Richard V. Morris, et al., report that tridymite, a SiO2 mineral that crystallizes at low pressures and high temperatures (greater than 870 degrees C) from high-SiO2 materials, was detected at high concentrations in a sedimentary mudstone in Gale crater on Mars.
Mineralogy and abundance were determined by X-ray diffraction using the Chemistry and Mineralogy instrument on the Mars Science Laboratory rover Curiosity. Terrestrial tridymite is commonly associated with silicic volcanism where high temperatures and high-silica magmas prevail, so this occurrence is the first in-situ mineralogical evidence for Martian silicic volcanism. Multistep processes, including high-temperature alteration of silica-rich residues of acid-sulfate leaching, are alternate formation pathways for Martian tridymite but are less likely. The unexpected discovery of tridymite is further evidence of the complexity of igneous petrogenesis on Mars, with igneous evolution to high-SiO2 compositions.
The amorphous mixed-cation salts and oxides and possibly the crystalline magnetite (otherwise detrital) are primary precipitates and/or their diagenesis products derived from multiple infiltrations of aqueous solutions having variable compositions, temperatures, and acidities. Anhydrite is post lithification fracture/vein fill.
The Buckskin tridymitic mudstone sample contains abundant SiO2 phases ( about 55 wt.% of bulk sample) consisting of 14 wt.% tridymite, 2 wt.% cristobalite, 6 wt.% opal-CT, 33 wt.% opal-A and/or high-SiO2 glass, and no quartz. The monoclinic tridymite is well crystallized and not a mineraloid, such as opal with partial tridymite stacking, which might be attributed to a low-temperature origin. Because the Martian crust is largely basaltic, we explored other options for the origin of Buckskin tridymite. Of all of the options, derivation from silicic volcanic rocks is the most parsimonious within available constraints, the most important being its high formation temperature.
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