PHYLLOSILICATES IN THE WOOLY PATCH OUTCROP INVESTIGATED BY THE SPIRIT ROVER AT WEST SPUR, GUSEV CRATER, MARS

“Wooly Patch” is an outcrop having unique characteristics, investigated by the Mars Exploration Rover, Spirit, along the rover's traverse to the Columbia Hills. It is the softest rock abraded by the Rock Abrasion Tool at Gusev through sol 291. It shows hardened material at the edges of surface fractures, potentially involving cementation/deposition by fluid. It shows cataclastic texture in the interior matrix and extremely fine porous cuttings. The rock interior has distinct Vis-NIR spectra, and a distinct Mössbauer spectrum with a paucity of Fe2+ in olivine and an intense Fe2+(VI) spectral doublet with parameters slightly different from the pyroxene/glass component within plains basalts. Compositionally, targets on Wooly Patch form the endmembers in three chemical trends of Gusev rocks, and its major silicate-related cation ratios [TC/ICT and ICS/(ICS+ICL)] suggest a medium degree of polymerization (e.g., phyllosilicates). A modified normative calculation based on igneous mineralogy indicates an excess of Al2O3 and SiO2 in its composition. Mass-balance mixing-model calculations suggest phyllosilicates plus remnants of primary basaltic minerals to be the essential constituents that make up this outcrop. Phyllosilicate groups possessing similar cation ratios to those implied by the Wooly Patch analysis spots include the kaolinite, serpentine, chlorite, and septechlorite groups. The potential existence of kaolinite type Al-rich phyllosilicates within the Wooly Patch outcrop suggests a mildly acidic environment (pH from 4 to 6) in the past, and an open hydrologic system.