Emily Lakdawalla of the Planetary Society has written some nice extensive summaries of the conclusions drawn on the enigmatic feature known as "White Rock" on Mars from the Mars orbiter missions:

White Rock through the ages: Mariner 9, 1972
By Emily Lakdawalla
Feb. 8, 2008 | 13:31 PST | 21:31 UTC
http://www.planetary.org/blog/article/00001322/

White Rock through the ages: Viking (1976-1980)
By Emily Lakdawalla
Feb. 13, 2008 | 12:10 PST | 20:10 UTC
http://www.planetary.org/blog/article/00001325/

White Rock through the ages: Mars Global Surveyor (1997-2006)
By Emily Lakdawalla
Feb. 22, 2008 | 10:24 PST | 18:24 UTC
http://www.planetary.org/blog/article/00001335/

White Rock through the ages: 2001 Mars Odyssey (2001-present)
By Emily Lakdawalla
Mar. 4, 2008 | 16:23 PST | Mar. 5 00:23 UTC
http://www.planetary.org/blog/article/00001347/

White Rock through the ages: Mars Express (2004-present)
By Emily Lakdawalla
Mar. 27, 2008 | 15:28 PDT | 22:28 UTC
http://www.planetary.org/blog/article/00001381/

Newly released archive of Mars Express HRSC data, part 2
By Emily Lakdawalla
May. 3, 2006 | 18:12 PDT | May. 4 01:12 UTC
http://www.planetary.org/blog/article/00000562/

The next to last link above, about the Mars Express observations, has a link to a nice Digital Terrain Model (DTM) 3D flyover of White Rock.

Emily's summary of the Mars Reconnaissance Orbiter results on White Rock is upcoming. However, some CRISM observations from MRO on White Rock are available here:

Pollack Crater's White Rock.
http://crism.jhuapl.edu/gallery/featuredImage/image.php?gallery_id=2&image_id=124

None of the spectrometer readings on any of the spacecraft show White Rock to be composed of evaporites or aqueous minerals such as carbonates, sulfates, clays, or salts. The CRISM observations also show it to be covered with olivine, although from my reading of the news release they are saying this is just a cover of some indurated material composed of sand or dust underneath.
Still the MER rovers did not initially see aqueous minerals either. These were revealed when the dust covers on the rocks were removed by the drills or when the wheels churned up the soil.
Another interesting fact mentioned is that "White Rock" is not really white but appears that way in comparison to the surrounding dark terrain within Pollack crater. However, I found it interesting that it is still lighter than most of the surrounding terrain even outside the crater. The exceptions might be notable. There are some small outcrops that may be the same material as White Rock. But there is a large area to the upper right of Pollack crater that has the same comparatively light color as White Rock. It's visible in the images in "Newly released archive of Mars Express HRSC data, part 2" article:

Anyone know if this area has the same thermal inertia readings as White Rock?

Bob Clark