Gypsum (Sulfates)

Gypsum hydrated calcium sulfate CaSO4-2H2O
transparent crystals of the variety selenite			golden-brown crystals of the variety selenite (photo courtesy of Amethyst Galleries, Inc.)
clear crystals of the variety selenite (photo courtesy of Amethyst Galleries, Inc.)

Diagnostic properties:

Color: colorless, white, gray; various shades of yellow, red, brown from impurities.
Streak: white.
Hardness: 2
Specific Gravity: 2.32
Luster: usually vitreous; sometimes, pearly or silky.
Cleavage: perfect into thin flexible but inelastic plates.
Transparency: transparent to translucent.
Occurrence: very common.
Crystal system: monoclinic.
Uses: in the making of Plaster of Paris and Portland cement; as a flux for pottery; as a fertilizer.
Name: from the Greek name for the mineral.
Varieties:
- Satin spar - fibrous with silky luster.
- Alabaster - fine-grained massive.
- Selenite - transparent crystals and cleavage fragments.

Some images © Copyright 1996 Friends of the Geology Museum and may not be copied for commercial purposes. Permission to copy is granted for personal and educational use only. All such copies must include this copyright notice and explicit references to the URL http://geology.wisc.edu/~museum.

Some images © Copyright 1995-1996 by Amethyst Galleries, Inc. and may not be copied for commercial purposes. Permission to copy is granted for personal and educational use only. All such copies must include this copyright notice and explicit references to the URL http://mineral.galleries.com/.