if the white rocks showing near by to the Opportunity rover are anhydrite. (Scans currently show no silicate, hematite or quartz.)
Most people would know anhydrite by another name, plaster of paris (Calcium Sulphate). It’s basically dehyrated gypsum.
If it is indeed anhydrite then that would be a good case for the presence of water at one time or another with the Calcium Sulphate precipitating out when the water evaporated. Such deposites on Earth are created when ground water evaporates at the surface and in evaporating lakes.