Albite - Soda-feldspar Composition, Crystallization & Structure
Composition. Sodium –aluminum silicat, NaAlSi3O8 = silica 68.7 alumina 19.5 soda 11.8. Calcium is usually present in small amount in the from of the anorthite molecule, CaAl2Si2O8.
Crystallization. Triclinic. Usually in tabular crystals parallel to brachypinacoid. Sometimes elongated parallel to b crystal axis. Twinning very common, according to the albite law (see above) and evidenced by fine striation lines on the better cleavage surface. Twinning according to the other laws frequent.
Structure. Commonly massive, either lamellar with lamellar often curved or in cleavable masses. Distinct crystals rare.
Physical Properties. Perfect cleavage parallel to base; good cleavage parallel to brachypinacoid. Cleavage angle 86° 24’. H. = 6. G. = 2.62. Vitreous luster; sometimes pearly on cleavage surface. Colorless, white, gray. Transparent to opaque.
Tests. Fusible at 4-4.5, giving yellow flame (sodium). Insoluble in acids. Characterized by its hardness, white color, cleavage, frequently curved lamellar structure, striations on better cleavage surface, etc.
Occurrence. Like orthoclase, a widely distributed and important rock-making mineral. It occurs in all classes of rocks, but particularly in those of igneous origin, such as granites syenites m porphyries and felsite lavas. Found commonly, also, in pegmatite veins. Frequently occurs in gneiss, less often in the crystalline schists. May occur in veins. At times in disseminated crystallized albite are to be found in Switzerland and the Tyrol; in the United States at Paris, Etc. Maine; Chesterfield, Massachusetts; Haddan and Branchville, Connecticut; Amelia Court House, Virginia, Etc.
Name. From the Latin albus, white, in allusion to its color.
Use. Has the same uses as orthoclase, but not so commonly employed. Some varieties, when polished, show an opalescent play of colors and are known as moonstones. Members of the plagioclase series and orthoclase show at times this same effect. The stones are usually cut in round or oval shapes. The finest moonstones come from Ceylon, but they are chiefly orthoclase.
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