The composition of White Rock still is not settled conclusively. This is because there is debate about the accuracy of the current orbiters for detecting carbonates on Mars, due to the effects of a dust cover and the roughness of the terrain.
This article presents the views of Steven Ruff who argues his measurements show it not to be made of carbonate, of Allan Treiman who believes that neither TES nor THEMIS has the accuracy to make that determination, and of Jack Farmer who takes the centrist view that TES doesn't but THEMIS does have the sensitivity to detect carbonate:

Where Are The Carbonates?
by Bruce Moomaw
Cameron Park - April 28, 2000
http://www.spacer.com/news/life-00v3.html

This article discusses that some scientists believe carbonates may already have been detected and that to make a definitive detection will require near-infrared spectrometers, such as those on the upcoming Mars Express and Mars Reconnaissance Orbiter missions:

Evidence Of Carbonates May Already Exist.
by Bruce Moomaw
Cameron Park - April 11, 2000
http://www.spacedaily.com/news/lunarplanet-2000-00c3.html

Here are some reports on the lack of sensitivity to carbonate of the current Mars orbiters:

A Carbonate Soil (Calcrete) in Nevada: Biogenicity, and Detectability
with Thermal Infrared Spectroscopy
http://www.astrobiology.com/asc2000/abstract.html?ascid=217

A Laboratory Study of Weathered Carbonates, with Implications for the
Infrared Remote Sensing of Carbonates on Mars
http://adsabs.harvard.edu/cgi-bin/bib_query?2000LPI....31.1915K [full text]

A Field Study of Thermal Infrared Spectra of Carbonates, with
Implications for Studies of Mars
http://adsabs.harvard.edu/cgi-bin/bib_query?2000LPI....31.1876K [full text]

Full color images of White Rock will at least allow us to evaluate the assertion of Ruff et.al. that its color is no different than the surrounding terrain. It may be for example that an overcovering of dust made it appear to be no brighter, as measured by albedo, than the surrounding terrain. Approximated color images from both Viking and MGS obtained by averaging of available color channels certainly give the appearance that it is brighter:

"White Rock" of Pollack Crater
MGS MOC Release No. MOC2-264, 4 December 2000.
http://www.msss.com/mars_images/moc/dec00_seds/pollack/

White Rock an Unusual Martian Feature.
http://www.solarviews.com/cap/mars/whiterk.htm

Martian crater lake may have harboured life.
http://www.gla.ac.uk/press/releases/originoflife.html

And here are some reports that show a dust covering of only 300 microns (that's about the thickness of three strands of human hair) can obscure the spectra of the underlying terrain:

Effects of Palagonitic Dust Coatings on Thermal Emission Spectra of Rocks and Minerals: Implications for Mineralogical Characterization of the Martian Surface by MGS-TES.
"Thermal emission measurements on dust-coated rocks and minerals show that a 300 um thick layer is required to mask emission from the substrate and that non-linear effects are present."
http://adsabs.harvard.edu/cgi-bin/bib_query?2001LPI....32.1899G [full text]

Effects of Palagonitic Dust Coatings on Visible, Near-IR, and Mössbauer Spectra of Rocks and Minerals: Implication for Mineralogical Remote Sensing of Mars.
"Visible, near-IR, and Mossbauer measurements on dust coated rocks and minerals show that a 300 µm thick layer is required to obscure the substrate for VNIR measurements and that a >2000-µm-thick layer is required to obscure the substrate for Mossbauer measurements."
http://adsabs.harvard.edu/cgi-bin/bib_query?2001LPI....32.1912M [full text]

Bob Clark