Download Minerals, Igneous and Metamorphic Rocks

Minerals
• A mineral is a naturally occurring, inorganic, usually non‐biologic, crystalline solid, which is physically and chemically distinctive.
• Form in the geosphere (most minerals), hydrosphere (e.g., halite, gypsum), biosphere (e.g., calcite, aragonite), and even the atmosphere (e.g., water ice, as snow)
• Consistent and recognizable physical and chemical properties
Composition of Earth’s Crust
• Common elements
– Nearly 98% of the atoms in Earth’s crust are
represented by the 8 most common elements
• O, Si, Al, Fe, Ca, Na, K, Mg
• Common mineral types
– Most minerals are silicates (contain Si and O
bonded together)
• Minerals have crystalline structures
– Regular 3-D arrangement of atoms
Silicate Structures
•
The Silicon-Oxygen tetrahedron
– Strongly bonded silicate ion
– Basic structure for silicate minerals
•
Sharing of O atoms in tetrahedra
– The more shared O atoms per
tetrahedron, the more complex the
silicate structure
• Isolated tetrahedra (none shared)
• Chain silicates (2 shared)
• Double-chain silicates (alternating
2 and 3 shared)
• Sheet silicates (3 shared)
• Framework silicates (4 shared)
Garnet
Augite (inosilicate)
Tremolite (amphibole)
Biotite (mica)
Quartz
Feldsapr (albite)
Non-silicate Minerals
• Carbonates
– Contain CO3 in their structures (e.g., calcite - CaCO3)
• Sulfates
– Contain SO4 in their structures (e.g., gypsum - CaSO4. 2H2O)
• Sulfides
– Contain S (but no O) in their structures (e.g., pyrite - FeS2)
• Oxides
– Contain O, but not bonded to Si, C or S (e.g., hematite - Fe2O3)
• Native elements
– Composed entirely of one element (e.g., diamond - C; gold - Au)
Minerals
• A mineral must meet the following criteria:
– Crystalline solid
• Atoms are arranged in a consistent and orderly geometric pattern
– Forms through natural geological processes
– Has a specific chemical composition
• May include some internal compositional variation,
such as the solid solution of Ca and Na in plagioclase)
• Rock-forming minerals
– Although over 4000 minerals have been identified, only a few hundred are
common enough to be generally important to geology (rock-forming minerals)
– Over 90% of Earth’s crust is composed of minerals from only 5 groups (feldspars,
pyroxenes, amphiboles, micas, quartz)
Minerals
• Ore minerals
– Minerals of commercial value
– Most are non-silicates (primary source of metals)
• Examples: magnetite and hematite (iron), chalcopyrite (copper),
galena (lead), sphalerite (zinc)
– Must be able to be extracted profitably to be considered current resources
• Gemstones
– Prized for their beauty
and (often) hardness
– May be commercially useful
• Diamond, corundum, garnet, and
quartz are used as abrasives
Mineral Properties
• Physical and chemical properties of minerals are closely
linked to their atomic structures and compositions
•
Color
– Visible hue of a mineral
•
Streak
– Color left behind when mineral
is scraped on unglazed porcelain
•
Luster
– Manner in which light reflects
off surface of a mineral
•
Hardness
– Scratch-resistance
•
Crystal form
– External geometric form
Mineral Properties
•
Cleavage
– Breakage along flat planes
•
Fracture
– Irregular breakage
•
Specific gravity
– Density relative to that of water
•
Magnetism
– Attracted to magnet
•
Chemical reaction
– Calcite fizzes in dilute HCl
Crystal Habit
“appearance in hand specimens”
Massive, Granular, Compact
find grained
Lamellar, Micaceous, Bladed
layered
Fibrous, Acicular, Radiating
needlelike
Dendritic
branching
Banded, Concentric, Geodes
bands
Botryoidal, Globular, Stalactitic
orbs etc.
Oölitic, Pisolitic
pea like
Now to Rocks.
What is the
difference between a
rock and a mineral?
The Rock Cycle
• A rock is a naturally formed,
consolidated material usually
composed of grains of one or
more minerals
• The rock cycle shows how one
type of rocky material gets
transformed into another
– Representation of how rocks are
formed, broken down, and processed
in response to changing conditions
– Processes may involve interactions
of geosphere with hydrosphere,
atmosphere and/or biosphere
– Arrows indicate possible process
paths within the cycle
Imagine the first rock and the cycles that it has been through.
