pink and white anhydrite from Brazil, dehydrated selenite

A specimen of pink and white anhydrite from Brazil, geologically classified as dehydrated selenite. A rare constellation: the original selenite (gypsum) lost its water of crystallization through diagenesis and transformed into anhydrite.

Anhydrite (CaSO₄) is the anhydrous form of calcium sulfate, in contrast to hydrous selenite (CaSO₄·2H₂O). The transformation of selenite to anhydrite occurs under pressure and elevated temperature over geological timeframes and is a classic diagenetic process.

The pink coloration comes from traces of manganese or iron oxides. Anhydrite is color-sensitive and can even revert to gypsum upon contact with water, which makes its mineralogy interesting as a carrier of a reversible phase transformation.

Brazilian deposits yield anhydrite pieces with preserved selenite remnants, which are considered didactic teaching objects for the sulfate mineralization sequence.

A collector's item with an instructive phase genesis and a rare carbonate mineral variant.