A mineral is a naturally occurring inorganic solid with a characteristic chemical composition and a crystalline structure.

Chemical composition and crystalline structure are the two most important properties of a mineral. They distinguish any mineral from all others.

NATURAL OCCURRENCE

A synthetic diamond can be identical to a natural one, but it is not a true mineral because a mineral must form by natural processes. Like diamond, most gems that occur naturally can also be manufactured by industrial processes. Natural gems are valued more highly that manufactured ones.

INORGANIC SOLID

Organic substances are made up mostly of carbon that is chemically bonded to hydrogen or other elements. Although organic compounds can be produced in laboratories and by industrial processes, plants and animals create most of the Earth’s organic material. In contrast, inorganic compounds do not contain carbon-hydrogen bonds and generally are not produced by living organisms. All minerals are inorganic and most form independently of life. An exception is the calcite that forms limestone. Limestone is commonly composed of the shells of dead corals, clams, and similar marine organisms. Shells, in turn, are made of the mineral calcite or a similar mineral called aragonite. Although produced by organisms and containing carbon, the calcite and aragonite are true minerals.

ELEMENTS, ATOMS, AND THE CHEMICAL COMPOSITION OF MINERALS

Chemical elements are the fundamental components of matter. An element cannot be broken into simpler particles by ordinary chemical processes. Most common mineral consist of a small number – usually two to five – of different chemical elements.

A total of 88 elements occur naturally in the Earth’s crust. However, eight elements – oxygen, silicon, aluminum, iron, calcium, magnesium, potassium, and sodium – make up more than 98 percent of the crust.

An atomis the basic unit of an element. An atom consists of a small, dense, positively charged center called a nucleus surrounded by negatively charged electrons.

An electron is a fundamental particle; it is not made up of smaller components. An electron orbits the nucleus, but not in a clearly defined path. An electron travels in a rapidly undulating path and is usually portrayed as a cloud of negative charge surrounding the nucleus. Electrons concentrate in spherical layers, or shells, around the nucleus. Each shell can hold a certain number or electrons.

The nucleus is made up of several kinds of particles: the two largest are positively charged protonsand uncharged neutrons. A neutral atom contains equal number of protons and electrons. Thus, the positive and negative charges balance each other so that a neutral atom has no overall electrical charge.

An atom is most stable when its outermost shell is completely filled with electrons.

But in their neutral states, most atoms do not have a filled outer shell. Such an atom may fill its outer shell by acquiring extra electrons until the shell becomes full. Alternatively, an atom may give up electrons until the outermost shell becomes empty. In this case, the next shell, which is full, then becomes the outermost shell. When an atom loses one or more electrons, its protons outnumber its electrons and it develops a positive charge. If an atom gains one or more extra electrons, it becomes negatively charged. A charged atom is called an ion.

A positively charged ion is a cation. All of the abundant crustal elements except oxygen release electrons to become cations. Atoms with negative charges are called anions.

Atoms and ions rarely exist independently. Instead, they unite to form compounds. The forces that hold atoms and ions together to form compounds are called chemical bonds. Most minerals are compounds.

The 88 elements that occur naturally in the Earth’s crust can combine in many ways to form many different minerals. In fact, about 3500 minerals are known. However, the eight abundant elements commonly combine in only a few ways. As a result, only nine rock-forming minerals (or mineral “groups”) make up most rocks of the Earth’s crust. They are feldspar, quartz, pyroxene, amphibole, mica, the clay minerals, olivine, calcite, and dolomite. The first seven on this list are silicates.

Although about 3500 minerals are known to exist in the Earth’s crust, only a small number – between 50 and 100 – are important because they are common or valuable.

ROCK-FORMING MINERALS

The rock-forming minerals make up the bulk of most rocks in the Earth’s crust. They are important to geologists simply because they are the most common minerals. They are olivine, pyroxene, amphibole, mica, the clay minerals, feldspar, quartz, calcite and dolomite. The first six minerals on this list are actually mineral “groups”, in which each group contains several varieties with very similar chemical compositions, crystalline structures, and appearances.

ACCESSORY MINERALS

Accessory minerals are minerals that are common but usually are found only in small amounts. Chlorite, garnet, hematite, limonite, magnetite, and pyrite are common accessory minerals.

GEMS

A gem is a mineral that is prized primarily for its beauty, although some gems, like diamonds, are also used industrially. Depending on its value, a gem can be either precious or semiprecious. Precious gems include diamond, emerald, ruby, and sapphire. Several varieties of quartz, including amethyst, agate, jasper, and tiger’s eye, are semiprecious gems. Garnet, olivine, topaz, turquoise, and many other minerals sometimes occur as aesthetically pleasing semiprecious gems.

ORE MINERALS

Ore minerals are minerals from which metals or other elements can be profitably recovered. A few, such as native gold and native silver, are composes of a single element. However, most metals are chemically bonded to anions. Copper, lead, and zinc are commonly bonded to sulfur to form the important ore minerals chalcopyrite, galena, and sphalerite.

INDUSTRIAL MINERALS

Several minerals are industrially important, although they are not considered are because they are mined for purposes other then the extraction of metals. Halite is mined for table salt, and gypsum is mined as the raw material for plaster and sheetrock. Apatite and other phosphorus minerals are sources of the phosphate fertilizers crucial to modern agriculture. Many limestones are made up of nearly pure calcite and are mined as the raw material of cement.

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