Sulfate Mineral
sulfate mineral
[′səl‚fāt ′min·rəl]Sulfate Mineral
any one of various minerals that are natural salts of sulfuric acid. The sulfate class includes about 150 mineral varieties. There are, however, few stable and common sulfate minerals in the earth’s crust; among such minerals are anhydrite, barite, gypsum, celestite, alunite, mirabilite, and alum. Chemically, the sulfate minerals are either simple anhydrous salts or mostly simple or double salts with water of crystallization or with additional anions. The generalized formulas for the salts with additional anions are of the type Am[SO4]pZq·xH2O and AmBn[SO4]pZq·xH2O, where A and B are cations and Z represents an additional anion, such as [OH]–, Cl–, or [CO3]2–. The most important variety-forming cations in natural sulfate minerals are Fe (especially Fe3+), Na+, K+, Cu2+, Mg2+, Al3+, Ca2+, Pb2+, and Ba2+.
The main crystal structural unit of sulfate minerals is the [SO4] tetrahedron, in which the sulfur atom lies in the center. The anions [SO4]2– are always separated by other anions and cations, and therefore insular structures are strongly predominant among sulfate minerals. Chained or layered structures result either from the presence of hydrogen or hydroxyl bonds or from the combination of such bonds with strong complexes, such as UO6. Most sulfate minerals have low symmetry; they are rhombic, monoclinic, or, less often, triclinic. Crystals of sulfate minerals usually have an isometric habit. The color of sulfate minerals is caused by the ionic chromophores, mineral inclusions, and the presence of defects. Their density ranges from 1,490 to 6,920 kg/m3. The minerals are characterized by low hardness (2–3.5 on Mohs’ scale) and high solubility in water; minerals with monovalent cations are particularly soluble.
Almost all sulfate minerals are formed in a supergene zone and during sedimentogenesis in large water basins. In hydrothermal deposits, sulfate minerals are represented by barite and, less frequently, by other anhydrous sulfates, such as anhydrite, celestite, and anglesite. Many sulfate minerals, among them gypsum, brochantite, jarosite, and vitriols, are formed in the oxidation zone of sulfide deposits and upon solfatara activity of volcanoes (for example, alunite); some rare natural sulfates, such as chalcocyanite, form as a result of fumarole activity.
Sulfate minerals are widely used in the chemical industry as raw materials for the extraction of various metals, including barium, strontium, lead, and aluminum. Some sulfate minerals, including barite, are used as weighting compounds in clay mortars. Sulfate minerals find application in the rubber, paper, paint-and-varnish, and sugar industries and in the production of glass and ceramics. Gypsum and anhydrite are used in construction for production of cement. Sulfate minerals are employed in the production of pharmaceuticals, leather, and textiles. Some sulfate minerals are also used to make potassium fertilizers.