Well, today, on the BBC News web site as part of the story of Opportunity’s initial results on the rock strata in the crater:-
Principal investigator Steve Squyres told a news conference in Pasadena, California that preliminary readings with the alpha-particle X-ray spectrometer (APXS) show rocks in the crater outcrop are loaded with the element sulphur.
“I’m not going to give you quantitative numbers, but it is maybe a few more times sulphur than we have seen at any location on Mars,” said Professor Squyres.
Well, now, what was my prediction for the white rock? That’s right, Calcium Sulphate. It’s looking more and more like that’s what they are.
Not only this, but looking at the close-up images on the rover web site, the rocks show polygonal cracking, suggesting being deposited in water and then drying out repeatedly. In addition to this there are balls of some other compound which suggest the possibility of wave action during precipitation of the salts. The possible cross bedding I thought I saw turned out to be an optical illusion caused by the mis-alignment of ajoining broken blocks.
Of course, this is still only speculation. I’m not even sure that the rovers have enough equipment to make the final resolution of what the rocks are really made of.