Mineral

Minerals are essential to the life of plants and animals. Most plants get minerals from the soil. Animals, including humans, obtain them from plants, vegetables, and fruits or from the milk, eggs, and meat of plant-eating animals  Industry is equally dependent upon an abundant supply of minerals. The science of mineralogy is concerned with the natural substances called minerals that make up the rocks, clays, sand, and similar materials of the Earth . Mineralogy includes the study of the physical and chemical properties of minerals, their forms, and the various ways in which they are distinguished from one another.

To be classified as a mineral, a substance must have an inorganic, or nonliving, origin and a composition that can be expressed by a chemical formula. Its elements may be metallic or nonmetallic. Most minerals have a crystalline structure
By this definition, the resin amber and the fuels coal, petroleum, and natural gas are not true minerals. These substances were formed from organic substances—animal or vegetable matter—that once lived on the Earth
The important metallic and nonmetallic mineral beds that are mined today were deposited over long periods of geologic time as a result of such natural phenomena as weathering and erosion. Other processes contributing to the concentration of minerals in the Earth are sedimentation, precipitation, and evaporation of water bodies, as well as the circulation of groundwater

Mineral Identification

The names of most minerals, usually ending in the suffix -ite, are a kind of shorthand description or history of their substance. Some are named for the scientists who discovered them, others for the locations where they were first found, and still others for outstanding physical or chemical properties.
Mineralogists identify minerals by certain important properties. These include color, hardness, and cleavage, the ability of the mineral to split along definite planes. Other means of distinguishing a mineral are its elasticity and strength, specific gravity, radioactivity, and thermal, electrical, and magnetic properties. Luminescence, or the emission of light, sometimes permits rapid detection of some minerals, including some uranium ores.
Related to the color of a mineral are its transparency, luster, and iridescence, or the play of colors in its interior or exterior. The streak, another property that identifies a mineral, is the color produced when a piece of the mineral is rubbed against the surface of rough, unglazed porcelain.
The hardness of minerals is usually stated in terms of the Mohs scale. On this scale, a comparison is made of ten typical minerals, which are arranged in order from the softest to the hardest. They are: (1) talc; (2) gypsum; (3) calcite; (4) fluorite; (5) apatite; (6) orthoclase; (7) quartz; (8) topaz; (9) corundum; and (10) diamond. The hardness of a mineral that can scratch orthoclase but is itself scratched by quartz is between 6 and 7 on the Mohs scale.
The crystal form of minerals determines their cleavage, or the way they split or break, as well as many other properties. Mica, for example, splits into thin, flat sheets.

Major Groups of Minerals

Minerals can be classified into 13 groups according to their constituents. The major groups are: (1) native elements; (2) sulfides; (3) sulfosalts; (4) oxides and hydroxides; (5) halides and borates; (6) carbonates; (7) nitrates and iodates; (8) phosphates, vanadates, and arsenates; (9) sulfates; (10) tungstates and molybdates; and (11) silicates.
About 1,500 separate mineral species have been identified by mineralogists. About 200 of these are recognized as common minerals; 30 occur frequently enough to be considered rock-forming minerals.

Native Elements

Most minerals are composed of two or more elements, but a few consist of only one element. The most abundant of the native metals are gold, silver, copper, and platinum. Native iron occurs in meteorites, but in the Earth it is combined with other elements.
Mercury, lead, tin, and zinc are also metals found in the native, or uncombined, form. Carbon exists in two native forms—graphite and diamond. The most common nonmetallic native element is sulfur.

