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Rocks and Mineralsposted version
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What is a Mineral?• Naturally occurring – not man-made• Solid - not liquid or gas• Inorganic- not part of a living thing• Chemical composition-composed of the
same type of atoms • Crystalline structure-orderly and
repeating arrangement of atoms
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Minerals
Classified by:
• Chemical composition- what they are made of
• Crystal structure- how atoms are arranged
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Mineral Properties
• Physical properties are used to identify minerals—Hardness—Cleavage and fracture—Color—Luster—Density—Crystal form
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HardnessResistance of a mineral to scratching • Dependent on the strength of a mineral’s
chemical bonds.• Bond strength is determined by ionic charge,
atom (or ion) size, and packing.• The Mohs Scale compares the hardness of
different minerals.
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Cleavage and Fracture
Cleavage - breaks along planes of weakness.– Determined by crystal structure
and bond strength.
Fracture - bond strength is generally the same in all directions. – Minerals that fracture do not
exhibit cleavage.
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Color• Most obvious feature- but it is not reliable for
mineral identification..—A mineral may occur in many color variations or
be colorless.—Caused by impurities (trace elements)
Luster-How a mineral reflect light (dull, glassy, metallic, etc)
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Density
Ratio of a mineral’s mass to its volume.
In simple terms, it is how heavy a mineral feels for its size (volume).
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Mineral Properties: Crystal Form
Crystal form (shape)- outward expression of internal arrangement of atoms.
Also affected by growth conditions:
—Temperature, pressure, space for growth
Well-formed minerals are rare in nature—most minerals grow in cramped confined spaces.
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Classification of Rock-Forming Minerals
Two:—Silicate minerals—Nonsilicate minerals
Silicate minerals make up more than 90% of the Earth’s crust.
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Nonsilicate Minerals
About 8% of Earth’s crust• Carbonate minerals (contain CO3)
• Calcite, dolomite
• Oxide minerals (metal bonded with 0)• Ore minerals—hematite, magnetite, chromite
• Sulfide minerals (metal bonded with S)• Ore minerals—pyrite, galena
• Sulfate minerals (contain SO4)• Gypsum, anhydrate
• Native elements (only 1 element)• Gold, platinum,
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The Formation of Minerals and Rock
• Minerals form by the process of crystallization.
• Minerals crystallize from:—Magma (molten rock)—Water solutions—Heat and pressure (metamorphism)
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Mineral Formation from Cooling Magma
Minerals crystallize systematically based on their respective melting points—First minerals - lowest amount of silica and
highest melting point—Last minerals - higher amounts of silica and lower
melting point
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Mineral Formation by Water
Hot water solutions from magma account for many important ore deposits
• As water solutions become chemically saturated, minerals form.
• Ore deposits can be deposited into cracks or into the matrix of the rock itself.
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• Water solutions can form chemical sediments such as carbonates and evaporites.
• This process is called precipitation (forming a solid within a liquid)
Mineral Formation by Water
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Rock TypesThe three categories of rock reflect how they
were formed:• Igneous
• Cooling and crystallization of magma or lava
• Sedimentary• Pieces (clasts) of rocks that have been glued
together
• Metamorphic• Preexisting rock transformed by heat, pressure, or
chemical fluids
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Igneous Rocks
• Formed from the cooling and crystallization of magma or lava.—Magma is molten rock that forms inside
Earth.—Lava is molten rock (magma) erupted at
Earth’s surface.
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Generation of Magma
• Heat:—Temperature increases within Earth’s
upper crust—the geothermal gradient—at an average rate of 30°C per kilometer.
—Rocks in the lower crust and upper mantle are near their melting points.
—Additional heat (rising up from the mantle) induces melting.
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Generation of Magma• Fluids
—Water causes rocks to melt at lower temperatures.
• Pressure:—Reduced pressure lowers the melting
temperature of rock.
• Example:—The solid inner core
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Classification of Igneous Rocks
• Composition– Percentage of mineral grains present– Silica content of mineral grains
• Texture– Size of mineral grains
• Small = fine grained (extrusive)• Large = coarse grained (intrusive)
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Composition
• Based on silica content (Silicon and Oxygen)– 45-52% - basaltic– 52-63% - andesitic– >63% - rhyolitic/granitic
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Cooling RateDepends on location
– Underground (plutonic) long time to cool; mineral grains grow large
– Above the ground (volcanic) extruded onto the surface, cooling quickly; mineral grains small
If a lava cools immediately it forms volcanic glass obsidian- no bubblespumice or scoria- many bubbles (vesicular)
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VolcanoesThere are three types of volcanoes:• Shield- low, gentle gradient sides• Cinder cone- small size, steep sides• Composite/stratacone- large, steep sides
Each erupts differently.
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Composition of lava determines how a volcano erupts
– Basaltic (low silica) flows easily & erupts quietly
– Andesitic/rhyolitic (high silica) is viscous and traps gas so it
erupts explosively
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Sedimentary Rocks
Sedimentary rocks are products of 4 processes:
• Weathering• Erosion• Deposition• Sedimentation
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Weathering—breaking down the rock
• Two types:—Mechanical weathering—breaking and
disintegration of rocks into smaller pieces (clasts).
—Chemical weathering— decomposition, dissolving and transformation of rock into different compounds.
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Erosion• Physical removal of material • Agents-- water, wind, ice, or gravity• Does not occur in place - involves
movement.
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Deposition
Deposition — eroded particles come to rest.
Larger particles - first Smaller particles are able
to remain with the flow.Sediments are sorted
according to size as they are deposited.
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Sedimentation
• Deposited horizontally layer by layer.• Changes into sedimentary rock by lithification
2 steps:—Compaction—Cementation
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Lithification
• Compaction—Weight of overlying material presses down upon deeper layers.
• Cementation— “pore water” rich in dissolved minerals acts as a glue to cement sediment particles together.
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Classifying Sedimentary Rocks
• 2 types:—Clastic rocks—transported sediment
particles—bits and pieces of weathered rock (shale, sandstone, conglomerate)
—Chemical rocks—sediments that were once in dissolved in water. (travertine, halite, limestone)
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Clastic Sedimentary Rocksclassified by particle size
Shale• Mud-sized particles, sometimes in thin layers
Sandstone • Composed of sand-sized particles, quartz common
Conglomerates • Composed of pebble sized, rounded gravels
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Metamorphic Rocks• Metamorphic rocks are produced from:
—Igneous rocks—Sedimentary rocks—Other metamorphic rocks
• Metamorphism occurs via recrystallization and mechanical deformation.
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Metamorphic Rocks
Contact metamorphism:• Intruded by magma • high temperatures • high water content
– lots of chemical activity, little mechanical deformation.
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Metamorphic Rocks
Regional metamorphism:• the alteration of rock by both heat and
pressure over an entire region • Subduction zone- 2 plates coming
together.
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The Rock Cycle, explained• A cycle of formation, change, and
destruction • Starts with molten rock (magma), which
cools and forms igneous rocks… uplift… weathering… erosion…deposition…burial and lithification into sedimentary rocks
• Then buried … heat, pressure, and fluids, and become metamorphic rocks
• Eventually, these metamorphic rocks may be heated to the point where they again melt into magma
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ShortcutsDoesn't have to work this way:• Igneous rocks metamorphose without ever
eroding • Sedimentary and metamorphic rocks uplift
and erode, rather than melting• Rocks remain un-eroded and unchanged in
stable regions for long periods of time
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The Rock Cycle
Molten rock rises from the depths of Earth, cools, solidifies, and eventually returns to become magma again.