Gypsum

Typical sedimentary mineral, of chemical origin: it forms in evaporitic environments by direct precipitation from water or by hydration of anhydrite. The tabular crystals, elongated, can reach up to a metre in length, and are often twinned in “swallowtail” or “spearhead” forms.

Gypsum is a soft mineral (2 on the Mohs scale), light and easily cleaved into sheets and flakes. It is sometimes luminescent under ultraviolet rays. Soluble in hydrochloric acid and also in hot water.