Geological Museum KIGAM

6. Sulfates, Chromates, Tungstates and Molybdates

Anionic groups with general formula (XO₄)^2- form the basic structure of sulfates, chromates, tungstates and molybdates, in which X represents S, Cr, W or Mo respectively. As their structures and properties are somewhat similar, they can be placed in the same group. They have low density, low hardness, nonmetallic luster and low birefringence.

Sulfates can be subdivided into anhydrous and hydrous ones. Anhydrous sulfates are barite, celestite and anhydrite. Hydrous sulfates are gypsum, antierite and alunite. Crocoite is an anhydrous chromate. Tungstates include wolframite and scheelite. Molybdates include wulfenite. Barite is a common and widespread mineral, which is isostructural with celestite and anglesite. Celestite and anglesite contain Sr and Pb respectively instead of Ba in Barite. When gypsum is dehydrated, it changes into anhydrite. Wolframite and scheelite are the main ores of tungsten. Scheelite fluoresces in ultraviolet light.

Gypsum, Australia

Gypsum is characterized by softness and three unequal cleavages. It is colorless or white with various shades of red, brown and yellow due to impurities. Anhydrite forms by dewatering of gypsum. Selenite is a colorless and transparent variety with pearly luster and moon-like glow. Fibrous gypsum with silky luster is called satin spar. Alabaster is a fine-grained massive variety. 'Desert rose' is a nickname to indicate gypsum with rosette-like forms as in the illustration. Gypsum is the main material used in the production of plaster of Paris.