell get there, hang on

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• Origin of sediments (the grains)
• From weathered, disintegrated, decomposed older
  rocks (of any type)
• In the beginning, they were igneous (most likely)
• Next, the evolution of an igneous rock of
  granite/granodiorite composition (a
  continental-type rock)

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These react with water dissolved carbon dioxide
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• Classifying sedimentary rocks
• Again, texture composition
• Texture the size, shape, arrangement of
  grains
• Composition the minerals
• Two big groups
• Clastic - made up of clasts (grains, broken
  fragments of minerals, rocks, fossils)
• Chemical/biochemical carbonates evaporites
• And a minor one organic
• coals

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• Clastics
• Derived from weathering of pre-existing rocks
  particles transported to a depositional basin
• Composed of clasts (the bigger pieces), matrix
  (finer-grained sediment surrounding the clasts),
  and cement
• The texture is related to the grain sizes, which
  have specific meanings geologically

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• Four grain sizes
• Gravel grains are larger than 2 mm
• Rounded clasts conglomerate
• Angular clasts breccia
• Sand grains are 1/16 to 2 mm
• Sandstone various types, more on them later
• Silt grains 1/256 to 1/16 mm
• Siltstone gritty (tooth test)
• Clay less than 1/256 mm
• Shale or claystone
• mud technically, a mixture of silt clay

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• Chemical/biochemical rocks
• Carbonates both chemical biochemical
  processes
• Limestones
• Contain calcite, aragonite, dolomite
• Many names some important ones
• Micrite very fine-grained basically, carbonate
  mud
• Oolitic limestone made of sand-sized, rounded
  grains
• Coquina a fossil hash made of pieces of shells
• Chalk made of microscopic shell pieces of
  planktonic organisms
• Evaporites
• Halite (rock salt)
• Gypsum
• Travertine (calcium carbonate)
• Technically a carbonate, but deposited in caves
  around hot springs from water solutions

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• Siliceous rocks (generally, biochemical)
• Dominated by silica
• Silica-secreting organisms such as diatoms,
  radiolarians, some sponges
• Chert, from reactions of silica in solution on
  limestones (e.g., by leaching from volcanic ash)
• Diatomite
• looks like chalk, but very low density
• Diatomaceous earth pool filters
• Can buy it powdered very fine powder, but feels
  very gritty
• Chert
• a massive, hard, microcrystalline form of quartz

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• Organic sedimentary rocks (coals)
• Formed from organic matter (mainly plant matter)
• Rocks CAN be organic in origin
• Four types based on depth of burial
  (temperature pressure)
• Peat brownish plant fragments resembling peat
  moss
• Lignite crumbly black
• Bituminous dull to shiny black burns sooty
  layers may be visible
• Anthracite extremely shiny and black low
  density no soot when burns
• Often considered a metamorphic rock due to higher
  temperatures pressures it has undergone
  surrounding rocks are still sedimentary

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Metamorphic rocks

• changed form texture and mineralogy of other
  rocks changed
• Caused by
• High temperatures (but not to the point of
  melting)
• High pressures
• Chemical reactions with solutions hot gasses
• Here, new elements may be added to the mix, often
  not

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• Two major types of metamorphism
• Contact metamorphism
• Rock altered by heat from adjacent lava or magma
• Regional metamorphism
• Rock altered over a large area by heat pressure
• Deep burial or tectonic pressure
• Creates the majority of metamorphic rocks

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• Metamorphic index minerals
• Minerals formed under specific temperature
  pressure conditions
• Allows us to decipher the growth history of old
  mountain regions
• grade the relative combination of temperature
  pressure

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• Textures of metamorphic rocks
• Foliated
• Parallel alignment of mineral grains
• Nonfoliated
• Granular equidimensional grains, no preferred
  orientation

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• Parent rock
• The rock type from which a given metamorphic rock
  was formed

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• Foliated rocks
• Slate
• very fine grained, parent rock shale
• Phyllite
• Slighter coarser grained often wrinkled surface
• Ultimate parent rock shale
• Schist
• Platy or needle-like minerals visible to unaided
  eye
• Named according to dominant mineral (i.e. mica
  schist)
• Ultimate parent rock shale, sometimes
  fine-grained volcanic rocks
• Gneiss
• Coarse grained, minerals segregated into bands
• Parent rocks often high-silica rocks or dirty
  sandstones

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• Nonfoliated rocks
• Marble
• Metamorphosed limestone
• Quartzite
• Metamorphosed quartz sandstone
• Greenstone
• A fine-grained, dark green rock
• Formed by low-grade metamorphism of basalt
• Hornfels
• Hard, fine-grained rock often composed of mica
  garnet
• Usually formed from contact metamorphism of shale
  or other fine-grained rocks

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• What metamorphic rocks indicate about Earths
  history
• Presence of these rocks indicate periods of past
  deformation, uplift, erosion
• Minerals textures indicate direction of
  compressional forces, amount of pressure, how
  much heat involved, type of pre-existing rock

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• Metamorphic rock may also indicate conditions
  prior to metamorphism
• Marble
• had to have limestone, thus conditions similar to
  those in some present seas
• Quartzite
• Some have relict structures, similar to those
  seen in sandstones (more on such structures
  later)
• Indicate depositional processes similar to those
  today occurred in the past