Remember, the dark soil is REALLY dark, dark as asphalt. Albedo of 0.1 or so if I remember from a couple of weeks ago.

So, I suspect that the bright stuff isn't white, but is just overexposed by comparaison. Remember the early photos where the lander parts were exposed correctly, and the soil was almost a featureless black? With the soil exposed to a medium grey, it wouldn't take much for something to register white in comparison.

So, I doubt it would look like chalk, or ice (not clean ice at least). Probably darker than that.

That being said, it does look like more than a trick of light on compressed soil -- the powder on the wheel, some bright chunks in the mound of soil to the left of the trench. I think there's something actually there.

I wonder if the wheel ground up and pulverized those bright little spherules, and that's part of what we're seeing?