Composition: A1O(OH) or A12O3.H2O = Alumina 85, water 15.
Crystallization: Orthorhombic. Usually in thin crystals, tabular parallel to the brachypinacoid.
Structure: Bladed; foliated massive.
Physical Properties: Perfect cleavage parallel to brachypinacoid. H. = 6.5-7. G. = 3.35-3.45. Vitreous luster except on cleavage face, where it is pearly. Color white, gray, yellowish, greenish.
Test. infusible. Insoluble: Fine powder wholly soluble in salt of phosphorus bead (absence of silica). Ignited with cobalt nitrate turns blue (aluminum). Gives water in C.T. Characterized by its good cleavage, scaly structure and its hardness (6.5-7).
Occurrence Usually a decomposition product of corundum and found associated with that mineral in dolomite, chlorite-schist, etc. Occurs similarly in bauxite deposits. Has been noted as an accessory mineral in metamorphic limestones. Found in the Urals; at Schmitz, Czechoslovakia; Campolungo in Switzerland. In the United States in Chester County, Pennsylvania; at Chester, Massachusetts; with alunite forming rock masses at Mt. Robinson, Rosita Hills, Colorado.