Evidence for Ferrous Phosphates in Paso Robles Soils, Gusev Crater, Mars

Light-toned soil deposits have been churned up at Paso Robles by the Mars Exploration Rover Spirit's wheels during its ascent of Husband Hill in Gusev crater. These deposits are rich in sulfur and ferric sulfate, and were possibly formed by hydrothermal processes. They share a relatively high abundance of phosphorous with surrounding rocks. Up to 10 % phosphate mineral phases are expected to be present in Paso Robles soils. Calcium phosphates have been suggested but have not been confirmed by visible/near-infrared or thermal emission data. Evidence for ferric phosphates has been put forward. Because the light-toned Paso Robles soils were churned up from underneath a sand layer of basaltic composition, doublets arising from ferrous phases in the Mössbauer spectra obtained from two light-toned soil deposits at the Paso Robles location were initially assigned to silicates as a result of presumed physical mixing of the light-toned materials and the basaltic sand. Upon more detailed evaluation of the Mössbauer data, an alternative interpretation may be the presence of ferrous or mixed-valence phosphates such as vivianite or other members of the homologous series Fe²⁺₃(PO₄)₂·n(H₂O), including anhydrous phases. A reasonable scenario for the formation of ferrous phosphates and ferric sulfates would be for sulfuric acid to react with basalt containing apatite (or merrillite). In such a reaction the Ca-phosphate would form CaSO₄ and phosphoric acid. The phosphoric and/or excess sulfuric acid would react with olivine, forming Fe²⁺ phosphate (e.g., vivianite) and sulfate (e.g., melanterite). The phosphate is less soluble and precipitates first. The ferrous sulfate remaining in solution could be oxidized as the pH of the solution increases. Rocks of the Wishstone class in the vicinity of Paso Robles, for example, contain olivine and Ca-phosphates, and represent possible parent material.