CaSO405H2O

Calcium sulfate pentahydrate, CaSO4·5H2O, is a hydrated calcium sulfate. It represents one of the hydration states of calcium sulfate; in nature the best known mineral form is calcium sulfate dihydrate, CaSO4·2H2O, called gypsum. The pentahydrate is less common as a naturally occurring mineral but can be formed and observed in laboratory crystallization and in certain solution environments. Calcium sulfate hydrates exist in a hydration series that also includes the hemihydrate (CaSO4·0.5H2O) and the anhydrous salt (CaSO4).

Occurrence and sources: gypsum, the dihydrate, is widespread in sedimentary rocks and is mined as a major industrial material. The pentahydrate is not a major commercial mineral but may appear in experimental settings or specialized synthetic systems. Industrially important calcium sulfate hydrates are typically dihydrate or hemihydrate, with the latter being produced by calcining gypsum.

Properties: CaSO4·5H2O forms white to colorless crystals and is relatively soft (Mohs hardness around 2). Its density is about 2.3 g/cm3, and it is only moderately soluble in water. Like other calcium sulfate hydrates, its stability and hydration level depend on temperature and humidity. The crystal structure and exact stability range are subjects of study in hydration chemistry.

Dehydration and transformations: upon heating, the pentahydrate progressively loses water to form lower hydrates, such as dihydrate and hemihydrate, and eventually anhydrous calcium sulfate. The hemihydrate is widely used in plaster technology and construction.

Uses and applications: in practice, the dihydrate (gypsum) and the hemihydrate (plaster of Paris) are the principal commercial forms. The pentahydrate is primarily of interest in research on hydration behavior and crystallography rather than as a major industrial material.

Safety: calcium sulfate hydrates are considered relatively low-toxicity; dusts can irritate the respiratory system and eyes with exposure.