The sulfates contain the sulfate anion, SO42-. Sulfates commonly form in evaporitic settings, where highly saline waters slowly evaporate, allowing the formation of sulfates and halides at the water-sediment interface. Sulfates also occur in hydrothermal vein systems. Common sulfates include anhydrite (calcium sulfate), celestite (strontium sulfate), barite (barium sulfate), and gypsum (hydrated calcium sulfate). The sulfate class also includes the chromate, molybdate, selenate, sulfite, tellurate, and tungstate minerals.