The Rock Cycle and Plate Tectonics
• Magma is created by melting of rock above a subduction zone
• Less dense magma rises and cools to form igneous rock
• Igneous rock exposed at surface gets weathered into sediment
Convergent plate boundary
• Sediments transported to low areas, buried and hardened into sedimentary rock
• Sedimentary rock heated and squeezed at depth to form metamorphic rock
• Metamorphic rock may heat up and melt at depth to form magma
Igneous Rocks
• Magma is molten rock
• Igneous rocks form when magma
cools and solidifies
– Intrusive igneous rocks form when
magma solidifies underground
Granite
• Granite is a common example
– Extrusive igneous rocks form when
magma solidifies at the Earth’s
surface (lava)
• Basalt is a common example
Basalt
Igneous Rock Textures
• Texture refers to the size, shape and
arrangement of grains or other constituents
within a rock
• Texture of igneous rocks is primarily
controlled by cooling rate
• Extrusive igneous rocks cool quickly at or
near Earth’s surface and are typically finegrained (most crystals <1 mm)
• Intrusive igneous rocks cool slowly deep
beneath Earth’s surface and are typically
coarse-grained (most crystals >1 mm)
Fine-grained igneous rock
Coarse-grained igneous roc
The basics of igneous rock textures
Fast cooling
small xtals
Slow cooling
large xtals
It is a function of viscosity of the melt, which is controlled by composition and temperature.
Coarse grained igneous rock
Fine grained igneous rock
Special Igneous Textures
• A pegmatite is an extremely coarse-grained
igneous rock (most crystals >5 cm) formed
when magma cools very slowly at depth
• A glassy texture contains no crystals at all,
and is formed by extremely rapid cooling
• A porphyritic texture includes two distinct
crystal sizes, with the larger having formed
first during slow cooling underground and
the small forming during more rapid cooling
at the Earth’s surface
Pegmatitic igneous rock
Porphyritic igneous rock
Pegmatite:
Very coarse grained igneous rock
Porphyritic igneous rock:
Big xtals in a fine grain matrix
Igneous Rock Identification
•
Igneous rock names are based on texture (grain size) and
mineralogic composition
Textural classification
•
–
–
•
Plutonic rocks (gabbro-diorite-granite) are coarse-grained and cooled
slowly at depth
Volcanic rocks (basalt-andesite-rhyolite) are typically fine-grained and
cooled rapidly at the Earth’s surface
Compositional classification
–
–
–
Mafic rocks (gabbro-basalt) contain abundant dark-colored
ferromagnesian minerals
Intermediate rocks (diorite-andesite) contain roughly equal amounts of
dark- and light-colored minerals
Felsic rocks (granite-rhyolite) contain abundant light-colored minerals
Igneous Rock Identification
•
Igneous rock names are based on texture (grain size) and
mineralogic composition
Devil’s Tower; a volcanic neck, a feeder pipe
Sill; parallels layers in the country rock
Dike; cuts across layers in the country rock
Half Dome; part of the Sierra Nevada batholith
Bowen’s Reaction Series
Six common Igneous Rocks
1000 C
Solidifying Temperature
500 C
Increasing Grain Size
Silica
Content
Minerals
Basalt
low
pyroxene,
olivine, feldspar,
& amphibole
Gabbro
Andesite
intermediate
feldspar,
amphibole,
pyroxene, biotite
mica
Diorite
Rhyolite
high
feldspar, quartz,
muscovite mica,
& amphibole
Granite
Present (in order of
abundance)
Plutonic
Rocks
Lighter Color
Volcanic
Rocks
Lessons from Bowen’s Reaction Series
•
•
•
•
•
Large variety of igneous rocks is produced by large
variety of magma compositions
Mafic magmas will crystallize into basalt or gabbro if
early-formed minerals are not removed from the magma
Intermediate magmas will similarly crystallize into
diorite or andesite if minerals are not removed
Separation of early-formed ferromagnesian minerals
from a magma body increases the silica content of the
remaining magma
Minerals melt in the reverse order of that in which they
crystallize from a magma >>>> partial melting!!!
