•Rocks and Minerals
• No other planet in the solar system has the unique
combination of fluids of Earth. Earth has a surface
that is mostly covered with liquid water, water vapor
in the atmosphere, and both frozen and liquid water
on the land.
• Solid Earth Materials
• Earth’s Molten Stage
– During the early formation of the Earth it was molten
– During this stage the heavier elements such as iron and
nickel, sank to the deeper interior of the Earth.
– This left a thin layer of lighter materials on the surface
that is mow called the crust.
– The majority of the Earth’s mass lies below the crust
• Chemical Analysis
– 8 elements make up 98.6% of the crust
– These 8 elements make up the solid materials of the
Earth’s crust and are known as rocks and minerals.
– A mineral is solid inorganic material of the Earth that
has both a known chemical composition and a crystalline
structure that is unique to that mineral
– A rock is a solid aggregate of one or more minerals that
have been cohesively brought together by a rock-forming
process.
• (A)The percentage by
weight of the elements
that make up Earth's
crust. (B) The
percentage by weight
of the elements that
make up the whole
Earth.
• Minerals
• Introduction
– Minerals
• A mineral is solid inorganic material of the Earth that has both a
known chemical composition and a crystalline structure that is
unique to that mineral
– Rocks
• A rock is a solid aggregate of one or more minerals that have
been cohesively brought together by a rock-forming process.
• Crystal Structures
– Can be made up of atoms of one or more kinds of
elements.
– Crystals are classified according to six major groups,
with subdivisions of each.
• A crystal is composed of a structural unit that is repeated in
three dimensions. This is the basic structural unit of a
crystal of sodium chloride, the mineral halite.
• The structural unit for a crystal of table salt, sodium
chloride, is cubic, as you can see in the individual grains.
• These quartz crystals are hexagonal prisms.
• Crystalline substances are
classified into six major systems:
isometric, hexagonal, tetragonal,
orthorhombic, monoclinic, and
triclinic. The six systems are
based on the arrangement of
crystal axes, which reflect how
the atoms or molecules are
arranged inside. For example,
crystals in the orthorhombic
system have three axes of
different lengths intersecting at
90O angles while crystals in the
hexagonal system have three
horizontal axes intersecting at 60O
and one vertical axis. Some
common examples in each system
• (A)The geometric shape
of a tetrahedron with
four equal sides. (B) A
silicon and four oxygen
atoms are arranged in the
shape of a tetrahedron
with the silicon in the
center. This is the basic
building block of all
silicate minerals.
• (A)Isolated silicon-oxygen tetrahedra do not share oxygens.
This structure occurs in the mineral olivine. (B) Single
chains of tetrahedra are formed by each silicon ion having
two oxygens all to itself and sharing two with other silicons
at the same time. This structure occurs in augite. (C) Double
chains of tetrahedra are formed by silicon ions sharing
either two or three oxygens. This structure occurs in
hornblende. (D) The sheet structure in which each silicon
shares three oxygens occurs in the micas, resulting in layers
that pull off easily because of cleavage between the sheets.
• Silicates and Nonsilicates
– Silicates – made of silicon and oxygen and make up 92 % of
Earth’s crust.
• Ferromagnesian Silicates
– made of iron, magnesium, and silicates
– Form a basic tetrahederal structure.
– Higher density and darker color than other silicates due to
the presence of iron and magnesium
• Nonferromagnesiam Silicates
– silicates that do not contain either iron or magnesium.
– Lower density and lighter color than the ferromagnesian
silicates.
• Compare the dark colors of the ferromagnesian silicates
augite (right), hornblende (left), and biotite to the lightcolored nonferromagnesian silicates.
• Compare the light colors of the nonferromagnesian silicates
mica (front center), white and pink orthoclase (top and
center), and quartz, to the dark-colored ferromagnesian
silicates.
– Structure of silicates
• Isolated tetrahedrons
• Chain silicates
• Sheet silicates
• Framework silicates
– Nonsilicates – make up 8% of Earth’s crust
• Carbonates
• Sulfates
• Oxides
• Sulfides
• Halides
• Phosphates
• Hydroxides
• Native elements
• Physical Properties of Minerals
– Color
• A visual measure.
• Not very useful for identification as color of minerals varies
considerably.
– Streak
• This is the color of the mineral when it is finely powdered.
• Rubbed across a piece of tile, leaving a fine powder of the
mineral on the tile.
– Hardness
• Resistance of the material to being scratched.
• Measured using the Mohs hardness scale, which compares the
hardness of the mineral to 10 reference minerals.
• (A)Gypsum, with a hardness of 2, is easily scratched
by a fingernail. (B) Quartz, with a hardness of 7, is
so hard that even a metal file will not scratch it.
– Crystal form
• Related to the internal geometric arrangement of the atoms that
make up the crystal structure.
– Cleavage
• the tendency of mineral to break along smooth planes.
• Depends upon zones of weakness in the crystal structure.
– Fracture
• The broken surface is irregular and not in a flat plane.
– Luster
• Surface sheen
• Metallic – like metal
• Pearly – like pearl
• Vitreous – like glass
• Earthy
– Density – ratio of the mass of a mineral to its volume.
• Specific gravity – ratio of mineral density to the
density of water
• Depends on:
– Kind of atoms which make up the mineral
– How the atoms are arranged in the crystal lattice.
• Mineral-forming Processes
• Introduction
– Magma
• Molten rock from which minerals are formed
– Lava
• Magma that is forced to the surface
– Influences on the mineral forming process
• Temperature
• Pressure
• Time
• Availability and concentration of ions that are in
solution
• Minerals Formed at High Temperatures
– Bowen’s Reaction Series
• Arranged with minerals at the top that crystallize at
higher temperature and minerals at the bottom that
crystallize out at lower temperature.
