Minerals that typically form nodules include calcite, chert, apatite (phosphorite), anhydrite, and pyrite. Salt domes contain anhydrite, gypsum, and native sulfur, in addition to halite and sylvite. Common calcium-containing minerals are chalk, limestone, gypsum, and anhydrite. These calcite fins were once crack filling gypsum and anhydrite. Sulfur will be obtained from pyrites, then gypsum or anhydrite. The anhydrite and gypsum react with organic material to form calcite. When gypsum dehydrates severely, anhydrite is formed. Hydrothermal anhydrite in veins is commonly hydrated to gypsum by groundwater in near-surface exposures. Its associated minerals include pyrite, chalcopyrite, quartz, anhydrite, fluorite, and scheelite. Below this is a gypsum mush where nodules of anhydrite and other sulfates may develop. It also forms as an evaporite mineral and as a hydration product of anhydrite. It occurs in association with halite, carnallite, polyhalite, anhydrite, boracite, sulfoborite, leonite, epsomite and celestine. BASF also acquired and began mining anhydrite for gypsum at the Kohnstein in 1917. Massive amounts of anhydrite occur when salt domes form a caprock. The seal is an evaporitic package of rocks including impermeable anhydrite.