Minerals
From energy, construction, and agriculture, to communications, transportation, and national
defense, at home, work or play. We all use or require minerals and mineral materials every
day. How do we get minerals? Minerals must be mined from the ground, either by surface or
underground mining or by drilling methods.
A mineral is naturally occurring inorganic compound or element found in the Earth. A
mineral, by definition, must satisfy five conditions:
This definition excludes compounds
Five Conditions to be a Mineral
invented by humans in laboratories because
1.
2.
3.
4.
5.
these compounds are not naturally
occurring. Compounds that are found in
only plants or animals are also excluded
because they are organic. Liquids are
It must be naturally occurring.
It must be inorganic (nonliving).
It must be a solid element or compound.
It must have a definite composition.
It must have a regular internal crystal
structure (not liquid).
excluded because they are not solids. Their
atoms are free to move. Minerals can be a single element, like diamond, which is made of
carbon. Minerals can also be made of compounds of two or more elements, like quartz, which
contains one silicon and two oxygen atoms. Definite composition indicates that a chemical
analysis of a given mineral will always produce the same ratio of elements. For example,
quartz will always have one silicon atom for every two oxygen atoms. Therefore, minerals
can be expressed by chemical formulas, such as SiO2 for quartz.
There are 91 naturally occurring elements on
earth but only eight elements make over 98% of
the minerals on the Earth's crust. They are
oxygen, silicon, aluminum, iron, calcium,
sodium, potassium, and magnesium. Table 1
shows you the amounts of these elements in the
Earth's crust.
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More than four thousand minerals have been discovered but only about one hundred are
abundant and common in the rocks of the Earth's crust. Geologists use the physical properties
of a mineral to identify it. A property is a characteristic of a mineral such as color, streak,
luster, cleavage, specific gravity (density) and hardness. Several properties must be used
together to properly identify a mineral. The properties we are going to study are luster,
hardness, cleavage and fracture, color, streak, and magnetism.
Luster
Luster is a property of a mineral that tells how the mineral reflects light. Luster gives you an
indication of how "shiny" a mineral is. The two main ways that geologists categorize a
mineral's luster is metallic and non-metallic. The luster of a mineral may differ from sample
to sample. Metallic minerals shine like metal, while non-metallic minerals vary greatly in
their appearance. There are many different descriptions of non-metallic luster. We are going
to discuss four. They are pearly, earthy, vitreous (glassy), and greasy. Pearly luster is
iridescent, glows like a pearl. Greasy luster looks like the mineral is covered with grease, the
mineral definitely shines. Minerals with an earthy luster have a dull look with no shine.
Minerals with an earthy luster look as though they are covered with dirt or dust. Minerals with
a vitreous luster (glassy) look like small pieces of a broken glass bottle.
Color
Color is the easiest of the properties to see, but it is not always the best way to identify a
mineral. Many minerals have more than one color because of impurities that were present
during the formation of the mineral. Quartz is an example of a mineral with many different
colors. Quartz can be clear, white, blue, brown, and almost black. Amethyst is a quartz crystal
with a purple color. The impurity that makes amethyst purple is manganese.
Streak
A better determinant of the true color of a mineral is its streak. Streak is a test used by a
geologist to see the color of the mineral under the top layer or coating on the mineral. The
mineral is rubbed on a "streak plate", which is a piece of unglazed porcelain. When the
mineral is rubbed across the streak plate some of the mineral is broken off and ground into a
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powder. This allows the geologist to see under the outer layer which could have a different
color due to the mineral being exposed to the atmosphere. When minerals are exposed to the
atmosphere, gases like oxygen can chemically combine with the mineral to change its color.
Hardness
In 1822 a German scientist by the name of Frederick Mohs (1773-1839) set up a scale to
determine the approximate hardness of minerals. He called it the Mohs Hardness Scale. He
arranged the minerals in his scale from softest (Talc) to hardest (Diamond). The minerals get
increasingly harder as you read down the scale, but they
do not increase in hardness at a constant rate.
(Example: Calcite is not twice as hard as talc and a
diamond is not 10 times harder than talc. In fact a
diamond is over 40 times harder than talc.) The scale is
not exact because the hardness of a mineral varies
slightly from one specimen to the next. We can
determine the approximate hardness of a mineral by
running a group of tests. Scratch the mineral on a fingernail, penny, iron nail, or glass slide. If
the mineral scratches the testing materials the mineral is said to be harder than the mineral it
scratched. Example: A piece of pink feldspar will scratch a fingernail, penny, and an iron nail,
but will not a glass slide. The feldspar is said to be harder than the first three testing materials
but not as hard as the glass slide.
