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Rocks and Minerals
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What is a Mineral?
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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
Minerals
Classified by:
• Chemical composition- what they are
made of
• Crystal structure- how atoms are arranged
Mineral Properties
• Physical properties are used to identify
minerals
—Hardness
—Cleavage and fracture
—Color
—Luster
—Density
—Crystal form
Hardness
Resistance 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.
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.
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)
Density
Ratio of a mineral’s mass to its volume.
In simple terms, it is how heavy a mineral
feels for its size (volume).
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.
Classification of Rock-Forming
Minerals
Two:
—Silicate minerals
—Nonsilicate minerals
Silicate minerals make up more than 90%
of the Earth’s crust.
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,
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)
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
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.
Mineral Formation by Water
• Water solutions can form chemical
sediments such as carbonates and
evaporites.
• This process is called precipitation
(forming a solid within a liquid)
Rock Types
The 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
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.
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.
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
Classification of Igneous Rocks
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Composition
– Percentage of mineral grains present
– Silica content of mineral grains
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Texture
– Size of mineral grains
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Small = fine grained (extrusive)
Large = coarse grained (intrusive)
Composition
• Based on silica content (Silicon and Oxygen)
– 45-52% - basaltic
– 52-63% - andesitic
– >63% - rhyolitic/granitic
Cooling Rate
Depends 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 bubbles
pumice or scoria- many bubbles (vesicular)
Volcanoes
There are three types of volcanoes:
• Shield- low, gentle gradient sides
• Cinder cone- small size, steep sides
• Composite/stratacone- large, steep sides
Each erupts differently.
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
Sedimentary Rocks
Sedimentary rocks are products of 4
processes:
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Weathering
Erosion
Deposition
Sedimentation
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.
Erosion
• Physical removal of material
• Agents-- water, wind, ice, or gravity
• Does not occur in place - involves
movement.
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.
Sedimentation
• Deposited horizontally layer by layer.
• Changes into sedimentary rock by
lithification
2 steps:
—Compaction
—Cementation
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.
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)
Clastic Sedimentary Rocks
classified 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
Metamorphic Rocks
• Metamorphic rocks are produced from:
—Igneous rocks
—Sedimentary rocks
—Other metamorphic rocks
• Metamorphism occurs via recrystallization
and mechanical deformation.
Metamorphic Rocks
Contact metamorphism:
• Intruded by magma
• high temperatures
• high water content
– lots of chemical activity,
little mechanical
deformation.
Metamorphic Rocks
Regional metamorphism:
• the alteration of rock by both heat and
pressure over an entire region
• Subduction zone- 2 plates coming
together.
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
Shortcuts
Doesn'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
The Rock Cycle
Molten rock rises from the depths of
Earth, cools, solidifies, and eventually
returns to become magma again.