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Themes for Today
• A huge variety of minerals, but relatively few
that make up most of the crust
• Rocks are composed of minerals
• The rock cycle describes the origin,
transformation, and erosion of the three types
of rocks:
–igneous
–sedimentary
–metamorphic
Earth Materials – Minerals and
Rocks
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Earth Materials – Minerals
• Minerals have many essential uses
– common minerals like quartz: glass, sandpaper,
optics
– not-so-common like corundum (ruby): nail files,
gemstones
– everything in between
• Basic units that make up most of Earth’s
materials
Earth Materials – Rocks
• Rocks are solid aggregates of minerals
– (few exceptions)
• Rocks also have many uses
– crushed for aggregate in cement and for
roadbeds
– sawed and polished rocks for buildings,
mantle pieces, and counter tops
– soils are formed by alteration of rocks
Building a rock...
Matter and Its Composition
• Matter
– is anything that has mass and occupies
space
– exists as solids, liquids, and gases
– consist of elements and atoms
• Element
– is a chemical substance that cannot be
chemically decomposed into simpler
substances and is composed of tiny
particles called atoms
Atoms
• Atoms are the smallest units of matter
that retain the characteristics of the
element
• Atoms have
– a compact nucleus containing
• protons – particles with a positive electrical
charge
• neutrons – electrically neutral particles
– particles orbiting the nucleus
• electrons – negatively charged particles
Structure of an Atom
• Dense
nucleus of an
atom
consisting of
protons and
neutrons
• Surrounded by
a cloud of
orbiting
electrons
Atoms
• Atomic number
= the number of protons
– is constant
• Atomic mass number
= number of protons + number of neutrons
– may vary due to varying numbers of neutrons
Isotopes
• An element that has varying numbers of
neutrons (and  different atomic
weights) has multiple isotopes
• Different isotopes of the same element
– have different atomic mass numbers
– behave the same chemically
• Isotopes are important in radiometric
dating
Carbon Isotopes
• Carbon atoms (with 6 protons)
– have 6 neutrons = Carbon 12 (12C)
– have 7 neutrons = Carbon 13 (13C)
– have 8 neutrons = Carbon 14 (14C)
– thereby making up three isotopes of
carbon.
Minerals—The Building
Blocks of Rocks
• A mineral’s composition is shown by a
chemical formula
– a shorthand way of indicating how many atoms
of different kinds it contains
– Quartz consists of
Quartz: SiO2
1 silicon atom for every
Ratio: 1: 2
2 oxygen atoms
– Orthoclase consists of
KAlSi3O8
1 potassium, 1 aluminum,
and 3 silicon for every
1: 1: 3: 8
8 oxygen atoms
Native Elements
• A few minerals
consist of only one
element
• They are not
compounds
• They are known as
native elements
• Examples:
– gold – formula: Au
– diamond – formula: C
Minerals
California Academy of Sciences in San Francisco
Minerals
• Geological definition of a mineral:
– naturally occurring
– crystalline solid
• crystalline means that minerals have an
ordered internal arrangement of their atoms
– a narrowly defined
chemical composition
– characteristic physical
properties such as
• density
• hardness
• color...
Crystalline Solids
• By definition, minerals are crystalline
solids
– with atoms arranged in a specific 3D
framework
• If given enough room to grow freely,
– minerals form perfect crystals with
– planar surfaces, called crystal faces
– sharp corners
– straight edges
Narrowly Defined
Chemical Composition
• Some minerals have very specific
compositions
– examples are halite (NaCl) or quartz (SiO2)
• Others have a range of compositions
– because one element can substitute for
another if the atoms of the two elements
have
• the same electrical charge
• and are about the same size
– example: olivine
• (Mg,Fe)2SiO4
• iron and magnesium substitution in any
proportion
Mineral Properties
• Mineral properties are controlled by
– Chemical composition
– Crystalline structure
• Mineral properties are used for mineral
identification and include:
•
•
•
•
color
streak
luster
crystal form
•
•
•
•
cleavage
fracture
hardness
specific gravity
How Many Minerals
Are There?
