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feldspar (fĕlˈspär, fĕldˈ–) or felspar (fĕlˈspär), an abundant group of rock-forming minerals which constitute 60% of the earth's crust. Chemically the feldspars are silicates of aluminum, containing sodium, potassium, iron, calcium, or barium or combinations of these elements. Feldspar is found in association with all rock types, including granite, gneiss, basalt, and other crystalline rocks, and are essential constituents of most igneous rocks. Feldspars weather to yield a large part of the clay found in soils.

Feldspar crystals are either monoclinic or triclinic (see crystal), and all show clean cleavage planes in two directions. Orthoclase feldspars have cleavage planes that intersect at right angles; triclinic feldspars, including the plagioclase feldspars (e.g., albite, anorthite, and labradorite) and microcline, have cleavage planes that form slightly oblique angles. Pure feldspar is colorless and transparent but the mineral is commonly opaque and found in a variety of colors.

Orthoclase and microcline are called potassium or potash feldspars. They usually range from flesh color to brick red, although other colors are found, and are used in the making of porcelain and as a source of aluminum in making glass. Moonstone is usually a milky, bluish variety of orthoclase used as a gem, and a green variety of microcline known as amazonite, or Amazon stone, is used for ornamental purposes. The plagioclase feldspars are most commonly gray and occasionally red. A milky variety of plagioclase feldspar oligoclase also is used as the gem moonstone, and a reddish or golden variety that exhibits flashes of reddish color is used as the gem sunstone. Another form of feldspar, labradorite, exhibits a play of colors, which makes it valuable for decorative purposes.

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the most widely distributed group of rock-forming minerals, constituting more than 50 percent of earth and lunar rocks; they are also found in meteorites. The composition of feldspars is basically determined by the component ratio in a ternary system: NaAlSi3O8—KAlSi3O8—CaAl2Si2O8; that is, feldspars are aluminosilicates of Na, K, and Ca, with admixtures of Ba, Sr, Pb, Fe, Li, Rb, Cs, Eu, Ce, and other elements. All feldspars have a basic three-dimensional framework composed of tetrahedral (Al, Si)O4 groups in which one-third to one-half of the Si atoms are replaced by Al. Univalent K+ and Na+ cations, with an Al/Si ratio of 1:3, or bivalent Ca2+ and Ba2+ cations, with an Al/Si ratio of 1: 2, are arranged in the large vacancies within this framework.

Two series of solid solutions are differentiated in the feldspar group: anorthoclases, or alkali feldspars (KAISi3O8—NaAlSi3O8), and plagioclases (NaAlSi3O8–CaAl2Si2O8). The barium feldspar BaAl2Si2O8, known as celsian, is rare, as are solid solutions with compositions between KAlSi3O0 and BaAl2Si2O8, known as hyalophane and containing up to 10–30 percent Ba.

Figure 1

Many feldspar varieties result from complex variation in composition (see Figure 1), the ordering of Al and Si distribution according to structural position, the decomposition of solid solutions, and submicroscopic twinning. The following are examples of potassium feldspars: (1) sanidine, with monoclinic symmetry and disordered Si and Al distribution; (2) maximum microcline (triclinic), with fully ordered Si and Al distribution; (3) intermediate microclines; and (4) orthoclase (assumed to be pseudomonoclinic), composed of submicroscopic twinned triclinic domains.

High-temperature anorthoclases are disordered and form a continuous series of solid solutions. Low-temperature anorthoclases decompose to yield perthites—regular intergrowths of microcline or orthoclase—and sodium feldspars, or albite. All plagioclase varieties are high-temperature (disordered with respect to Al and Si distribution), low-temperature (ordered), or intermediate.

Changes in the degree of ordering and the composition of plagioclases occur with the retention of triclinic symmetry with extremely complex structural changes and the formation of two unmixed regions, which in many oligoclases and labradorites is accompanied by iridescence.

Precise determination of the composition and structural state (ordering) of feldspars is carried out by means of optical orientation diagrams and diagrams of optic axial angles, measured on a Fedorov, or universal, stage, as well as by X-ray methods (diffractometry).

Plagioclases and microclines are nearly always polysynthetically twinned; that is, they form microscopic intergrowths of several elements in accordance with various characteristic laws of twinning.

The tabular or prismatic habit of feldspars in rocks is determined by well-developed {010} and {001} faces, along which perfect cleavage is formed at a right or nearly right angle, as well as by {110} faces. Feldspars have a hardness of 6–6.5 on Mohs’ scale and a density of 2,500-2,800 kg/m3. They have no color of their own; the varied coloration (gray, pink, red, green, black) is due to the presence of very fine inclusions of hematite, iron hydroxides, hornblende, pyroxene, and other minerals; the bluish green color of amazonite and the green color of microcline are associated with the electron center of Pb, substituting for K. Bands of Pb2+, Fe3+, Ce3+, and Eu2+ are distinguished in the luminescence spectra of feldspars. Electron paramagnetic resonance spectra of feldspars are used to determine the electron centers Ti3+ and the hole centers Al—O-—A1, formed through the entrapment of an electron or hole, respectively, by lattice defects.

Feldspars serve as the basis for rock classification. The most important types of rocks are chiefly composed of feldspars: (1) intrusive rocks—granites, syenites (alkali feldspars and plagioclases), gabbros, and diorites (plagioclases); (2) effusive rocks—andesites and basalts; and (3) metamorphic rocks—gneiss, crystalline schists, contact- and regional-metamorphosed rocks, and pegmatites. In sedimentary rocks, feldspars occur as fragmented grains and new formations (authigenic feldspars). Only plagioclases are present in lunar rocks (lunar basalts, gabbros, anorthosites).

Because of the wide variations in composition and properties, feldspars are valuable in the geological and petrographie studies of magmatic and metamorphic rock masses. The isotope 40.K/40Ar ratio of anorthoclases is used to determine the absolute age of rocks.

Alkali feldspars in pegmatites and small-veined rocks are used in the production of ceramics, glass, porcelain, and faience. Feldspar rocks (labradorites) are used as facing material. Amazonite and moonstone (iridescent oligoclase) are decorative stones.


Deer, W. A., R. A. Howie, and J. Zussmann. Porodoobrazu-iushchie mineraly, vol. 4. Moscow, 1966. (Translated from English.)
Marfunin, A. S. Polevye shpaty—fazovye vzaimootnosheniia, opticheskie svoistva, geologicheskoe raspredelenie. Moscow, 1962.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A group of silicate minerals that make up about 60% of the outer 9 miles (15 kilometers) of the earth's crust; they are silicates of aluminum with the metals potassium, sodium, and calcium, and rarely, barium.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


A group of igneous minerals, all of which are softer than quartz, having the chemical composition of calcium silicates, potassium silicates, or sodium-aluminum silicates.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.


, felspar
any of a group of hard rock-forming minerals consisting of aluminium silicates of potassium, sodium, calcium, or barium: the principal constituents of igneous rocks. The group includes orthoclase, microcline, and the plagioclase minerals
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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