Minerals

In ordinary usage, minerals are the natural, non-living materials that compose rocks and are mined from Earth. Examples are metals, gemstones, clays, and ores.

The scientific definition of a mineral is more limited. To be considered a mineral, a substance must be solid under ordinary conditions, thus excluding petroleum and water. Minerals must be single, homogeneous (uniform) substances. Therefore quartz is a mineral, but rocks such as granite, which contain quartz mixed together with other minerals, are not considered minerals. Minerals must have definite chemical formulas, allowing only slight variations. Therefore, a sample of a particular mineral will have essentially the same composition no matter where it is from—Earth, the moon, or beyond. Minerals must be of nonbiological, or inorganic, origin, which excludes coal and peat. Finally, the atoms of which minerals are made must be arranged in orderly rows and stacks; that is, minerals must be made of crystals. Thus, to summarize the scientific definition, a mineral is a naturally occurring, inorganic, homogenous solid with a definite range of chemical composition and an ordered atomic arrangement.

With these restrictions, almost 4,000 different minerals are known, with several dozen new minerals identified each year. Every mineral possesses a combination of chemical composition and crystal structure that makes it unique, and by which it is classified (grouped with similar minerals) and identified. These minerals make up the solid Earth, the moon, and even meteorites. However, only 20 or so minerals compose the bulk of Earth's crust, that is, the part of the solid Earth accessible to human beings, extending from the surface downward to a maximum depth of about 55 mi (90 km). These minerals are often called the "rock-forming" minerals.

Naming mineral groups

Anions, because of their extra electrons, tend to be much larger than cations. Ionic crystals therefore are

built mainly of stacks of anions, with the much smaller cations filling spaces between them. Minerals more closely resemble each other in structure or behavior if minerals with the same anions (rather than cations) are compared. That is why minerals are generally grouped according to their anions, even if the cations may be of greater practical interest. For example, ore minerals are mined for the metals (cations) they contain, which can be changed from ions to neutral atoms of pure metal by a chemical process called smelting. Nevertheless, ore minerals (for example, oxides or sulfides) are grouped according to their non-metal elements.

Silicate minerals and the role of structure

Oxygen and silicon together make up almost three fourths of the mass of Earth's crust. The silicate minerals, a group containing silicon and oxygen atoms, are the most abundant minerals and are the major component of nearly every kind of rock. Silicate compounds make up over 90% of the weight of Earth's crust. Most silicate minerals contain other elements in their formulas; therefore, there is a great variety of silicate minerals. In some rocks such as granite, the different silicate minerals can be seen as the small interlocking crystals of various colors. In other rocks, the mineral grains may be too small to distinguish, but they are usually silicates.

Regardless of composition, all silicates have the same basic building unit, the silica tetrahedron. This consists of a silicon atom bound covalently to four oxygen atoms. The oxygen atoms occupy the corners of a geometrical shape called a tetrahedron. The silicon atom is at its center. The entire unit bears a negative electrical charge, enabling it to form compounds with cations.

Silica tetrahedra can join together by sharing oxygen atoms. The simplest result is two tetrahedra joined at one point or six tetrahedra forming a ring. Ribbons or sheets of silica tetrahedra can be millions of units long. If all four oxygen atoms are shared with neighbors, the tetrahedra form rigid networks that extend over the entire crystal. Such large silicates are inorganic polymers, large molecules built up of a great many similar small units. (The only other element known to form polymers is carbon, and carbon-based polymers are the basis of living things.) The arrangements of tetrahedra affect the properties of the silicate minerals. Garnets are very dense and hard, because their tetrahedra stand alone, bound by strong ionic charge to nearby cations. Beryl forms long, six-sided crystals, which may be colored by traces of metal to form precious emeralds and aquamarines. On the atomic level, beryl contains rings of six tetrahedra, the rings stacked one upon the other with their holes aligned. In muscovite, the tetrahedra are arranged in sheets, with alternating layers of aluminum and potassium atoms between them. The result is flat, flaky crystals, which can easily be separated by hand.

