From Energy, Construction, and Agriculture, to Communications, Transportation, and National

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 invented by humans in laboratories because these compounds are not naturally occurring. Compounds that are found in only plants or animals are also excluded because they are organic. Liquids are 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.

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 metallicand 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 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 HardnessScale. 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.

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!

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 basedon 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.

Igneous rocks can be classified in several ways. One of which is by their chemicalcomposition 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 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 course-grained 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 feature.Very large underground areas of solidified magma are called batholiths. Theycan 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 the surface of the Earth, either in water or on land. Sedimentary rocks 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. 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 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 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.

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.