feldspar

(redirected from Feldspars)
Also found in: Dictionary, Thesaurus, Medical.

feldspar

(fĕl`spär, fĕld`–) or

felspar

(fĕl`spär), 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 granitegranite,
coarse-grained igneous rock of even texture and light color, composed chiefly of quartz and feldspars. It usually contains small quantities of mica or hornblende, and minor accessory minerals may be present.
..... Click the link for more information.
, gneissgneiss
, coarse-grained, imperfectly foliated, or layered, metamorphic rock. Gneiss is characterized by alternating light and dark bands differing in mineral composition and having coarser grains than those of schist.
..... Click the link for more information.
, basaltbasalt
, fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color. Basalt is an igneous rock, i.e., one that has congealed from a molten state.
..... Click the link for more information.
, and other crystalline rocks, and are essential constituents of most igneous rocks. Feldspars weatherweather,
state of the atmosphere at a given time and place with regard to temperature, air pressure (see barometer), wind, humidity, cloudiness, and precipitation. The term weather
..... Click the link for more information.
 to yield a large part of the clay found in soils.

Feldspar crystals are either monoclinic or triclinic (see crystalcrystal,
a solid body bounded by natural plane faces that are the external expression of a regular internal arrangement of constituent atoms, molecules, or ions. The formation of a crystal by a substance passing from a gas or liquid to a solid state, or going out of solution (by
..... Click the link for more information.
), 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.

Feldspar

 

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.

REFERENCES

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.

A. S. MARFUNIN

feldspar

[′fel‚spär]
(mineralogy)
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.

feldspar

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.

feldspar

, 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
References in periodicals archive ?
Hakro [8] reported the sediments of Bara Formation from Ranikot area are composed of quartz (low), hematite, potash feldspar, gypsum and goethite minerals.
John Attanasio, Co-founder of Feldspar Studios, said, We learned a lot about the challenges of the current Hollywood process during the decades we spent in traditional content development and commercialization.
To date, Fortispar has proven to be a substitute for the commonly used potassium feldspars Custer and G-200 EU.
The results indicate that SiO2 and Al2O3 are most abundant with dominance of quartz, feldspar and clay.
2], quartz, feldspars or clay and mica minerals, was due to formation of concretions isolated from the post-diagenetic and epigenetic changes.
In a pocket which had been opened shortly before, good crystals of smoky quartz, feldspar and garnet rested on a thin feldspar layer.
Importantly, in the MWG feldspars with variable textures are observed commonly juxtaposed in the same hand sample or thin section (see Figs.
Quartz and feldspar were the predominant mineral grains identified within the clay matrix.
The study of the efficiency of (optical) response by stimulation means the measurements of the OSL stimulation spectra of quartz and alkali feldspars in the widest possible spectral region.
2) It has been used as a direct one-for-one substitute in clay body and glaze formulas for G-200 and G-200 HP as they all are potassium based feldspars.