Say people,

Mars is a dusty place, and the rocks are covered in dust and other indurated coatings. We still don't know the composition of those rocks with absolute certainty. So, White Rock may well be a hydromagnesite deposit covered by layers of dust causing it to appear "brownish" or (doubtfully) "reddish". The only thing the spectrometers are picking up from orbit are the outer dust coatings and layers covering rocks. This is why the MER's have an abrasion tool and Beagle 2 a rock coring tool -- to grind off and get underneath the coatings on the rocks to see what they are really made of...until then we won't know.

Mars could have carbonates all over the place, but if they are covered by indurated dust deposits and rock varnish, you wouldn't be able to tell with an IR spectrometer. Both MER and Beagle 2 are going to be critical in this regard. We will finally know what some of the rocks on Mars are really made of.

BD