Silicic volcanism on Mars evidenced by tridymite in high-SiO2 sedimentary rock at Gale crater

Tridymite, a low-pressure, high-temperature (>870 degrees C) SiO2 polymorph, was detected in a drill sample of laminated mudstone (Buckskin) at Marias Pass in Gale crater, Mars, by the Chemistry and Mineralogy X-ray diffraction instrument onboard the Mars Science Laboratory rover Curiosity. The tridymitic mudstone has similar to 40 wt.% crystalline and similar to 60 wt.% X-ray amorphous material and a bulk composition with similar to 74 wt.% SiO2 (Alpha Particle X-Ray Spectrometer analysis). Plagioclase (similar to 17 wt.% of bulk sample), tridymite (similar to 14 wt.%), sanidine (similar to 3 wt.%), cation-deficient magnetite (similar to 3 wt.%), cristobalite (similar to 2 wt.%), and anhydrite (similar to 1 wt.%) are the mudstone crystalline minerals. Amorphous material is silica-rich (similar to 39 wt.% opal-A and/or high-SiO2 glass and opal-CT), volatile-bearing (16 wt.% mixed cation sulfates, phosphates, and chlorides-perchlorates-chlorates), and has minor TiO2 and Fe2O3T oxides (similar to 5 wt.%). Rietveld refinement yielded a monoclinic structural model for a well-crystalline tridymite, consistent with high formation temperatures. Terrestrial tridymite is commonly associated with silicic volcanism, and detritus from such volcanism in a Lake Gale catchment environment can account for Buckskin's tridymite, cristobalite, feldspar, and any residual high-SiO2 glass. These cogenetic detrital phases are possibly sourced from the Gale crater wall/rim/central peak. Opaline silica could form during diagenesis from high-SiO2 glass, as amorphous precipitated silica, or as a residue of acidic leaching in the sediment source region or at Marias Pass. 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.