Cleavage/Fracture
Some minerals have a tendency to split or crack along parallel or flat planes. This property is
easily seen in some minerals and you can test the mineral by breaking it with a hammer or
splitting off sheets with a pen knife. The planes along which the mineral breaks are called
cleavage planes. If the mineral splits easily along these planes the mineral is said to have
perfect cleavage. Mica is a good example of perfect cleavage. Feldspar is an example of a
mineral with cleavage in more than one direction. Quartz is a mineral that has no cleavage at
all. Quartz shatters likes glass when struck with a hammer so it has fracture. Fracture occurs
when a mineral breaks at random lines instead of at consistent cleavage planes. Many
minerals that have no cleavage or poor cleavage have fracture.
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Special Properties
Only two minerals on Earth are magnetic. They both have high quantities of iron. Magnetite
is one of the magnetic minerals and Pyrrhotite is the other. Magnetite was used by ancient
sailors for making compasses. They would chip off needles of magnetite and float them on
water and watch the needle point to the North. Smell is another special property. Some
minerals can be identified by their odor. Sulfur smells like rotten eggs. Taste is a special
property. Certain minerals have familiar tastes. However, do not taste the minerals because
some contain poisons like arsenic (used in rat poison). Licking the minerals is also disgusting
because they have been handled by many people. Halite (NaCl) would taste familiar. It tastes
very salty because salt is actually made from Halite. Hydrochloric Acid (HCl) can be used to
identify calcite or the rock limestone. Calcite is a mineral that contains calcium carbonate
which bubbles furiously when HCl acid is applied. Double refraction is a dead give away
property of Iceland spar Calcite. When this mineral is placed on paper you see double!
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Rocks
Rocks are all around us. They make up the backbones of hills and mountains and the
foundations of plains and valleys. Rocks are simply a mixture of minerals and possibly
organic matter. The rocks you will read about are used in many ways to build our present-day
world. Glass windows, concrete highways, building stones, fertilizers, metal objects, and
thousands of other objects around us came originally from rocks. We really are living in a
"modern stone age”.
Another important material that comes from rocks is topsoil. Topsoil is a mixture of rock
particles and the remains of plants and animals. The thin layer of topsoil on Earth's surface
provides the environment in which plants live and grow. Water and minerals that plants need
for proper growth are stored in the topsoil. The plants, in turn, are used as food by people and
many other animals. In fact, life on Earth would be impossible without topsoil and therefore
impossible without rocks.
Geologists classify rocks into three groups based on the major Earth processes that formed
them. The three rock groups are igneous, sedimentary and metamorphic rocks.
Igneous Rocks
Igneous rocks are formed from melted rock that has cooled and solidified. Igneous rocks are
the most abundant type of rock on Earth. Igneous rocks are classified by their chemical
composition (amount of silica) and crystal size (extrusive or intrusive). Rocks melt when they
are buried deep within the Earth because of the high pressure and temperature. The molten
rock inside the Earth, called magma, can then flow upward or even be erupted from a volcano
creating lava which can flow across the Earth's surface. A wide variety of rocks are formed
by different cooling rates and different chemical compositions of the original magma.
Obsidian (volcanic glass), granite, basalt, and andesite porphyry are four of the many types of
igneous rock.
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Igneous rocks can be classified in several ways. One of which is by their chemical
composition or percentage of SiO2. Igneous rocks can be basaltic ( also known as mafic),
granitic (also known as felsic), or andesitic.
Basaltic (mafic) igneous rocks are dense, heavy, dark-colored rocks that form from fluid
basaltic magma or lava. The eruptions they are come from are relatively gentle. Balsaltic
magma and lava are rich in iron (Fe) and magnesium (Mg). These elements make the molten
materials dense and dark colored. Basaltic rocks tend to have lower percentages of silica (less
than 52%). Examples of basaltic rocks we will study are Basalt, Gabbro and Scoria. Basaltic
rocks can be found in Hawaii.
Granitic (felsic) igneous rocks are light-colored rocks (pink, gray, white, green) of a lower
density than basaltic rocks. Granitic magmas are thick and stiff and contain a lot of silicon
(Si) and oxygen (O). Granitic magma can build up a great deal of pressure, which is released
during a violent volcanic eruption. Granitic rocks tend to have percentages of silica (higher
than 60%). The granitic rocks we will study are Granite, Rhyolite and Pumice.
