CaSO₄

These samples of anhydrite are displayed in the Smithsonian Museum of Natural History. Anhydrite is a sulfate mineral of calcium with the composition CaSO₄. The sample at left is about 5 cm across and is from Naica, Chihuahua, Mexico.

The name anhydrite, or "no water" alludes to the fact that there is no water in the structure, in contrast to gypsum, which is also calcium sulfate but incorporates water in its structure.

This strange sample is decribed as quartz after anhydrite. This means that the sample is quartz which has taken the place of the anhydrite, but retains the geometry of the anhydrite. It is a large sample, some 40 to 50 cm across. It is from Danialo mine, Rio Grande do Sul, Brazil.

"Quartz after anhydrite" means that it initially formed as anhydrite crystals and then was chemically replaced by quartz while retaining the crystal shape of the anhydrite precursor.

This sample is about 20cm across and is from Irai, Rio Grande do Sul, Brazil.