FELDSPAR, feVspiir ( Ger. FcRspath, from Feld, field + Spath,M1I13. spat, laminated stone: connected by popular etymology with spar). A group of minerals, all the species of which con sist of silicate of aluminum in combination with one or more of the following metals: Sodium, potassium, calcium, or barium. The various species and their numerous varieties, all of which occur originally in igmeous or metamorphic rocks. resemble each other closely in respect of general form, texture,. etc. Their hardness is not quite, that of quartz. and their specific gravity varies between 2.5 and 2.9. while their colors range from white, through pink, yellow, green, anal red, In dark greenish-brown. Aecord ing to their crystalline form, the different varie ties ail' divided into monoclinic and triclinic feld spar. and under each of these beads they are We species, according to their compo sition. Orthorlase, a potash feldspar, is the most eommon kind, occurring in monoelinie erys tills of glassy, white, pink, and Ilesh-red color, in granite. 11111.1sA, syenite, and many voleanie reeks. .Ida/ria, or moonstone, is a white variety of orthoclase that, because of its pearly, opalescent refiectiens, is eat as it gem. Ilir•arline, a tri clinic potash feldspar, has a green variety. amazon stone, found in granite at Pike's l'eak, Colo., and in the Ural Mountains; this variety is also cut as a gem stone.
The plagioclase feldspars, embracing albite. oligoelase, andesine, labradorite, anorthite, and their several varieties, are soda-lime feldspars that vary in composition between the albite, which is high in soda and low in lime, and anor Mite, which is low In soda and high in lime. Among the members of this group. labradorite is the most important, as it forms a large part of the volcanic rocks of Pre-Cambrian age, such as those in the Adirondack region of New York and in many other similar localities of America and Europe. Some varieties of lahradorite ex hibit on the polished surface a beautiful play of iridescent colors, and have on this account been employed to a considerable extent as ornamental stones.
All the feldspars weather readily under the action of both atmospheric agencies and acidu lated surface waters, and yield a form of clay known as kaolin, which is of great economic importance in the pottery indnstries. Unaltered feldspar, especially such as occurs in veins in granite and gneiss. is quarried, crushed, washed. and made into a paste that is applied to the sur face of pottery to form the glaze. See Kaoust; PORCELAIN : POTTERY and the names of the different forms of feldspar mentioned above.