CaSO42H2O

Calcium sulfate dihydrate, CaSO4·2H2O, is a common mineral widely known as gypsum. It consists of calcium ions and sulfate ions with two water molecules per formula unit, forming soft crystals that are white to colorless and can appear in massive, fibrous, or granular forms.

Gypsum occurs extensively in sedimentary rocks and evaporite deposits. It forms by the evaporation of seawater and by diagenetic alteration of anhydrite, as well as through precipitation from sulfate-rich waters in various environments. It is quarried worldwide for industrial use.

Physical properties include a hardness of about 2 on the Mohs scale and a density around 2.3 g/cm3. It is relatively low in color and can be transparent to translucent in well-formed crystals. In water, it is moderately soluble, yielding calcium and sulfate ions.

Chemical behavior and processing are notable: when heated, gypsum undergoes dehydration. At around 150°C, it loses water to form calcium sulfate hemihydrate (CaSO4·0.5H2O), commonly called plaster of Paris. Further heating yields anhydrite (CaSO4), the anhydrous form.

Industrial and practical uses are diverse. Gypsum is the source material for plaster and plasterboard, and it acts as a set regulator in cement production. Its agricultural use includes soil conditioning to improve structure and reduce dispersion of clay minerals. Gypsum-derived products also appear in art, sculpture, dentistry, and various construction formulations.

Safety notes indicate that gypsum is non-toxic, but its fine dust can irritate the respiratory system if inhaled.