Sulfide Minerals

The sulfides include the majority of ore minerals from which metals are obtained. Thus, lead sulfide forms the mineral galena; silver sulfide, argentite; and zinc sulfide, sphalerite. These minerals are often found together as ores.
Two important sulfides of arsenic are realgar and orpiment. Antimony sulfide is called stibnite. Antimony is alloyed with lead for casting the type metal used in printing. The mineral cinnabar is a sulfide of mercury. Molybdenum, a metal that is used in alloy steels, is obtained from molybdenite, a sulfide of the metal (see
Three other iron sulfides, all differing slightly in chemical composition, are pyrite, marcasite, and pyrrhotite. Pyrite is known as fool's gold because its brilliant yellow luster somewhat resembles that of flakes of native gold.
The copper sulfides exist as minerals called chalcocite and covellite. Many ore bodies consist of sulfides containing both copper and iron. These include the minerals bornite and chalcopyrite.
The group of minerals classified as sulfosalts is similar to the sulfides. These are made up of sulfur, plus the metals copper, lead, or silver, combined with arsenic, antimony, or bismuth. An example is enargite, a copper arsenic sulfide.

Oxide and Hydroxide Minerals

The oxide group includes the silicon oxide quartz, also called silica. One of the most common minerals, quartz occurs in many areas in a variety of forms. Semiprecious gem stones of quartz include amethyst, tigereye, agate, and onyx . Siliceous sinter, or geyserite, is an impure quartz deposited by hot springs and is a form of opal. The fire opal has an internal iridescence of intense orange to red.
Diatomaceous earth, or diatomite, was formed from the siliceous shells of diatoms, microscopic algae found in fresh water and seawater. It is also called kieselguhr and tripolite. The powdery substance is used for insulating and filtering material and in the manufacture of polishing and scouring powders. Another abundant oxide of silicon is tridymite.
Among the oxides of metals that exist as minerals are cuprite, or copper oxide; zincite, or zinc oxide; cassiterite, or tin oxide; and rutile, or titanium oxide. Pyrolusite, or manganese oxide, is the chief ore of manganese. Among the ores of iron are the oxides hematite and magnetite. Lodestone, a form of magnetite, is a natural magnet. Ilmenite, which exists in large deposits, is a mixed oxide of iron and titanium. It is a chief source of the titanium used as a paint pigment and as a purifier in alloys
Aluminum oxide, known in mineralogy as corundum (and in an impure form as bauxite), exists in two transparent and colored gem forms—sapphire and ruby. Emery, a black granular corundum mixed with iron minerals, is used in a powdered form for grinding and polishing. Spinel is a mixed oxide of magnesium and aluminum, and chromite is an iron and chromium oxide that makes up the chief ore of chromium. Chromium is one of the major components of stainless steels.
The leading radioactive minerals, sources of such elements as radium, thorium, and uranium, include uraninite, carnotite, and autunite. They are all complex oxides of the radioactive and other elements and usually contain lead . Pitchblende, in which mineral radioactivity was first discovered, is an impure form of uraninite
Hydroxides are low-temperature minerals typically formed from products of aqueous alteration or from hydrothermal vents. Among the hydroxide minerals are the aluminum ore bauxite and limonite, an iron ore containing hydrated iron oxides.

Halide and Borate Minerals

The halides, which are soluble in water, account for relatively few minerals, but some of them have commercial value. The most outstanding of these is sodium chloride, or common salt. It is known in its mineral form as halite.
Potassium chloride, called sylvite, is one of the most important minerals found in the famous Stassfurt mines in eastern Germany. These deposits are one of the world's chief sources of potash for commercial fertilizers  Other essential minerals in the Stassfurt deposits are carnallite, which is a hydrous chloride of magnesium and potassium, and kainite, which contains hydrated potassium chloride and magnesium sulfate.
Fluorite, sold commercially as fluorspar, is composed of calcium fluoride. It is used as a flux in metallurgical processes. The rare, clear crystals of the mineral are valuable in making lenses and prisms for optical systems used with ultraviolet light. Cryolite is a fluoride of aluminum and sodium.
A large deposit of halite at Searles Lake, in California, contains also millions of tons of a brine composed of potash and borax (a hydrous sodium borate). This area is the chief source of these minerals in the United States. Other minerals from which borax is obtained are kernite, another hydrous sodium borate, and colemanite, a hydrous calcium borate. Most borate minerals occur in dried-up basins fed by waters rich in volcanic material.