• More than 3500 minerals are known
• Only about 2 dozen are particularly
common
• Many others are important resources
• Mineral groups:
– minerals with the same negatively charged
ion or ion group belong to the same mineral
group
• Most minerals in the crust
– belong to the group called silicates
Silicates
• Silicates are minerals containing silica
– Si and O
• They make up perhaps 95% of Earth’s
crust
– and account for about 1/3 of all known
minerals
Types of Silicates
• Ferromagnesian silicates
– contain iron (Fe), magnesium (Mg), or both
• Nonferromagnesian silicates
– do not contain iron or magnesium
Ferromagnesian Silicates
• Common ferromagnesian silicates include:
– olivine
– hornblende,
an
amphibolegroup
mineral
– augite, a pyroxenegroup mineral
– biotite
mica
Nonferromagnesian Silicates
Quartz
Potassium
feldspar
Plagioclase feldspar
Muscovite
Other Mineral Groups
• Carbonates
– minerals with carbonate ion (CO3)-2
– as in calcite (CaCO3),
• found in limestone
– and dolomite [CaMg(CO3)2],
• found in dolostone
• Other mineral groups are important, but
more as resources than as constituents
of rocks
Rock-Forming Minerals
• Most rocks are solid aggregates of one or
more minerals
• Thousands of minerals occur in rocks,
– but only a few are common: rock-forming
minerals
• Most rock-forming minerals are silicates,
– carbonates are also important
• Accessory minerals are present in small
amounts
– largely ignored in classifying rocks
Rock Cycle
• The rock cycle represents events
leading to
– the origin, destruction, change, and
reformation of rocks
• Rocks belong to 3 major families
– igneous
– sedimentary
– metamorphic
• The rock cycle shows
– how these rock families are interrelated
and can be derived from one another
Igneous Rocks
• All igneous rocks
– cool and crystallize from magma,
– solidify from lava,
– or consolidate from pyroclastic materials
• Magma is molten material below the
surface
• Lava is molten material on the surface
• Pyroclastic materials are particles such
as volcanic ash
Igneous Part of the Rock
Cycle
Pyroclastic
material
Lava
Categories of Igneous Rocks
• Extrusive or volcanic rocks
– formed at the surface
– from lava or pyroclastic materials
• Intrusive or plutonic rocks
– formed from magma injected into the crust
– or formed in place in the crust
Igneous Rocks
Igneous Rock Textures
• Texture
– is the size, shape and arrangement
– of crystals, grains and other constituents of
a rock
• Igneous rocks have 4 textures
– that relate to cooling rate of magma or lava
4 Cooling-Rate Textures
• Phaneritic,
– with visible grains
• cooled slowly
• Aphanitic,
– with grains too small to see without
magnification
• cooled quickly
• Porphyritic,
– with larger grains surrounded by a finergrained groundmass
• cooled slowly first, then more quickly
• Glassy,
– with no grains
• cooled too quickly for minerals to grow
Igneous Rock Textures
• Other textures reveal further details of
the formation of the rock
• Vesicular texture, with holes (vesicles)
– indicates the rock formed as water vapor
and other gases became trapped during
cooling of lava
• Pyroclastic or fragmental texture
– containing fragments,
– formed by consolidation of volcanic ash or
other pyroclastic material
Igneous Rock Textures
Rapid cooling
Slow cooling
2-stage cooling
Aphanitic
texture
Phaneritic
texture
Porphyritic
texture
Igneous Rock Textures
Glassy texture
cooling was too
rapid for mineral
growth
Vesicular texture Pyroclastic texture
gasses trapped in
cooling lava
particles fragmented
during eruption
Classifying Igneous Rocks
• Texture and composition are the criteria
– used to classify most igneous rocks
• Composition categories are based on silica
content
– felsic (>65% silica)
– intermediate (53-65% silica)
– mafic (45-52% silica)
• More felsic magmas have higher Na, K, Al
• More mafic magmas have higher Ca, Fe,
Mg
Classifying Igneous Rocks
Common Igneous Rocks
Basalt
Gabbro
Andesite
Diorite
Common Igneous Rocks
Rhyolite
Granite
Classifying Igneous Rocks
with Special Textures
Composition
Texture
Vesicular
Glassy
Pyroclastic or
Fragmental
Felsic
Mafic
Pumice
Scoria
Obsidian
Volcanic breccia
Tuff/welded tuff
Igneous Rocks with
Special Textures
Tuff has
pyroclastic
texture.
Pumice is glassy
and extremely
vesicular.
