FELDSPAR

any of a group of aluminosilicate minerals that contain calcium, sodium, or potassium. Feldspars are the most common minerals in the Earth's crust. Feldspars constitute the major component in nearly all igneous rocks found on the Earth, on the Moon, and in meteorites. They also are common in metamorphic and clastic sedimentary rocks. The complex chemical and structural properties of feldspars make them useful for interpreting the origins of rocks. The basic structural unit of a feldspar mineral is a tetrahedron composed of four oxygen atoms surrounding either a silicon or aluminum atom. Each tetrahedral unit shares an oxygen atom with another tetrahedral unit to form a three-dimensional framework. If all tetrahedral units contained silicon, the framework would be neutral; however, in fact from one-fourth to one-half of the units contain aluminum, resulting in a net negative charge. To balance this charge either monovalent sodium (Na) and potassium (K) or divalent calcium (Ca) are located in the cavities. Natural feldspars can be represented by two compositional series known as the alkali feldspars and the plagioclase feldspars. The alkali feldspars show a continuous composition series between orthoclase (KAlSi3O8) and albite (NaAlSi3O8), while the plagioclase series is continuous between albite and anorthite (CaAl2Si2O8). Only limited substitution occurs between the potassium- and calcium-rich members. An important property of the feldspars is the complex ordering of silicon and aluminum atoms into the nonequivalent tetrahedral sites. At high temperatures the distribution of atoms is random or disordered, but with decreasing temperature the aluminum and silicon atoms tend to order into certain sites (K-feldspar: sanidine microcline; Na-feldspar: high albite low albite). The degree at which the feldspars are ordered represents an important measurable parameter for interpreting the thermal history of a feldspar. any of a group of aluminosilicate minerals that contain calcium, sodium, or potassium. Feldspars make up more than 50 percent of the Earth's crust, and professional literature about them constitutes a large percentage of the literature of mineralogy. Of the more than 3,000 known mineral species, less than 0.1 percent make up the bulk of the Earth's crust and mantle. These and an additional score of minerals serve as the basis for naming most of the rocks exposed on the Earth's surface (Table 16). Nearly all common rocks can be named by determining the presence (versus absence) of one, two, or a few of the minerals whose names are given in boldface in Table 16. Most of the less common rocks can be named by similarly identifying the additional half dozen minerals whose names are given in regular type. Essentially all rocks can be named as professional geologists name them if, in addition, the presence of the minerals whose names are in italics is known. Each of the common rock-forming minerals can be identified on the basis of its chemical composition and its crystal structure ( i.e., the arrangement of its constituent atoms and ions). The nonopaque minerals can also be identified by their optical properties. Fairly expensive equipment and sophisticated procedures, however, are required for such determinations. Therefore, it is fortunate that macroscopic examination, along with one or more tests, are sufficient to identify these minerals as they occur in most rocks. The following descriptions include basic chemical and structural data and the properties used in macroscopically based identifications. Optical data, which are not included in these descriptions, are available in mineralogy books. Two important rock-forming materials that are not minerals are major components of a few rocks. These are glass and macerals. Glass forms when magma (molten rock material) is quenchedi.e., cooled so rapidly that the constituent atoms do not have time to arrange themselves into the regular arrays characteristic of minerals. Natural glass is the major constituent of a few volcanic rockse.g., obsidian. Macerals are macerated bits of organic matter, primarily plant materials; one or more of the macerals are the chief original constituents of all the diverse coals and several other organic-rich rocks such as oil shales. In the classification of igneous rocks of the International Union of Geological Sciences (IUGS), the feldspars are treated as two groups: the alkali feldspars and the plagioclase feldspars. The alkali feldspars include orthoclase, microcline, sanidine, anorthoclase, and the two-phase intermixtures called perthite. The plagioclase feldspars include members of the albite-anorthite solid-solution series. Strictly speaking, however, albite is an alkali feldspar as well as a plagioclase feldspar. Additional reading William L. Brown (ed.), Feldspars and Feldspathoids: Structures, Properties, and Occurrences (1984); Joseph V. Smith and William L. Brown, Feldspar Minerals, 2nd rev. and extended ed. (1988), the most comprehensive, well-illustrated treatment. R.V. Dietrich