Magma Evolution
•
•
•
•
A change in the composition of a magma
body is known as magma evolution
Magma evolution can occur by
differentiation, partial melting,
assimilation, or magma mixing
Differentiation involves the changing of
magma composition by the removal of
denser early-formed ferromagnesian
minerals by crystal settling
Partial melting produces magmas less
mafic than their source rocks, because
lower melting point minerals are more
felsic in composition
Differentiation
Magma chamber fills
Early formed mafic minerals crystallize and settle (or are otherwise separated from the residual melt)
The remaining melt is enriched in silica
Magma Evolution I
•
Assimilation occurs
when a hot magma
melts and
incorporates more
felsic surrounding
country rock
Xenolith
Insert new
Fig. 3.22 here
Magma Evolution II
•
Magma mixing
involves the
mixing of more
and less mafic
magmas to produce
one of intermediate
composition
Metamorphism
• The transformation of rock by
temperature and pressure
• Metamorphic rocks are produced by
transformation of:
• Igneous, sedimentary and igneous rxs
Thanks to CU Boulder Geology Dept for use of some of these slides
Metamorphism
• Metamorphism progresses from low to
high grades
• Rocks remain solid during metamorphism
What causes metamorphism?
• Heat
• Most important agent
• Heat drives recrystallization - creates new,
stable minerals
• Pressure (stress)
• Increases with depth
• Pressure can be applied equally in all
directions or differentially
Main factor affecting metamorphism
• Parent rock
• Metamorphic rocks typically have the same
chemical composition as the rock they were
formed from
• Different minerals, but made of the same
stuff.
• Exception: gases (carbon dioxide, CO2) and
water (H2O) may be released
Progressive metamorphism of a shale
Shale
Progressive metamorphism of a shale
Slate
Progressive metamorphism of a shale
Phyllite
Progressive metamorphism of a shale
Schist
Progressive metamorphism of a shale
Gneiss
Metamorphism
• Three types of metamorphic settings:
• Contact metamorphism – from a rise in
temperature within host rock
• Hydrothermal metamorphism – chemical
alterations from hot, ion-rich water
• Regional metamorphism -- Occurs in the
cores of mountain belts and makes great
volumes of metamorphic rock
Common metamorphic rocks
• Nonfoliated rocks
• Quartzite
– Formed from a parent rock of quartz-rich
sandstone
– Quartz grains are fused together
– Forms in intermediate T, P conditions
Sample of
quartzite
Thin section
of quartzite
Common metamorphic rocks
• Nonfoliated rocks
• Marble
– Coarse, crystalline
– Parent rock usually limestone
– Composed of calcite crystals
– Fabric can be random or oriented
Marble (Random fabric = annealing; nonfoliated)
Change in metamorphic grade with depth
Common metamorphic rocks
• Foliated rocks
• Slate
– Very fine-grained
– Excellent rock cleavage
– Made by low-grade metamorphism of shale
Example of slate
Slate roof
Common metamorphic rocks
• Foliated rocks
• Phyllite
– Grade of metamorphism between slate and schist
– Made of small platy minerals
– Glossy sheen with rock cleavage
– Composed mainly of muscovite and/or chlorite
Phyllite (left) and Slate (right) lack visible mineral grains
Common metamorphic rocks
• Foliated rocks
• Schist
– Medium- to coarse-grained
– Comprised of platy minerals (micas)
– The term schist describes the texture
– To indicate composition, mineral names are
used (such as mica schist)
Mica Schist - note well developed foliation
A mica garnet schist
Common metamorphic rocks
• Foliated rocks
• Gneiss
– Medium- to coarse-grained
– Banded appearance
– High-grade metamorphism
– Composed of light-colored feldspar layers with
bands of dark mafic minerals
Gneiss displays bands of light and dark minerals
What are metamorphic textures?
• Texture refers to the size, shape, and
arrangement of mineral grains within a
rock
• Foliation – planar arrangement of
mineral grains within a rock
Outcrop of foliated gneiss
Metamorphic textures
• Foliation
• Foliation can form in various ways:
– Rotation of platy or elongated minerals
– Recrystallization of minerals in a preferred
orientation
– Changing the shape of equidimensional
grains into elongated and aligned shapes
Flattened Pebble Conglomerate = flattening
Development of foliation due to directed pressure