FELDSPARS, the most important group of silicate minerals. They are characterized by monoclinic or closely related triclinic crystal, lization, the frequent occurrence of twin crystals, cleavage in two similar directions in clined at an angle of 90 degrees or nearly 90 degrees, a hardness of about 6, a specific gravity between 2.5 and 2.9, a light color, usually white, pale yellow or green, or flesh-pink, a white streak and a low relief and low order of inter ference colors as seen in thin sections with the polarizing microscope.
Uses.— The chief usefulness of the potash and potash-soda feldspars results from the fact that within certain limits of temperature they melt without becoming quite fluid and on cool ing form a strong colorless or very light colored glass. Because of this quality, feldspar is ex tensively used in the manufacture of porcelain, serving as a flux to bind together the other constituents, clay and flint. It is also one of the principal ingredients in the glaze for china ware and tiles. In almost all forms of pottery it is used both in the body and in the glaze in the former in the proportion of 10 to 30 per cent, in the latter, 35 to 50 per cent. In the manufacture of corundum and emery wheels, feldspar is used as a binder. Small quantities of the purest grade are used in making artificial teeth. It is one of the essen tial ingredients of opalescent glass. In powdered form it supplies the abrasive in scouring soaps and window cleaners. In crushed form it be comes poultry grit and a surface for the ((gran itoiclp varieties of ready roofing. Many at tempts have been made to extract the potash from feldspar for fertilizing purposes, but with out success from a commercial point of view. The beautiful play of colors in labradorite has led to its use for table tops and inlaid work The gem moonstone is a variety of feldspar, usually oligoclase; the sunstone is averturine feldspar. The peculiar radiance of these gems is caused by oriented lamellar inclusions which reflect the light with great intensity.
Production.— The total amount of feldspar sold in the open market in the United States in 1916 was 118,465 long tons, valued at $404, 689. There was, besides, a large production which was ground and consumed by the in dustrial concerns mining it and of which no report was made. The largest production was recorded for North Carolina — 30,955 tons. In Maine the total marketed was 25,000 tons; and in Maryland, 21,364 tons. The Maryland prod uct brought much the highest price. About
82 per cent of the total output was used in the ceramic industries; about 7 per cent in the manufacture of roofing and for the facing of concrete; and about 3 per cent in the makng of glass. The 8 per cent not accounted for is probably that used in experimenting in the endeavor i to release the potash from its com bination n feldspar, so as to render it avail able as a fertilizer in the agricultural industry.
Occurrence.— The feldspars are the most abundant mineral constituent of the majority of the igneous rocks and are abundant in many metamorphic rocks. They are also present in some sedimentary rocks. The limesoda feld spars are more abundant in the igneous rocks, and the potash feldspars in the meta morphic rocks. In the sedimentary rocks they occur as fragments, especially in the arkose sandstones and graywackes. They occur also in veins, where they were deposited from aqueous solutions. The great pegmatite veins or dikes are believed by some geologists to have been deposited in this way. Feldspars are known also to have been deposited from the gaseous exhalations from volcanic vents.
The feldspars now occupy a most important position from a scientific standpoint, since the present scheme of classification of igneous rocks is chiefly based on the identity of the contained feldspar (see Rocxs). Orthoclase is an es sential constitutent of granite, syenite and porphyry, while it is also one of the constituents of gneiss and the other granitoid rocks. Its variety sanidine is the feldspar of trachyte and phonolite. Microcline occurs similarly and is not easily distinguished from orthoclase except by an optical examination, though its two prominent varieties, amazonstone and chester lite, are quite distinct. Albite is an essential constituent of diorite and is one of the com ponent minerals of many crystalline rocks, oc curring associated with orthoclase and micro line in much granite and gneiss. Albitic granite is often the matrix of the rarer minerals, and especially of some of the gems. Oligoclase abounds in granite, syenite, gneiss, diorite, trachyte, andesite and diabase, often being as sociated with orthoclase. Labradorite is an es sential constituent of various basic, eruptive rocks, in which it is commonly associated with sotne member of the pyroxene or amphibole groups, as in norite, gabbro, diabase and basalt. Anorthite occurs in gabbro, basalt and porphyry.