Sedimentary Rocks
• Sedimentary rocks form
– by the lithification of sediment
• In the rock cycle, sediment originates
when:
– mechanical and chemical weathering
• breaks rocks down into smaller particles and
into solution
– transport removes sediment from its source
area and carries it elsewhere
• running water, glaciers, wind, and waves
– deposition:
• settling of particles,
• chemical and biological extraction of minerals
from solution
Sedimentary Part of the
Rock Cycle
Lithification
• Lithification means
– turning loose sediment into rock
• Lithification occurs by
– burial
• when additional sediment accumulates on top
– compaction
• reduction of the amount of pore space between
particles
• because of the weight of overlying sediment
– cementation
• precipitation of minerals within pores
• that effectively binds sediment together
– calcium carbonate (CaCO3) cement is common
– silica (SiO2) cement is common
– iron oxide (Fe2O3) cement is less common
Categories of Sedimentary Rocks
• Detrital sedimentary rocks
– consist of solid particles derived from
preexisting rocks (detritus)
• Chemical sedimentary rocks
– consist of minerals derived from materials
in solution and extracted by either
• inorganic chemical processes
• or by the activities of organisms
– biochemical sedimentary rocks
Detrital Sedimentary Rocks
• are composed of fragments or particles
known as clasts = clastic texture
• These rocks are defined primarily by size
of clasts
• conglomerate
– composed of gravel (>2mm)
– with rounded clasts
• sedimentary breccia
– also composed of gravel (>2mm)
– but clasts are angular
• sandstone
– composed of sand
Detrital Sedimentary Rocks
• Mudrocks consist of particles < 1/16 mm
– siltstone
• composed of silt-sized particles - 1/16-1/256 mm,
• feel slightly gritty,
• but not visible without magnification
– mudstone
• composed of a mixture of silt- and clay-sized particles
– claystone
• composed of clay-sized particles
– <1/256 mm, feel smooth even to the teeth
– shale
• mudstone or claystone that
• breaks along closely spaced parallel planes (fissile)
Chemical Sedimentary Rocks
• Inorganic chemical processes or
organisms extract minerals from
solution
• This can result in different textures
– crystalline texture
• has an interlocking mosaic of mineral crystals
• results from chemical precipitation
– clastic texture
• has an accumulation of broken pieces of shells
Chemical Sedimentary Rocks
• Limestone – carbonate rock made of calcite
precipitated chemically or by organisms
• Evaporites formed by inorganic chemical
precipitation during evaporation
– rock salt – evaporite made of halite
– rock gypsum – evaporite made of gypsum
• Chert – compact, hard, fine grained silica, formed
by chemical or biological precipitation (some
consisting of microscopic shells of silica-secreting
organisms)
• Coal – made of partially altered, compressed
remains of land plants accumulated in swamps
Common Sedimentary Rocks
Conglomerate
Quartz sandstone
Sedimentary breccia
Shale
Common Sedimentary Rocks
Fossiliferous limestone
Rock gypsum
Rock
salt
Chert
Coal
Metamorphic Rocks
• Metamorphic rocks
– result from transformation of other rocks in
the solid state, without melting
• Changes resulting from metamorphism
– compositional
• new minerals form
– textural
• minerals become reoriented
• minerals recrystallize
– or both
Metamorphic Part of the
Rock Cycle
Agents of Metamorphism
• Heat provides new conditions
– where different minerals may be stable
– and increases the rate of chemical reactions
• Pressure
– Lithostatic pressure provides new conditions
• where different minerals may be stable
• and forms smaller denser minerals
– Differential pressure
• exerts force more intensely from one direction
• causing deformation
• and development of foliation.
• Fluid activity enhances metamorphism
– by increasing the rate of chemical reactions
– by transporting ions in solution
Types of Metamorphism
• Contact metamorphism
– heat and chemical fluids from an igneous
body
• alter rocks adjacent to the magma
• Regional metamorphism
– large, elongated area
– tremendous pressure
– elevated temperatures
– fluid activity
Metamorphic Textures
• Foliated texture
– platy and elongate minerals aligned
parallel to one another
– caused by differential pressure
• Nonfoliated texture
– mosaic of roughly equidimensional
minerals or platy and elongate minerals
– with random orientations
Formation of Foliation
• When rocks are subjected to differential
pressure
– the minerals typically rearrange or grow
parallel to each other
Formation of Foliation
• Foliated
metamorphic
rock showing
mineral
alignment
visible under
microscope
Foliated Metamorphic Rocks
• Slate
– very fine-grained, breaks in flat pieces
• Phyllite
– fine-grained (coarser than slate but grains are
still too small to see without magnification)
– breaks in flat pieces
• Schist
– clearly visible platy and/or elongate minerals
• Gneiss
– alternating dark and light bands of minerals
Nonfoliated Metamorphic Rocks
• Marble
– made of calcite or dolomite from limestone
or dolostone
• Quartzite
– made of quartz from quartz sandstone
• Hornfels
– results from contact metamorphism
• Anthracite
– made of black lustrous carbon from coal
Common Metamorphic Rocks
Slate
Gneiss
Schist
Marble
Quartzite
Earth Materials and
Historical Geology
• Our record of Earth’s history is preserved in
rocks
– sedimentary rocks are especially useful in
preserving a historical record
• Igneous and metamorphic rocks provide
information
– about processes deep in the crust
– history of plate activity
Themes for Today
• A huge variety of minerals, but relatively few
that make up most of the crust
• Rocks are composed of minerals
• The rock cycle describes the origin,
transformation, and erosion of the three types
of rocks:
–igneous
–sedimentary
–metamorphic