selenite

Selenite is a transparent to translucent variety of gypsum, the mineral form of calcium sulfate dihydrate (CaSO4·2H2O). The name derives from the Greek selene, meaning moon, in reference to its moonlike luster and translucence.

Crystal structure and appearance: Selenite is monoclinic. It typically forms colorless, transparent crystals that are soft (hardness ~2 on the Mohs scale) with perfect cleavage in three directions. It exhibits a vitreous to pearly luster and occurs as tabular or prismatic crystals, as well as fibrous masses known as satin spar. A closely related, more opaque variety is alabaster, usually microcrystalline gypsum.

Formation and occurrence: Gypsum forms in evaporite settings as water evaporates from lakes and seas, and it can precipitate in sedimentary basins and caves. Selenite crystals may develop when mineral-rich waters crystallize gypsum; cave environments can produce delicate, well-formed selenite crystals, including some of the largest gypsum crystals known from places such as gypsum mines and cave systems.

Varieties and related terms: Selenite refers specifically to transparent crystals of gypsum. Satin spar denotes the silky, fibrous variety, while alabaster refers to a fine-grained, typically opaque form of gypsum used historically for sculpture and ornamental carvings.

Uses and handling: Selenite is valued by mineral collectors and for decorative carvings. Gypsum, including selenite crystals, is relatively soft and soluble in water, so specimens are handled with care and kept dry to prevent damage.