Anhydrite
Water-free calcium sulphate, found in Martian nakhlites and as a terrestrial alteration product.
Key facts
- Formula
- CaSO4
- Family
- Carbonates, sulphates, salts and weathering products
- Crystal system
- Orthorhombic
What it is
Anhydrite is calcium sulphate without water. It is closely related to gypsum, which contains water, and to bassanite, which holds a little. On Earth it forms where seawater evaporates and in hot hydrothermal systems.
Where it is found
It occurs in small amounts with other salts in the nakhlites, where it may come from salty water on Mars, and as a weathering product in some hot desert meteorites. Sulphate minerals have been found widely on Mars by orbiters and rovers.
How it is recognised
It is identified by microprobe analysis and Raman spectroscopy, which separates it from gypsum.
Did you know?
The Chicxulub impact struck rocks rich in anhydrite, releasing sulphur that cooled the climate.
Anhydrite turns into gypsum as it absorbs water.