FELDSPAR (Ger. feldspath, field-spar), a mineral extremely abundant in almost all parts of the world. It is a principal constituent of many rocks, as granite; gneiss, greenstone, trachyte, etc.; and clays seem very generally to have resulted, at least in great part, from its decomposition, It occurs both massive and crystallized, in rhomboidal, pyra midal, and prismatic crystals, often having their edges and angles truncated, and thus very variously modified. There are many different kinds of F., which mineralogists have recently attempted to arrange in mineral species, distinguished by physical and chemical characters, and also by geognostic position, and by the groups of minerals with which they are associated. For these mineral species new names have been invented, orthoclase, oligoclase, albite, labradorite, etc. All the feldspars are anhydrous silicates of alumina, and of an alkali or lime. Orthoclase, and the other more silicious feldspars containing potash, abound chiefly in granite and the plutonic rocks; the less silicious, containing soda and lime, characterize the volcanic rocks—" as labradorite the basaltic group, glassy feldspar the trachytic." All the kinds of P. are so hard as not to be easily scratched with a knife, and are fused with difficulty. Some of them are solu ble, some insoluble in acids.—The kind known as COMMON 'F.—referred to orthoclase— is generally white or flesh-colored, has a glassy and somewhat pearly luster, is translu cent at least on the edges, and has an uneven or splintery fracture. Crystals four or five inches long are found in Aberdeenshire. This variety, under the name of petunse or petuntze, is used by the Chinese in the manufacture of porcelain; along with some of the quartz which is associated with it in the rock. It is used, with other materials, as a
flux; and alone to form an enamel or glassy covering, without which the porcelain would absorb moisture and grease, and would be unfit for any except mere ornamental purposes.—ADULARIA is a transparent and almost colorless variety of F., often cut as an ornamental stone, the finest varieties, of which one is known as MooxsroNE, being prized almost as gems. A variety, found among rolled stones in Ceylon, and remark able for the reflection of a pearly light, has been sometimes confounded with cat's eye,— AVANTUMNE F. is similar to the variety of quartz called avanturine (q.v.) in the play of light which it exhibits, and which is said to be owing to minute crystals of specular or titanic iron. It is much esteemed as an ornamental stone. A variety with golden yellow specks, called SIINSTONE, is very rare and very beautiful: it sells at a high price.—LAB RADORITE exhibits rich colors and a beautiful opalescence, on account of which it is much used for ornamental purposes.—A blue variety of F., found only in Styria, and a green variety, sometimes called amazon atone, arc also esteemed as precious stones.—All the finer varieties of F. are characterized by a soft beauty, which well compensates for the want of that brilliancy which belongs to the true gems.
Kaolin, or porcelain clay, is regarded as a decomposed feldspar.—To F. also are referred, as chiefly composed of it, or apparently derived from it, felstone, trachyte, claystone, clinkstone, pitchstone, obsidian, and pumice.