Carbonate Minerals

The carbonates make up one of the largest groups of minerals. Among these is the plentiful mineral called calcite, or calcium carbonate. Large transparent crystals of calcite are called Iceland spar. This chemically pure, clear calcite—capable of producing double refraction of light—is used in prisms for polarizing microscopes and similar optical instruments
Ordinary limestone consists largely of calcite . Marble is composed of crystalline, metamorphosed limestone. The various colors in marbles are created by chemical impurities or by veins of other minerals. The stalagmites and stalactites found in caves usually consist of calcite
Marl is an impure limestone that is imperfectly hardened. Chalk is a soft, fine-grained limestone formed in the oceans from deposits of the shells of tiny sea animals
Magnesite is a magnesium carbonate. It frequently occurs mixed with calcite, forming a calcium magnesium carbonate called dolomite, or dolomitic limestone. Two copper carbonates are malachite and azurite. An increase in the water content of azurite, which is blue, can change it into malachite, which is green. Both crystals are used in jewelry.
The carbonate of iron appears as the mineral siderite. It is the major commercial iron ore in Great Britain. Manganese carbonate, called rhodochrosite, occurs usually as a gangue mineral with other ores.

Nitrate, Iodate, and Phosphate Minerals

Nitrates and iodates are structurally related to carbonates. The nitrates of potassium and of sodium exist as minerals known respectively as niter, or saltpeter, and soda niter, or Chile saltpeter. Nitrates are used in the manufacture of explosives and fertilizers. Much rarer than nitrates, iodates are yellow minerals. Among the iodates are lautarite and dietzeite.
The mineral salts of phosphoric acid that contain the element phosphorus are called phosphates. Apatite is an abundant phosphate of calcium that contains fluorine. An impure form of apatite is the phosphate rock used in fertilizers. A complicated phosphate mineral is monazite, found in beach and river sands. It is a source of the rare earth metal cerium, which is used in solid-state electronic devices. The gem turquoise is a basic hydrous phosphate of aluminum and copper. Vanadates, compounds of vanadium and oxygen, and arsenates resemble phosphates in their crystal structure.

Sulfate Minerals

Sulfates, which are insoluble in water and are related chemically to the saline minerals, include a barium compound called barite; celestite, an ore of strontium; and alunite, a basic aluminum and potassium sulfate. Gypsum, a hydrous calcium sulfate, is the source of plaster of Paris. Transparent crystals of gypsum, known as selenite, split in thin cracks. When calcium sulfate is found uncombined with water, it is called anhydrite. Polyhalite, a hydrous sulfate of potassium, calcium, and magnesium, is found in the Stassfurt deposits in eastern Germany.
Sodium sulfate, sold commercially as Glauber's salt, exists in the hydrated form as the natural mineral mirabilite. Epsom salts—hydrous magnesium sulfate—occurs as the mineral epsomite. For commercial purposes, however, Epsom salts is manufactured from other magnesium minerals.

Tungstate and Molybdate Minerals

Salts of tungstic acid and molybdic acid are respectively called tungstates and molybdates. Tungsten, a metal used in the manufacture of electric light bulbs, steel alloys, and magnets, is obtained chiefly from two minerals. These are scheelite, or calcium tungstate, and wolframite, which is a mixed tungstate of the elements manganese and iron.
Wulfenite is a lead molybdate. It is a minor ore of lead and molybdenum.

