CaSO₄

These samples of acanthite 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. It is a large sample, some 40 to 50 cm across. It is from Danialo mine, Rio Grande do Sul, Brazil.