Wherever we have standing water on Earth, diatoms, an algae, can also be found. Diatoms have tests or outer shells composed of silica (quartz). Diatomaceous earth is used for swimming pool water filters and as the abrasive ingrediant in many cleansers. NASA used diatomacous earth along with sand in creating a martian soil in their tests to free one the rovers when it was stuck. Diatomaceous earth is white to creamy, very fine grained and looks very much like the white material exposed by the rover's wheel. I believe that diatoms in martian water is the most plausable mechanism for the production of this material.