X. Class of Minerals

Sulfates are natural salts of sulfuric acid in which sulfate anion groups with a tetrahedral shape are always isolated, unlike those in borates and silicates. Over 200 species of sulfate minerals are known, but only a few appear more frequently in nature, such as barite, celestine, and gypsum. They can form as products of oxidation of sulfide ore deposits, through the evaporation process, by crystallization from seawater, or their formation can be associated with hydrothermal solutions (hot aqueous solutions) and volcanic gases. The sulfate class also includes the oxy-salts of chromic acid.