Andesitic rocks have mineral compositions between those of granitic and basaltic rocks.
Many volcanoes of the Pacific Ocean are andesitic. The andesitic rocks we will study will be
Diorite, Andesite and Scoria.
Extrusive Igneous Rocks
Igneous rocks can also be classified by their crystal size, which is determined by the way they
cool. Extrusive igneous rocks cool very quickly at or near the Earth’s surface. Because the
cooling occurs very rapidly, crystal size is very small. The crystals cannot be seen with the
unaided eye. Imagine standing at the base of the Kilauea Volcano. Hardened lava flow covers
the land around it. There in the hardened lava is basalt, one of the world's most common
igneous rocks. Basalt is a mafic extrusive rock composed mostly of pyroxene, feldspar, and
possibly olivine. The individual crystals of extrusive rocks such as basalt are too small to be
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seen with the unaided eye because of the rapid cooling that extrusive magmas undergo to
form rocks.
Another common extrusive igneous rock that forms under conditions similar to those for
basalt is rhyolite. But rhyolite is a felsic extrusive rock, because its main minerals are quartz,
feldspar, and light-colored micas. Unlike basaltic magmas, rhyolitic magmas are often
associated with volcanic eruptions of great violence. Also like basalt, rhyolite crystals cannot
be observed with the naked eye.
Intrusive Igneous Rocks
When magma cools slowly, usually deep within the Earth, the crystals are large and a coursegrained intrusive igneous rock results. Think of magma that has the same minerals as basalt
or rhyolite, but it solidifies (turns solid) deep inside the Earth instead of at the surface.
Cooling very slowly over many years allows the crystals to grow very large. In fact, the large
mineral crystals of intrusive igneous rocks can be seen without a lens or microscope.
Although intrusive rocks develop from the same types of magma as extrusive rocks, intrusive
rocks look quite a bit different because they cool more slowly and thus have larger crystals.
They are often coarse textured and have a rough feel, unlike extrusive rocks, which feel
smooth.
Intrusive igneous rocks may create underground features. A dike is an intrusive body of
magma that pushes its way through layers of sediments forming a vertical column. A sill is an
intrusive body of magma that
pushes its way between layers of
sediments forming a horizontal
layer. A laccolith is formed when
magma is pushed between
horizontal sedimentary rock layers
creating a dome or mushroom-like
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feature. Very large underground areas of solidified magma are called batholiths. They can
cover areas larger than 100 square kilometers. Stocks are solidified magma areas smaller than
100 kilometers.
One type of intrusive igneous rock that forms from the same type of magma as basalt is
gabbro. Gabbro contains no quartz. It is a mafic rock whose main minerals are pyroxene,
feldspar, and some olivine.
The best known of the igneous rocks is granite, an intrusive rock that forms from the same
type of magma as rhyolite. Like rhyolite, granite is a felsic rock, and is composed almost
entirely of feldspar and quartz. But unlike rhyolite, granite is rough textured and has obvious
crystals. In certain cases where the granite forms very slowly, crystals can be up to several
feet long. Because granite is a hard and tough rock, it is used for many construction projects.
Both extrusive and intrusive igneous rocks can become exposed to Earth's surface after they
form. There, they are vulnerable to attack from environmental agents such as wind, water, ice,
and gravity. These forces of nature break rocks into pieces that are later deposited and
cemented to form sedimentary rocks, the next type of rock to be considered.
Sedimentary Rocks
Sedimentary rocks are formed at
All sedimentary rocks have three things in common
the surface of the Earth, either in
water or on land. Sedimentary rocks
1) The sediments making them moved downward.
are the most common type of rock
on the surface of the Earth. They get
their name from the sediments, or
small pieces, they are made of.
There are three methods of
formation for sedimentary rocks.
2) The sediments settle in layers called strata. The oldest
layers are on the bottom. This is called Superposition.
3) The erosion and deposition processes that made the
sediments are still at work today making new sedimentary
rocks.
Layered accumulations of
sediments, fragments of rocks, minerals, or animal or plant material can be pressed together.
Compaction occurs when these materials are pressed together. Cementation occurs when
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sedimentary rocks become cemented together by minerals. Quartz and calcite are the two
natural glues. Some sedimentary rocks are created when chemicals or minerals are deposited
after falling out of solution. This is the third method of formation for sedimentary rocks.
