This graph presents information from the NASA Curiosity Mars rover's onboard analysis of rock powder drilled from the "Buckskin" and "Greenhorn" target locations on lower Mount Sharp.
Buckskin, in the "Marias Pass" area, and Greenhorn, in the "Bridger Basin" area, both contain high concentrations of silica. X-ray diffraction analysis of powered samples inside Curiosity's Chemistry and Mineralogy (CheMin) instrument revealed that each of them contains silica in the form of noncrystalline opal. The broad hump in the two X-ray diffraction patterns is diagnostic of opaline silica. Some of the silica in Buckskin is in the form of tridymite.
More information about Curiosity is online at http://www.nasa.gov/msl and http://mars.jpl.nasa.gov/msl/.