From the picture below, it looks like there are two types of
"blueberry" - one is light color/high reflectivity (tan) and the other
is darker (blue-gray). The images are from the 26 Feb press release
for Opportunity. it looks like the two materials have formed
blueberries in the same size ranges, I would assume similiar creation
processes. The page also includes some spectra for the Charlie Flats
area.

http://marsrovers.jpl.nasa.gov/galle...ats-B033R1.jpg

http://marsrovers.jpl.nasa.gov/galle...20040226a.html



Josh