|colorless, white, gray.
|Transparent to translucent
The chemical substance CaSO4 is commonly called gypsum. But it is a rather strange mineral, which is able to hold a certain amount of water. The waterless rock is called anhydrite (from the latin, without water). When it incoropartes water its volume increases and several physical properties change.
Anhydrite is a sedimentary rock, typically formed by the evaporation of sea water in huge depressions, especially in aride areas. Layers of gypsum and salt are formed. During diagenesis, when the temperature gets higher, the water leaves the gypsam and it is transformed into anhydrite. As soon as surface water reaches the anhydrite, it is transformed into gypsum. So typically people know only gypsum.
Gypsum is often karstified, and gypsum caves often contain gypsum speleothems. Sometimes the caverns are filled by ground water for a long time and selenite crystals grow.
Other caves, which exist in limestone, sometimes also contain a huge amount of gypsum speleothems. The reasons are typically sulfur thermal springs in the caves and the similarity of limestone and gypsum. The sulfur springs are the basis of sulfur based microorganisms living in cracks. This cave bugs living on sulfur produce several substances like sulphuric acid. This reaction increases cave formation as it provides corrosive agents, and it alows the transformation of calcite into gypsums, by adding sulfur.
The typical gypsum mineral is selenite. It is often clear like glas, and the crystals form sometimes large plates. This plates were used for a very long time, for glass windows and for vitrines. In Germany it was called Marienglas (Mary's glas) as it was used to cover small pictures of Mary.