Some, however, remain loose and unconsolidated. The layers (strata) of sediment are
normally parallel or nearly parallel to the Earth's surface. If they are at high angles to the
surface or are twisted or broken, some kind of Earth movement has occurred since the rock
was formed. Sedimentary rocks are forming around us all the time. Sand and gravel on
beaches or in river bars look like the sandstone and conglomerate they will become.
Compacted and dried mud flats harden into shale.
Sedimentary rocks often preserve some of the
Ripple
Marks
features of the depositional process. For
example, little waves or ripple marks may
develop in the sand on a beach. The rock that
forms from that sand may have rippled lines.
Another common feature is mud cracks, which
develop when the sediment at a bottom of a
river or stream goes through cycles of flooding
and drying.
Sometimes, sedimentary rocks contain features called concretions, hard, rounded objects that
become enclosed in the rock. A special type of concretion is a geode, whose hollow interior is
lined with crystals.
How do sediments accumulate to begin with? As gravity, water, or wind carries them to a
destination, sediments are deposited, usually in layers called strata. For example, many
horizontal bands can be seen in the Navajo Sandstone Formation. Those bands are the strata
of the rock, and each one represents a new layer of sand that was deposited. The oldest layer
is the one on the bottom since it was deposited first. This is referred to as the Law of
Superposition.
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Sedimentary rocks are classified according to their sediment type. Sediments form rocks
classified as clastic, chemical and organic sedimentary rocks. Note that the term chemical is
used to describe both sediments and a sedimentary rock type.
Clastic sedimentary rocks are formed by compaction and cementation of fragments of plants,
animals and/or rocks that are compacted or glued together by quartz or calcite. We will study
shale, sandstone, conglomerate and breccia. A common clastic sedimentary rock is shale, the
most abundant of all sedimentary rocks. Shale forms when mud and clay harden. Because the
clay sediments are extremely small, they settle out slowly. In fact, shale formations can take
up to 5 million years to form. When shale is wet it has a muddy smell. Sandstone, another
clastic sedimentary rock, is composed mainly of cemented grains of sand. Its main mineral is
quartz. It is known to be very hard and abrasive. Conglomerate is another common clastic
rock. It looks like a mixture of different-sized rounded pebbles cemented together.
Conglomerate rock forms when rapidly moving water drops pebbles into sand at the bottom
of a river or stream. These become rounded due to the running water. Over time, the pebbles
and sand became cemented together. Conglomerate can also be formed at the base of a glacier
in a similar way. Breccia looks similar to conglomerate but it is formed in a dry environment.
The pebbles forming breccia are angular instead of rounded like the conglomerate.
Chemical sedimentary rocks are formed from minerals and chemicals that were once
dissolved in a solution. Their method of formation is falling out of solution. The chemical
sedimentary rocks we will study are rock salt and limestone from calcite (50% calcite). Rock
salt is formed from the mineral halite being dissolved in water and then being deposited as the
water evaporates. The Great Salt Flats in Utah were formed in this manner.
The most common chemical sedimentary rock is limestone. The limestone is at least 50
percent calcite. The limestone is made from calcite that was dissolve from surrounding rocks
and soil as the water slowly passed through it. The water dissolves the calcite and then
deposits it later. Stalagmites and stalactites in caves are examples of chemical limestone.
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Limestone can be distinguished from sandstone by dripping hydrochloric acid on it. The acid
will react with the calcite creating bubbles. It will not react with the quartz in the sandstone.
The color of sedimentary rock is a clue to the chemical composition of the sediments from
which it formed. The bands of red and pink rocks in the Grand Canyon, for example, come
from iron-bearing minerals such as hematite.
Organic sedimentary rocks are formed from the remains of plants or animals. They are
compacted and cemented together. The organic sedimentary rocks we will study are
fossiliferous limestone (chalk, coquina) and coal. Notice that coal is a rock but not a mineral.
Rocks can be organic in origin, whereas, minerals must be inorganic.
Most limestone rocks are organic. They develop from the remains of organisms. Much
limestone is made up of marine animals and contains pieces of shells, corals, and mollusks.
Natural chalk is formed from very fine particles of marine animals whereas coquina is formed
from large marine shells.
Coal, which is often found in layers with other sedimentary rocks, is another organic
sedimentary rock. As plant remains are deposited in layers and slowly alter into hydrocarbons over millions of years to form coal. It takes ~20 feet of plant matter to make 1 foot of
coal.
Sedimentary rocks are also the source of most fossils because they do not undergo the
extreme heat and pressure of igneous or metamorphic rocks. The age of sedimentary rocks
can be estimated by looking at any fossils in it.
