Minerals

Characteristics and Classification

The characteristic criteria of minerals are implicit in their geological definition: Minerals are naturally occurring, are solid, have a definite chemical composition and an orderly crystal structure, and are typically inorganic. Quartz (hexagonal SiO2), halite (or rock salt, cubic NaCl; see first photo), and gold (cubic Au) are familiar examples. There are, however, a few exceptions to the rule. For example, native mercury is considered a mineral even though it is normally liquid at ordinary temperatures, and opal is a mineral, although it is frequently amorphous and contains a variable amount of water. But for the vast majority of minerals, the IMA definition works well. Thus, diamond is a mineral, but coal is not; ice is a mineral, but water is not.

The outward appearance of these halite crystals is a consequence of their internal cubic structure.

Source: Photograph taken by author of specimen at the A. E. Seaman Mineral Museum.

Together, the chemical composition and crystal structure of a mineral define it as a particular species and determine all of its chemical and physical properties. Mineralogists classify minerals according to their chemistry (e.g., sulfides, oxides, carbonates). Within each chemical division are individual groups of minerals based on similarity of their atomic structures. The classification hierarchy is simple, consisting of only three levels: (1) group, (2) species, and (3) variety. A common example is the calcite group, which includes various species such as calcite, siderite, magnesite, and rhodochrosite, among others (see second photo). These minerals all have similar structures and a general formula M2+CO3, where M2+ is a divalent metal cation, such as Ca2+, Fe2+, Mg2+, or Mn2+. When Ca2+ is the dominant cation, the species is calcite; if Fe2+ predominates, the species is siderite; and Mg2+ and Mn2+ result in magnesite and rhodochrosite, respectively. Sometimes a particular color variant or other distinguishing feature earns a species a varietal name. Thus, colorless calcite is known as “Iceland spar,” whereas red corundum is ruby, and purple quartz is amethyst.

Minerals derive their names from several sources, and those proposed for new species being described today must be approved by the IMA before they are generally accepted by the scientific community. There are few hard-and-fast rules dictating how a mineral should be named. Some are named after people (e.g., mill-erite [see third photo] for W. H. Miller, a famous British mineralogist), while others are named for geographic places (e.g., calumetite for Calumet, Michigan, where it was first found). Minerals such as cavansite (a calcium vanadium silicate) or nahpoite (NaHPO4) derive their names from their chemical compositions, while others, such as hematite, are named for their physical [Page 1907]appearance. Hematite commonly occurs in earthy red masses (in Greek, haimatitis means blood red). Magnetite is highly magnetic, and azurite is azure blue. One species, minrecordite, is even named after a popular journal, the Mineralogical Record.

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