Minerals are naturally occurring solid substances formed by geologic movements in the Earth. Their main defining characteristics are that 1) they are inorganic (composed of non-living matter); 2) they have a crystal structure; and 3) they have a unique chemical composition. The type of mineral is determined both by its crystal structure and its chemical composition. A crystal structure occurs when the atoms inside the mineral are ordered in a geometric pattern that repeats itself throughout the mineral. All crystal structures fit into one of 14 possible "lattice"-regular pattern arrangements of atoms, which lattices can be detected by X-rays.
A mineral's physical traits are influenced by its crystal form. For example, both diamond and graphite are composed of the same element (carbon), but the former is the hardest mineral while the latter is soft. This is because graphite's crystal structure arranges the carbon atoms in sheets that can slide past each other, while diamond's carbon atoms are arrayed in a strong, interlocking network.
Two minerals with identical crystal structures can have different chemical compositions. Thus, halite and galena share the same crystal structure but are composed of different chemicals. Conversely, two minerals with the same chemical ingredients can differ in their crystal structure. For example, pyrite and marcasite both are made of iron sulfide, but the arrangement of their atoms differs.
According to the International Mineralogical Association, 4,000 minerals have been identified to date. Only about 150 of them are plentiful, and about 50 are classified as "occasional." The remainder are rarely found, some consisting of only small grains of rock.
Minerals are often found as components of rocks, which may contain organic matter as well Some rocks consist wholly of one mineral, such as calcite in limestone rock. Other rocks may host many minerals. Almost all of the rocks visible today contain one or more of a group of about 15 minerals, including quartz, mica, and felspar.
The kinds of minerals found in any given rock are determined by three factors. First, the rock's chemical composition must be hospitable to a particular mineral. For example, rocks containing silicon will likely contain quartz. Second, the conditions under which the rock was formed will influence the kinds of minerals found in the rock. Thus, rock born from volcanic movements at high temperatures and pressures may contain granite. Third, mineral distribution is affected by the geological stages through which the rock passed before reaching its present state. For example, exposure to moisture and acids may decay some minerals and cause others to take their place. During the changes from one ecological stage to another, the rock may disintegrate into sand or soil.
Mineralogists classify minerals according to either physical properties or chemical composition. Minerals have numerous measurable physical properties. Hardness is measured on the "Mohs scale," which ranks hardness from one to 10. Any mineral can be cut or marked by a mineral with a higher ranking on the Mohs scale. Thus a diamond, with a rank of 10, can cut into quartz, with a rank of 7. Luster measures the reflection of light by the surface of the mineral. Metals have a higher luster than gypsum, which has a porous surface. Cleavage refers to the way a mineral splits apart along its natural grain, and fracture refers to its breakage against its natural cleavage planes. Streak is the color of the residue left by a mineral as it is rubbed across a special plate. Specific gravity measures the density of the mineral; it is computed by comparing the mass of the mineral to the mass of an equal volume of water.
Minerals can also be classified by their chemical characteristics. The most frequently occurring minerals are called silicates because of their large shares of silicon and oxygen. Almost all rocks fit this category. The second most common minerals are carbonates, which contain carbon and oxygen. Carbonates are found on the ocean floor as the deposits of decayed plankton. Another grouping, halides, are found where water has evaporated, as in dried lake beds and landlocked seas such as the Great Salt Lake in Utah. Other common classes include sulfates, oxides, sulfides, and phosphates.