Anhydrite Composition, Crystallization & Structure

Composition. Anhydous calcium sulphate, CaSO4 = Sulfur trioxide 58.8, lime 41.2.

Crystallization. Orthohombic. Crystals rare; when observed are thick tabular, also prismatic parallel to the macro-axis.

Structure. Usually in crystalline masses, with rectangular cleavage. Fibrous, granular.

Physical Properties. Cleavage parallel to the three pinacoids, so yielding rectangular blocks. H. = 3-3.5. G. = 2.89-2.98. Luster vitreous to pearly. Color white sometimes a faint gray , blue or red tinge. Transparent to translucent.

Test. Fusible at 3-3.5. After ignition gives an alkaline reaction on moistened test paper. Moistened with hydrochloric acid and ignited gives orange-red flame of calcium. Soluble in hot hydrochloric acid and dilute solution with barium chloride gives white precipitate of barium sulphate.

Occurrence. Occurs in much the same manners gypsum, and often associated with that mineral but is not nearly as common. Found in beds associated with salt deposits and in limestones rocks. Found at times in an ygdaloidal cavities in basalt. Occurs at Wieliczka, Poland; Aussee in Styria; at Stassfurs, Prussia; Bavaria; Hall in Tyrol; Bex, Switzerland; in the United States at Lockport, New York, Nashville, Tennessee. Found large beds in Nova Scotia.

Lead chromate, PbCrO₄. Monoclinic. In slender prismatic crystals, vertically striate. Also granular. H. = 2.5-3. G. = 5.9-6.1. Adamantine luster. Color bright red. Orange yellow-streak. Fusible at 1.5. fused with sodium carbonate on charcoal gives a lead globule. With borax gives a green bead in O. F. A rare mineral found in the oxidized zones of lead veins. Fine crystals come from Mount Dundas, Trasmania.