Non-silicate minerals

The so-called non-silicate minerals consist of a variety of different mineral groups each named for a particular anion. Only a few of these minerals contribute much volume to Earth's crust, but many of them are very important minerals for manufacturing and other industrial uses. Most mineralogists recognize ten or so major non-silicate groups and a variable number of lesser groups. Table 2 lists several of the major non-silicate groups.

Native elements

A few minerals, called native elements, contain only one element. These include the so-called native metals, gold, silver and copper, which occur in lumps, veins, or flakes scattered in rocks. Diamond and graphite are both naturally occurring forms of pure carbon. Sulfur, a yellow non-metal, is sometimes found pure in underground deposits formed by hot springs. Although not common, these minerals are economically important.

Hardness

A mineral's hardness is defined as its ability to scratch another mineral. This is usually measured using a comparative scale devised about 200 years ago by Friedrich Mohs. The Mohs scale lists ten common minerals, assigning to each a hardness from 1 (talc) to 10 (diamond). A mineral can scratch all those minerals having a lower Mohs hardness number. For example, calcite (hardness three) can scratch gypsum (hardness two) and talc (hardness one), but it cannot scratch fluorite (hardness four).

Color and streak

Although some minerals can be identified by their color, this property can be misleading because mineral color is often affected by traces of impurities. Streak, however, is a very reliable identifying feature. Streak refers to the color of the powder produced when one mineral is scratched by another, harder mineral. Fluorite, for example, comes in a great range of colors, yet its streak is always white.

Luster

Luster refers to a mineral's appearance when light reflects off its surface. There are various kinds of luster, all having descriptive names. Thus, metals have a metallic luster, quartz has a vitreous, or glassy luster, and chalk has a dull, or earthy luster.

Cleavage and fracture

Some minerals, when struck with force, will cleanly break parallel to planes of weakness in their atomic structure. This breakage is called cleavage. Muscovite cleaves in one direction only, producing thin flat sheets. Halite cleaves in three directions, all perpendicular to each other, forming cubes. A mineral's cleavage directions may reveal the crystal system to which it belongs.

However, most minerals fracture rather than cleave. Fracture is breakage that does not follow a flat surface. Some fracture surfaces are rough and uneven. Others show smooth, concentric depressions, called conchoidal fractures. Conchoidal fracture typically occurs in glasses, which are non-crystalline solids. However, it also occurs in many common crystalline minerals, for example garnet and quartz.

Specific gravity

Two minerals can look alike, yet a piece of one may be much heavier than an identical-sized piece of the other. The heavier one has a higher specific gravity. When pure, each mineral has a predictable specific gravity. Therefore, this property is a very reliable clue to a mineral's identity. A mineral's specific gravity can be thought of as a ratio of its weight to that of an equal volume of water. For example, the specific gravity of gold is 19.3 (19.3 times that of water), while quartz is 2.65.

To determine a mineral's specific gravity, it is necessary to weigh a sample (using grams), then measure its volume (in cubic centimeters). The weight divided by volume is the density. To calculate specific gravity, divide the mineral's density by that of water. Since the density of water is one gram per cubic centimeter, specific gravity and density are equal (provided all measurements are made using the metric system.)

Other identifying properties

Some minerals have unusual properties that further aid identification. Fluorescent minerals viewed under ultraviolet light glow with various colors. Phosphorescent minerals glow in the dark after exposure to ordinary light. Triboluminescent minerals give off light when crushed or hit. Several minerals containing iron, nickel, or cobalt are magnetic. Over 100 minerals contain uranium, thorium, or other radioactive elements and are therefore radioactive. These are only a few of the unique properties that can be used to identify minerals. Finally, an experienced mineralogist will take into account the location in which an unknown mineral is found. The nature of the surrounding rocks and the presence of other minerals and elements all provide clues to help in identification.