MINERAL

county, west-central Nevada, U.S., on the California border (southwest). It consists mostly of arid mountains (including the Wassuk Range and the Excelsior Mountains) and valleys, but Walker Lake lies in the west-central part of the county and part of Toiyabe National Forest in the south. The large Walker River Indian Reservation is in the northwest. The county was formed in 1911, and the county seat is Hawthorne. The economy depends on gold and silver mining and various military- and government-related activities. Area 3,757 square miles (9,730 square km). Pop. (1990) 6,475. any naturally occurring homogeneous solid that has a definite chemical composition and a distinctive internal crystal structure. Minerals are usually formed by inorganic processes. While this definition is applicable in most cases, there are certain notable exceptions. The term mineral is also applied to certain organic substances, such as coal, graphite, oil, and natural gas, that are obtained from the Earth for commercial use. Synthetic equivalents of various minerals, such as emeralds and diamonds, can be produced in the laboratory for experimental or commercial purposes. Although most minerals are chemical compounds, a small number (e.g., sulfur, copper, gold) are elements. The composition of a mineral can be defined by its chemical formula, and the identity of its anionic group determines the group into which the mineral is classified. For example, the mineral halite (NaCl) is composed of two elements, sodium (Na) and chlorine (Cl), in a 1:1 ratio; its anionic group is chloride (Cl-)a halideso halite is classified as a halide. Minerals can thus be classified into the following major groups: native elements, sulfides, sulfosalts, oxides and hydroxides, halides, carbonates, nitrates, borates, sulfates, phosphates, and silicates. Silicates are the most commonly occurring minerals because silica is the most abundant constituent of the Earth's crust (about 59 percent). A mineral crystallizes in an orderly, three-dimensional geometric form, so that it is considered to be a crystalline material. Along with its chemical composition, the crystalline structure of a mineral helps determine such physical properties as hardness, colour, and cleavage. Minerals combine with each other to form rocks. For example, granite consists of the minerals feldspar, quartz, mica, and amphibole in varying chemical ratios. Rocks are thus distinguished from minerals by their heterogeneous composition. A mere 100 of the several thousand known types of minerals constitute the main components of rocks. Minerals are treated in a number of articles. For the composition, physical properties, occurrence, and classification of important rock-forming minerals, see mineral and rock. For the characteristics, origins, and distribution of the major mineral fuels, see coal; natural gas; petroleum. For the methods of removing and refining or treating industrially important minerals, see copper processing; iron; metallurgy; mineral deposit; mineral processing; mining; steel. For minerals used primarily for personal adornment, see jewelry. For mineralogy, the scientific study of minerals, see geologic science. Additional reading Standard mineralogical reference works include W.A. Deer, R.A. Howie, and J. Zussman, Rock-forming Minerals, 5 vol. (196263), with a 2nd ed. in progress (1978 ); Manual of Mineralogy (after James D. Dana), 20th ed. by Cornelis Klein and Cornelius S. Hurlbut, Jr. (1985); Annibale Mottana, Rodolfo Crespi, and Giuseppe Liborio, Simon and Schuster's Guide to Rocks and Minerals (also published as The Macdonald Encyclopedia of Rocks and Minerals, 1978; originally published in Italian, 1977); and Michael Fleischer and Joseph A. Mandarino, Glossary of Mineral Species, 1991, 6th ed. (1991). Useful texts and monographs include Harvey Blatt, Sedimentary Petrology (1982); Richard V. Dietrich and Brian J. Skinner, Rocks and Rock Minerals (1979); Robert M. Garrels and Charles L. Christ, Solutions, Minerals, and Equilibria, 2nd ed. (1990); Paul C. Hess, Origins of Igneous Rocks (1989); Cornelis Klein, Minerals and Rocks: Exercises in Crystallography, Mineralogy, and Hand Specimen Petrology (1989); Anthony R. Philpotts, Principles of Igneous and Metamorphic Petrology (1990); and Tibor Zoltai and James H. Stout, Mineralogy: Concepts and Principles (1984). Cornelis Klein Classification of minerals Since the middle of the 19th century, minerals have been classified on the basis of their chemical composition. Under this scheme, they are divided into classes according to their dominant anion or anionic group (e.g., halides, oxides, and sulfides). Several reasons justify use of this criterion as the distinguishing factor at the highest level of mineral classification. First, the similarities in properties of minerals with identical anionic groups are generally more pronounced than those with the same dominant cation. For example, carbonates have stronger resemblances to one another than do copper minerals. Secondly, minerals that have identical dominant anions are likely to be found in the same or similar geologic environments. Therefore, sulfides tend to occur together in vein or replacement deposits, while silicate-bearing rocks make up much of the Earth's crust. Third, current chemical practice employs a nomenclature and classification scheme for inorganic compounds based on similar principles. Investigators have found, however, that chemical composition alone is insufficient for classifying minerals. Determination of internal structures, accomplished through the use of X rays, allows a more complete appreciation of the nature of minerals. Chemical composition and internal structure together constitute the essence of a mineral and determine its physical properties; thus, classification should rely on both. Crystallochemical principlesi.e., those relating to both chemical composition and crystal structurewere first applied by the British physicist W. Lawrence Bragg and the Norwegian mineralogist Victor Moritz Goldschmidt in the study of silicate minerals. The silicate group was subdivided in part on the basis of composition but mainly according to internal structure. Based on the topology of the SiO4 tetrahedrons, the subclasses include framework, chain, and sheet silicates, among others. Such mineral classifications are logical and well-defined. The broadest divisions of the classification used in the present discussion are (1) native elements, (2) sulfides, (3) sulfosalts, (4) oxides and hydroxides, (5) halides, (6) carbonates, (7) nitrates, (8) borates, (9) sulfates, (10) phosphates, and (11) silicates. Native elements Apart from the free gases in the Earth's atmosphere, some 20 elements occur in nature in a pure (i.e., uncombined) or nearly pure form. Known as the native elements, they are partitioned into three families: metals, semimetals, and nonmetals (see Table 4). The most common native metals, which are characterized by simple crystal structures, make up three groups: the gold group, consisting of gold, silver, copper, and lead; the platinum group, composed of platinum, palladium, iridium, and osmium; and the iron group, containing iron and nickel-iron. Mercury, tantalum, tin, and zinc are other metals that have been found in the native state. The native semimetals are divided into two isostructural groups (those whose members share a common structure type): (1) antimony, arsenic, and bismuth, with the latter two being more common in nature, and (2) the rather uncommon selenium and tellurium. Carbon, in the form of diamond and graphite, and sulfur are the most important native nonmetals.