I actually had to look caliche up.
New name for calcretes?
But now I see your point - if it caliche then it forms by precipitating calcium carbonate out from a solution. Like gypsum deposits, water would have to be present.....

BTW, caliche = CaC03, calcite - CaC03.
Howzit harder?

Tell you what Robert, since you are convinced already that these are water related - Wanna make a bet?
I'll say the white stuff is not evaporitic/precipitational.
You'll say it is.
Note - I'm not even sticking you to gypsum or calcite - anything that came out of a water solution will do.

What shall we wager? (note - my tastes and depth of pocket are not in the same league as Nick's) :-)

Will keep some fun/interest going as we wait for te rovers to do their stuff.
What say you?