Feldspar (Ger. Feldspath, from Feld, field, and Spath, spar), a species of aluminous minerals very abundantly distributed, principally in plutonic and volcanic rocks, as granite, gneiss, greenstone, and trachyte. The different species were formerly confounded, but they are now distinctly classified, not only by the different crystalline forms which they present, but, when these are the same, by distinct chemical composition. The feldspars are in all cases anhydrous double silicates, consisting of a silicate of alumina combined with a silicate of some one or more of the protoxides of potash, soda, lithia, baryta, or lime. The proportion between the aluminous or sesquioxide base and the protoxide bases is constant, being one equivalent of each, making the oxygen ratio 1 to 3; but the proportion of silica varies, causing considerable variation in the density and hardness. The amount of silica corresponds much to that in the rock in which the feldspar is found, and to the minerals associated with it, the more highly silicated kinds occurring in granite, and the less silicated in basalts. When a granite has large crystals of feldspar disseminated through it, it is called porphyritic granite, and sometimes porphyry, particularly when the proportion of feldspar is large.

The various species of feldspar are given in the following table, as classified by Prof. Dana, with their systems of crystallization, and also their composition as indicated by the oxygon ratios of constituents; the first column of figures showing the protoxide, and the second the aluminous base, while the last column gives the proportion of silica according to the same ratio:

All the feldspars may be fused before the blowpipe, with more or less difficulty, to a vitreous enamel, and this property causes them to be extensively used for. glazing porcelain. The crystals of the several varieties range in hardness from 6 to 7 upon a scale of 10, being harder than glass, but less so than quartz. Their specific gravity varies from 2.5 in orthoclase to 2.7 in labradorite. The crystals of some species exhibit a beautiful play of colors; labradorite, the lime-soda feldspar, first discovered by the Moravian missionaries on the shores of St. Paul's isle off the coast of Labrador, being the most beautiful. The splendid opalescent and chatoyant reflections of this mineral have made it much prized as an article of jewelry. The cause of the play of colors has been satisfactorily explained by Reuseh, who finds a cleavage structure of extreme delicacy transverse to the median section. He therefore regards the color as that of thin plates, produced by the interference of the rays of light. The more common feldspars are orthoclase, or common potash feldspar, and albite, or soda feldspar. The potash species is the one most frequently met with, and is the usual associate of mica and quartz in ordinary granite, and of hornblende and quartz in sy-enitic granite.

Fine crystals of orthoclase are found at Carlsbad and Elnbogen in Bohemia; at St. Agnes in Cornwall; in the Mourne mountains in Ireland, associated with beryl and topaz; in great abundance in trachyte at Drachenfels, on the Rhine; and also, in the lavas of Vesuvius, in the valley called Fossa Grande. In the United States, it is found at Mt. Desert on the coast of Maine, of a fine green; in Massachusetts, at South Royalton and Barre, in large crystals; in Connecticut, in the gneiss quarries at Haddam, and the feldspar quarries at Middletown, in crystals a foot long and from 6 to 8 in. thick; in New York, at Potsdam, St. Lawrence co., in crystals a foot thick, at Warwick, Orange co., associated with tourmaline and zircon, and in many other places. The formula of orthoclase is K20, AI4O3, 6Si02. The old formula, regarding silica as Si03 and using the small atomic weight of oxygen, is KOSi03, Al2033Si03. Albite, or cleavelandite, the soda feldspar, often replaces orthoclase as a constituent of granite, and in some instances is associated with it, as in Pompey's pillar, when it generally has a whiter color. Veins of albite granite often contain the rarer granite minerals, such as beryl and tourmaline. In its compact state as felsite, it is the base of albite porphyry.

It is found in Maine, at Paris, with red and blue tourmalines; in Massachusetts, at Chesterfield; in Connecticut, at Haddam, with beryl, columbite, and black tourmaline, and in other localities; in New York, at Granville, Washington co., in white transparent crystals; in Pennsylvania, at Unionville, Delaware co., where a granular variety is a matrix for corundum; and in Calaveras co., California, with native gold and auriferous pyrites. Albite has the same composition as that of orthoclase or potash feldspar, substituting soda in place of potash. Soda feldspars yield more rapidly than potash feldspars to the decomposing action of water and carbonic acid; and accordingly Prof. T. Sterry Hunt finds in the more recent crystalline rocks of Canada a less development of soda feldspar than of any other kinds, and conceives the carbonate of sodium resulting from the decomposition of the albite and similar minerals of the older rocks to have reacted with the chloride of calcium of the palaeozoic ocean, producing deposits of carbonate of calcium and the chloride of sodium which is held in solution.

In general, the decomposition of the feldspathic rocks has furnished the principal mass of the various clays, those containing the largest proportion of feldspar affording the finest deposits, such as kaolin, of which porcelain is made. The soil derived from them, particularly the common potash species, is noted for its fertility when under good cultivation, on account of their furnishing a large supply of potash, an important constituent of plants. The application of caustic lime to such soils, when they are worn, has the effect of liberating a portion of the potash, with the formation of silicate of lime; this accounts for the great difference often noticed in the fertilizing effects of the application of lime, depending upon the mineral character of the soil and upon the condition of the lime.

NAME OF FELDSPAR.

System of Crystallization.

Proportion of Constituents.

Anorthite, lime feldspar.....................

Triclinic

1:3:4

Labradorite, lime-soda feldspar........

,,

1:3:6

Hyalophane, baryta-soda ,,...............

Monoclinic

1:3:8

Andesite, soda-lime ,,.............

Triclinic

1:3:8

Oligoclase, ,, ,, ,,............

,,

1:3:9

Albite, Soda feldspar...................

,,

1:3:12

Orthoclase, Potash feldspar.............

Monoclinic.

1:3:12