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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
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
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 on
Minerals display
• at the
California
Academy of
Sciences in
San Francisco
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
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
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
1 silicon atom for every
Quartz: SiO2
2 oxygen atoms
Ratio: 1: 2
– Orthoclase consists of
1 potassium, 1
KAlSi3O8
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
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
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 silicon oxygen 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 (dark)
– contain iron (Fe), magnesium (Mg), or both
• Nonferromagnesian silicates (light)
– do not contain iron or magnesium
Ferromagnesian Silicates
• Common ferromagnesian silicates include
– olivine
– Hornblende,
an
amphibolegroup
mineral
– augite, a
pyroxene-group
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
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
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
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
Igneous Rock Textures
• Texture
– is the size (coarse or fine)
– 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
Aphanitic
texture
Phaneritic
texture
2-stage
cooling(mixed)
• 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 or % of
light to dark minerals (non-ferromagnesian to ferromagnesian)
– felsic
– intermediate
– mafic
– More felsic magmas have higher Si, and higher Na, K, Al
• More mafic magmas have lower Si, and higher Ca, Fe, Mg
Classifying Igneous Rocks
Common Igneous Rocks
Basalt
Andesite
Gabbro
Diorite
Common Igneous Rocks
Rhyolite
Granite
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.
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—”foliated”
• minerals recrystallize
– or both
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
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 (directional
stress)
– 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
• Anthracite
– made of black lustrous carbon from coal
– Also Greenstone is a metamorphic rock
associated with plate tectonics
Common Metamorphic Rocks
Slate
Gneiss
Schist
Marble
Quartzite