Sedimentary rocks cover almost all of the ocean floor and about three-fourths of Earth's land
surface area. Like igneous rocks, sedimentary rocks may undergo changes that transform
them into new rocks.
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Metamorphic Rocks
Sometimes sedimentary and igneous rocks are subjected to pressures so intense or heat so
high that they are completely changed. They become metamorphic rocks, which form while
deeply buried within the Earth's crust. The process of metamorphism does not melt the rocks,
but instead transforms them into denser, more compact rocks. New minerals are created either
by rearrangement of mineral components or by reactions with fluids that enter the rocks.
Metamorphic rocks are classified as foliated and non-foliated. We will study slate, gneiss,
quartzite, marble and schist.
Some kinds of metamorphic rocks like gneiss and slate are strongly banded or foliated.
Foliated metamorphic rocks are those in which the minerals have been flattened and pushed
down into parallel layers. The bands in foliated metamorphic rock look like pages in a book.
Slate is one of the most common foliated metamorphic rocks and splits easily into thin slabs.
Slate is derived from shale. Slate is commonly used for pool tables, chalkboards and shingles.
Gneiss is a rock that started as granite. Under the extreme pressure and heat the mineral grains
are flattened. Schist can be a foliated metamorphic rock. It can be made from metamorphosed
basalt, shale or slate.
Non-Foliated metamorphic rocks do not display layers. Rather, they are massive structures
with no obvious banding. A good example of a non-foliated rock is quartzite, the smoothtextured, metamorphosed form of the mineral quartz from sandstone. A coarse-textured nonfoliated rock is marble. Marble starts as the sedimentary rock limestone. Anthracite, or hard
coal, is a non-foliated rock that forms when intense pressure drives gases out of soft coal,
causing it to harden.
Pressure or temperature can even change
previously metamorphosed rocks into new
types.
There are several types of metamorphism, the
process by which rocks change into new rocks.
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The two most general types are contact metamorphism and regional metamorphism.
CONTACT METAMORPHISM occurs when a rock is exposed to hot magma inside the
Earth. The intense heat of the magma alters the rock, often causing its minerals to recrystallize. Thus, the new rock has new or larger mineral crystals than the older rock.
Sometimes, the hot magma will even introduce new minerals and modify the entire chemical
composition of the original rock. The area of rock affected by contact metamorphism is
appropriately known as the baked zone.
REGIONAL METAMORPHISM occurs during the formation of mountain ranges. As
tectonic plates collide and converge, intense pressure deforms and alters sedimentary and
igneous rocks already buried in the Earth. The mountain ranges of east Greenland are an
example of where this has taken place. Often, folds or curves in the rocks indicate the
direction of the intense pressure.
Whether metamorphic rocks are formed by contact with hot magma or by pressure from
colliding plates in the Earth, the result is that mineral crystals in the original rock are
rearranged.
THE ROCK CYCLE
Rock-forming and rock-destroying processes have been active for billions of years. In the
Guadalupe Mountains of western Texas, a person can stand on limestone rock that was once a
coral reef in a tropical sea about 250 million years ago. In Vermont's Green Mountains a
person can see schist that was once mud in a shallow sea. Half Dome in Yosemite Valley,
Calif., standing nearly 8,800 feet above sea level, is composed of an igneous rock that
solidified several thousand feet inside the Earth.
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The three types of rocks in the crust are continually being created, changed, destroyed and
recreated. This constant, gradual process of change is called the rock cycle. In the rock cycle,
igneous rocks are formed from the cooling and hardening of molten materials. Sedimentary
rocks are produced from sediments that are compressed and cemented in layers. And
metamorphic rocks are produced when sedimentary and igneous rocks are subjected to great
heat and pressure, deep inside
Earth.
As soon as an igneous,
sedimentary, or metamorphic
rock is exposed at Earth's
surface, the rock is subjected to
weathering and erosion. The
actions of plants, water, ice, heat,
and wind break rocks down into
sediments. The sediments are
transported by running water and
eventually deposited in lakes and
oceans.
Sediments harden into sedimentary rocks. Later, movements of Earth's crust may force the
sedimentary rocks downward to great depths, where they are heated and compressed into
metamorphic rock. Similar movements of the crust also can change igneous rock into
metamorphic rock. Any of the three rock types may melt if pushed sufficiently far into Earth's
interior. Then, upon hardening, the molten material becomes igneous rock once again
renewing the rock cycle.
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