Chapter 2
Earth Materials—
Minerals and Rocks
Earth Materials – Minerals
• Gemstones and other minerals,
– such as gold,
– have fascinated people for
thousands of years
– and have been supposed
– to have mystical or curative
powers
• Minerals have many essential
uses
– in industrial societies
• Minerals are the basic units
– that make up most of Earth’s
materials
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
Minerals
• Geological definition of a mineral:
– naturally occurring
– crystalline solid
• crystalline means that minerals
• have an ordered internal arrangement of their
atoms
– minerals have a narrowly
defined chemical composition
– and characteristic physical
properties such as
• density
• hardness
• color...
Minerals
Minerals on
display
– at the
California
Academy of
Sciences in
San Francisco
Earth Materials – Rocks
• With only a few exceptions rocks
– are solid aggregates of minerals
• Rocks too find many uses
– rocks crushed for aggregate in cement and for
roadbeds
– sawed and polished rocks for tombstones,
monuments, mantle pieces and counter tops
– Even the soils we depend on
• for most of our food
• formed by alteration of rocks
Earth Materials
• Some materials
formed by the Earth
– are interesting and
attractive
– such as this
metamorphic rock
• from the shoreline of
Lake Superior at
Marquette, Michigan
Rock Cycle
• The rock cycle is a pictorial representation
– of 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
Rock
Cycle
Pyroclastic
material
Lava
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
• Plutons are intrusive bodies
– consisting of plutonic rock
Plutons
Common Igneous Rocks
Basalt
Gabbro
Andesite
Diorite
Common Igneous Rocks
Rhyolite
Granite
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
• transport sediment
– Deposition involves settling of particles,
• and 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
– subcategory biochemical sedimentary rocks, for
which
• the activities of organisms are important
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
Chemical Sedimentary Rocks
• Limestone – carbonate rock made of calcite
precipitated chemically or by organisms
• Dolostone – carbonate rock made of dolomite usually
formed from limestone
• 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
Rock gypsum
Fossiliferous limestone
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
chemical fluids
from an igneous body
alter rocks adjacent to the magma
• Regional metamorphism
–
–
–
–
–
large, elongated area
tremendous pressure
elevated temperatures
fluid activity
occurs at convergent and divergent plate
boundaries
Summary
• Elements consist of atoms,
– which have a nucleus
• of protons and neutrons
– surrounded by orbiting electrons in shells
• The number of protons = the atomic number
• and the number of protons + neutrons = the
atomic mass number
• Bonding of atoms occurs
– by transfer of electrons to make ions
• ionic bonding
– or by sharing electrons
• covalent bonding
Summary
• Most minerals are compounds
– of two or more elements bonded together
• The most common minerals are silicates,
• which contain silicon and oxygen,
– but carbonate minerals,
• with carbon and oxygen
– are abundant in some rocks
• Two broad groups of igneous rocks
– are intrusive (or plutonic)
– and extrusive (or volcanic)
• Igneous rocks are classified primarily
– by composition and texture
Summary
• Sedimentary rocks also have two broad
groups
– detrital,
• which consist of solid particles of preexisting rocks,
– and chemical/biochemical,
• which consist of minerals derived
• by inorganic or organic (involving organisms)
processes
• Compaction and cementation
– transform sediment into sedimentary rock,
– in a process called lithification
Summary
• Metamorphic rocks form
– when composition and/or texture
– of another rock changes
– by heat, pressure and fluid activity
• Metamorphism imparts a foliation
– to many rocks,
• which is a parallel alignment of minerals
• Some metamorphic rocks have a mosaic
– of equidimensional minerals
– and are nonfoliated
• Texture or composition
– are the primary considerations
– in classifying many metamorphic rocks
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
• The dense
nucleus of an
atom
– consisting of
protons and
neutrons
– is surrounded
by a cloud of
orbiting
electrons
Atoms
• Atomic number
= the number of protons
• Atomic mass number
= number of protons + number of neutrons
• The number of neutrons in an atom
– may vary
Isotopes
• The different forms of an element’s atoms
– with varying numbers of neutrons
– are called 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)
or have 8 neutrons = Carbon 14 (14C)
thereby making up three isotopes of carbon.
Electrons and Shells
• Electrons orbit the nucleus in one or more shells
• The outermost shell participates
– in chemical bonding
– and contains up to 8 electrons
• Noble gas configuration of 8 electrons
• or 2 for helium
– completes the outermost shell
• Other atoms attain
– a noble gas configuration
– in the process of bonding
Bonding and Compounds
• Bonding
– the process whereby atoms join to other atoms
• Compound
– a substance resulting from the bonding
– of two or more elements
• Oxygen gas (O2) is and element
• Ice is a compound
– made up of hydrogen and oxygen (H2O)
• Most minerals are compounds
Ionic Bonding
• Ion
– an atom that has gained or lost one or more
electrons
– and thus has a negative or positive charge
• One way for atoms to attain the noble gas
configuration
– is by transferring electrons, producing ions
• Ionic bonding
– attraction between two ions of opposite charge
Covalent Bonding
• Another way for atoms
– to attain the noble gas configuration
– is by sharing electrons
• Covalent bonding
– results from
sharing electrons
shared electrons
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
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)
• but 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 particularly useful
– 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
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
– and called rock-forming minerals
• Most rock-forming minerals are silicates,
– but carbonates are also important
• Accessory minerals are present in small amounts
– and are ignored in classifying rocks
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
• The basic building block of silicates
– is the silica tetrahedron
• which consists of one silicon atom
• surrounded by four oxygen atoms
Types of Silicates
• Silica tetrahedra can be
– isolated units bonded to
other elements
– arranged in chains (single or
double)
– arranged in sheets
– arranged in complex
3D networks
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
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 finer-grained
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
Vesicular texture
Pyroclastic texture
cooling was too
rapid for mineral
growth
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
Classifying Igneous Rocks with
Special Textures
Texture
Composition
Felsic
Vesicular
Pumice
Glassy
Obsidian
Pyroclastic or
Fragmental
Mafic
Volcanic breccia
Tuff/welded tuff
Igneous Rocks with
Special Textures
Tuff has pyroclastic
texture.
Pumice is glassy and
extremely vesicular.
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
• Recall that these rocks result
– when 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
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
arranged in a helter-skelter fashion
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
• Microscopic
view
– of a
metamorphic
rock
– with foliation
– showing the
parallel
arrangement of
minerals
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
• Greenstone
– made of green mafic igneous rock
• Hornfels
– results from contact metamorphism
• Anthracite
– made of black lustrous carbon from coal
Common Metamorphic Rocks
Slate
Gneiss
Schist
Marble
Quartzite
Plate Tectonics
and the Rock Cycle
• The atmosphere, hydrosphere and biosphere
– act on earth materials
– and cause weathering erosion and deposition
• Earth’s internal heat
– aids melting and metamorphism
• Plate tectonics recycles Earth materials
– heat and pressure at convergent plate boundaries
• lead to metamorphism and igneous activity
– resulting deformation makes mountains
• that in turn weather and erode to form sediment
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
– and will be covered in more detail later in the term
• Igneous rocks reveal much
– about the history of plate activity
• Metamorphic rocks provide information
– about processes deep in the crust
Rocks
• Granite cliff of
El Capitan,
Yosemite,
California
– rises 900 m
above the
valley floor
– highest
unbroken cliff
in the world