• Bowen's reaction series. Minerals at the top of the series
(olivine, augite, and calcium-rich plagioclase) crystallize at
higher temperatures, leaving the magma enriched in silica.
Later, the residual magma cools and lighter-colored, less
dense minerals (orthoclase feldspar, quartz, and white mica)
crystallize. Thus, granitelike rocks can form from a magma
that would have produced basaltic rocks had it cooled
quickly.
• Minerals Formed at Normal Temperatures
– These form at normal temperatures and pressures and in
contact with atmospheric gases such as oxygen, carbon
dioxide, and water.
– There are most of the non-silicates; carbonates, sulfates,
oxides, halides, and sulfides.
• Altered Minerals
– These minerals undergo changes in chemistry or crystal
structure as a result of pressure, temperature, or chemical
solutions
– Similar to minerals that form under high temperatures
with similar physical properties.
• Ore Minerals
– Some minerals are left over after the crystallizing of
magma
– These elements are flushed away in hot water solutions
as the magma crystallizes.
– Usually crystallize in rock fractures to form thin, flat
bodies of mineral material called veins.
– If these minerals have some economic value they are
called ore minerals.
• Rocks
• Introduction
– Elements are chemically combined to form minerals
– Minerals are physically combined to form rocks.
• Igneous Rocks
– Form from molten rock material
– Intrusive igneous rock
• Formed when magma cools deep within the Earth’s surface
• Cools very slowly as it is in contact with molten rock.
• Produces course-grained igneous rock.
– Extrusive igneous rock
• Magma that cools above the Earth’s surface.
• Produces fine-grained igneous rocks.
• This rapid cooling does not allow time for crystals to form.
• Igneous rock classification scheme based on mineral
composition and texture. There are other blends of minerals
with various textures, many of which have specific names.
• Granite is a coarse-grained igneous rock composed mostly
of light-colored, light-density, nonferromagnesian minerals.
The earth's continental areas are dominated by granite and
by rocks with the same mineral composition of granite.
• This is a piece of obsidian, which has the same chemical
composition as the granite. Obsidian has a different texture
because it does not have crystals and is a volcanic glass. The
curved fracture surface is common in noncrystalline
substances such as glass.
• Sedimentary Rocks
– Form from material from previously existing rock
• Material is provided by weathering of previously
existing rock
– Sediments
• Weathered rock materials
• Dissolved rock materials
– Clastic sediments
• Another name for weathered rock materials
• This is a sample of breccia, a coarse-grained
sedimentary rock with coarse, angular fragments.
Compare the grain sizes to the centimeter scale.
• This is a sample of sandstone, a sedimentary rock that
formed from sand grains in a matrix of very fine-grained
silt, clay, or other materials. The grains in this sample are
mostly the feldspar and quartz minerals, which probably
accumulated near the granite from which they were eroded.
• This is a sample of limestone, a sedimentary rock made of
calcium carbonate that formed under water directly or
indirectly from the actions of plants and animals. This finegrained limestone formed indirectly from the remains of
tiny marine organisms.
– Chemical sediments
• Another name for dissolved rock material.
• The dissolved materials are ions from mineral and
rocks that have been completely broken down.
• Removed from solution by:
– Chemical precipitation from the solution
– Crystallization from evaporating water.
– Biological sediments.
– Compaction
• As sediments are laid down grain by grain, the mass becomes
greater.
• The increasing mass of the sediment layer above creates
pressure on the layers below.
• Eventually this pressure becomes great enough to compact the
existing layers into a cohesive rock layer.
– Cementation
• After, or during, the compaction process, the spaces between
the sediment particles become filled with a chemical deposit.
• This deposit holds the compacted layers into a cohesive mass of
sedimentary rock.
• (A)In compaction, the
sediment grains are
packed more tightly
together, often by
overlying sediments,
as represented by the
bricks. (B) In
cementation, fluids
contain dissolved
minerals that are
precipitated in the
space between the
grains, cementing
them together into a
rigid, solid mass.
• Metamorphic Rocks
– Rocks changed by heat, pressure, or hot solutions due to:
• Movement of the Earth’s crust
• Heat generated by intrusion of hot magma
• Pressure can change rock by flattening, deforming, or
realigning mineral grains.
– Foliation
• When the pressure on flat crystal flakes tends to align the flakes
into parallel sheets.
• Gives the rock the property of breaking along the planes
between the aligned mineral grains in what is known as rock
cleavage.
• Increasing metamorphic change occurs with
increasing temperatures and pressures. If the melting
point is reached, the change is no longer
metamorphic, and igneous rocks are formed.
• This is a sample of marble, a coarse-grained metamorphic
rock with interlocking calcite crystals. The calcite crystals
were recrystallized from limestone during metamorphism.
• This banded
metamorphic rock is
very old; at an age of
3.8 billion years, it is
probably among the
oldest rocks on the
surface of the earth.
• The Rock Cycle
• Earth is a dynamic planet with the surface and
interior in a constant state of flux.
– Internal changes alter the surface by moving the Earth’s
plates, building mountains.
– Seas advance and retreat over the continents brining in
new materials and taking other materials away.
– Rocks are continually being changed by Earth’s forces.
• The Rock Cycle describes the continually changing
structure of rocks.
– Igneous, sedimentary, or metamorphic rock are just
temporary stages in the continuing changes that all rocks
undergo.
• A schematic diagram of the rock cycle concept,
which states that geologic processes act
continuously to produce new rocks from old ones.