The four crystalline varieties of gypsum are sometimes grouped together and called selenite. Gypsum is a common, rock-forming mineral, with thick, extensive beds formed by the evaporation of extremely saline waters. It is often associated with other minerals, such as halite and sulfur. It is deposited from lakes, seawater, hot springs, volcanic vapors, and sulfate solutions in geological veins. Hydrothermal anhydrite in veins is commonly hydrated to gypsum by groundwater in near-surface areas.