The most abundant group of minerals in the earth's crust. Quite a lot of them have been used for jewelry; most of them are translucent or opaque but a few form transparent crystals. It can take chemical analysis to tell exactly what particular type of feldspar a material is, since they all are in the 6 to 6.5 range on the Mohs hardness scale and have other characteristics in common.

Feldspars are divided into subfamilies such as the plagioclase (including labradorite, spectrolite) and the orthoclase (including moonstone, adularia) and the oligoclase (including sunstone) families.

Feld"spar` (?), Feld"spath` (?), n. [G. feldspath; feld field + spath spar.] Min.

A name given to a group of minerals, closely related in crystalline form, and all silicates of alumina with either potash, soda, lime, or, in one case, baryta. They occur in crystals and crystalline masses, vitreous in luster, and breaking rather easily in two directions at right angles to each other, or nearly so. The colors are usually white or nearly white, flesh-red, bluish, or greenish.

The group includes the monoclinic (orthoclastic) species orthoclase or common potash feldspar, and the rare hyalophane or baryta feldspar; also the triclinic species (called in general plagioclase) microcline, like orthoclase a potash feldspar; anorthite or lime feldspar; albite or soda feldspar; also intermediate between the last two species, labradorite, andesine, oligoclase, containing both lime and soda in varying amounts. The feldspars are essential constituents of nearly all crystalline rocks, as granite, gneiss, mica, slate, most kinds of basalt and trachyte, etc. The decomposition of feldspar has yielded a large part of the clay of the soil, also the mineral kaolin, an essential material in the making of fine pottery. Common feldspar is itself largely used for the same purpose.


© Webster 1913.

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