Silicate Minerals

The most widespread and numerous minerals are the silicates. They consist of silicon and oxygen combined with potassium, sodium, magnesium, aluminum, and many other elements.
Feldspars make up the most prominent group of silicates. They include orthoclase, a potassium and aluminum silicate; albite, which contains sodium instead of potassium; and oligoclase, which contains calcium in addition to sodium.
Another important silicate group includes the micas. Muscovite is the transparent mica used as an insulating material in the manufacture of electrical equipment. It consists primarily of silicate of potassium and aluminum. As isinglass it is used in devices such as stove doors and lantern shields.
A second common mica is biotite, which contains magnesium and iron; it is usually dark green, brown, or black. Another mica is lepidolite, a fluosilicate of potassium, aluminum, and lithium. Lepidolite is one of the few ores that contains the metal lithium.
The pyroxene family contains a series of rock-forming minerals, as do the feldspar and mica groups. Two common pyroxenes are diopside, a silicate of calcium and magnesium, and augite, which contains some iron and aluminum. One variety of pyroxene is spodumene, a lithium aluminum silicate. It is sometimes found in a clear, pink crystal form called kunzite, which is used as a gem. A green variety is called hiddenite. Jadeite is another pyroxene. A true jade, it is sometimes called Chinese jade.
Other silicates form such gems as tourmaline, zircon, and topaz. Beryl, the chief ore of beryllium, is an aluminum and beryllium silicate. The emerald, whose green color is due to chromium traces, and the aquamarine are crystal forms of beryl.
A more complicated group of silicates, closely related to the pyroxenes, is the amphibole family. A variety called hornblende, containing aluminum, may occur in long, fiberlike crystals to form one kind of asbestos. Another kind of asbestos is a fibrous variety of the mineral serpentine, called chrysotile. It is the chief commercial source of asbestos. Serpentine is a hydrous magnesium silicate.
Similar to serpentine in composition is talc, the source of talcum powder. Soapstone, or steatite, is an impure variety of talc. Slabs of it are used for laboratory tabletops. Talc and soapstone are ingredients of paint, ceramics, and paper.
The mineral kaolinite is a hydrous silicate, somewhat like talc, but containing aluminum instead of magnesium. The majority of clays consist of impure kaolinite, formed from the decomposition of feldspar rocks. Bentonite is a type of clay that was formed from the decomposition of volcanic ash.
Zinc silicates include willemite, which is employed in laboratory experiments because ultraviolet light renders it fluorescent. Hemimorphite, or calamine, is a hydrous zinc silicate sometimes utilized as an ore of zinc. Zeolites such as stilbite are hydrous silicates of aluminum with calcium and sodium bases. They are used as molecular sieves to separate chemicals.

The Rocks of the Earth

Geologists classify the Earth's rocks roughly into sedimentary, igneous, and metamorphic rocks. The sedimentary rocks were formed from sand, clay, or other materials that were deposited by water, wind, or glacial action
Igneous rocks were formed from volcanic magma, or molten rock within the Earth. The metamorphic rocks were originally sedimentary or igneous but were later modified by heat and pressure or other natural processes
The most common sedimentary rocks are sandstone and shale. The chief mineral of sandstone is quartz. If the rock contains significant amounts of feldspar, it is called arkose. The shales are composed chiefly of the clay minerals such as kaolinite, with the addition of quartz and mica. When calcite or aragonite is present, the shales are graded as limestones.
Conglomerates are sedimentary rocks that may be considered consolidated gravels. Alluvium is the term for surface sedimentary deposits not yet hardened into rock. Till, a claylike alluvium, is believed to have been deposited by glaciers. Loess is the term for a fine-grained material that was probably laid down as windblown dust during the glacial period
One of the chief igneous rocks is granite, which is made up of crystals of quartz and feldspar, usually mixed with other minerals. It is one of the most frequently used building stones. Gneiss is the most common metamorphic rock. It is derived from either sedimentary rock, such as conglomerate, or igneous rock, such as granite. Gneiss is composed of quartz, feldspar, and mica or hornblende.
Schists are the second most common metamorphic rocks. They were apparently formed by the partial recrystallization of shales under the action of heat and pressure. A major type is mica schist. It is composed essentially of quartz, combined usually with muscovite or biotite mica.
When the magma within the Earth is extruded through a volcano or fissure at the Earth's surface, it is called lava. There are two common types of lava—a dark, heavy variety called basalt and the lighter rhyolite, usually a pale shade of green, red, or gray. Pumice is a porous, frothy rhyolite that is produced when the melted rock contains gas bubbles.
Obsidian is a glasslike rock that is produced when certain kinds of lava cool so rapidly that the individual minerals do not crystallize. The rock is usually black, but it may be red, brown, or green. The volcanic material called tuff consists of tiny fragments of obsidian or pumice blown into the air during volcanic eruptions