Rocks and Mineralsposted version

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

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

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.

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.

• 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

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

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

• Composition– Percentage of mineral grains present– Silica content of mineral grains

• Texture– Size of mineral grains

• 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 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)

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.

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:

• 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 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

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

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

The Rock Cycle

Molten rock rises from the depths of Earth, cools, solidifies, and eventually returns to become magma again.