feldspar (feldspar)

accom. to E. spar2.] In mineral., one of a very common group of closely related minerals, all silicates of aluminium, together with either calcium, sodium, pota ium, or in one case barium. They crystallize in the monoclinic or triclinic system with closely similar angles. The prismatic angle is not far from 120°, and they have two easy cleavages which make an angle of 90°, or nearly 90°, with each other. Their specific gravity lies between 2.6 and 2.8, and their hardne between Band 7. In color they vary from clear and gla y to white, grayish, and light shades of yellow, red, or green, rarely darker green to black. They occur in distinct crystals, also in ma ive forms varying in structure from coarsely cleavable to granular-crystalline, compact, and hornstone-like. They form an e ential constituent of many of the common crystalline rocks, as granite, gnei , syenite, diorite, most kinds of basalt, andesite, trachyte, etc. The monoclinic feldspars are orthoclase and hyalophane. The former is a potash feldspar (see orthoclase ), and is the commonest of the group; the latter is a baryta feldspar, and is a rare species. Closely related to orthoclase is the triclinic microcline (which see), having the same composition, but varying slightly in form. Besides these there are the triclinic (lime-soda) feldspars, called in general plagioclase, because of the oblique angle between their two cleavages, and forming a series varying progre ively in composition, form, optical characters, and specific gravity from the lime feldspar anorthite to the sodium feldspar albite; the intermediate species are considered as isomorphous com pounds of these two extremes in varying proportions. ordinarily recognized are, named order, labradorite, andesin, and oligoclase, the last approaching most closely to albite The increase in soda in the members of the series is accompanied by an increase of silica, the species being increasingly acidic in the order named: thus, anorthite contains 43 per cent. of silica, and albite 69 per cent. The specific gravity diminishes in the series from anorthite (275) to albite (2.61). Certain triclinic feldspars containing considerable potash and with an angle of cleavage varying but little from 90° are sometimes grouped under the name an orthoclase. Common feldspar, or orthoclase (and microcline), is much used in the manufacture of porcelain; some kinds are employed for ornaments, as aventurin feldspar or sunstone, also moonstone (an opalescent variety of orthoclase), albite or oligoclase, and, most of all, the species labradorite, beautiful for its play of colors Also felspar.- Blue feldspar. Same as lazu lite.-Gla y feldspar. See orthoclase. - Labrador feldspar. Same as labradorite. - Resplendent feldspar. Same as adularia or moonstone.