Dissolution Rates of Allophane, Fe-Allophane, and Hisingerite and Implications for Aqueous Alteration on Mars and in Potential Returned Martian Samples

Recent measurements from Mars document X-ray amorphous/nano-crystalline materials in multiple locations across the planet. However, despite their prevalence, little is known about these materials or what their presence implies for the history of Mars. The amorphous component of the martian soil in Gale Crater has an X-ray diffraction pattern that can be partially fit with allophane (approximately Al2O3•(SiO2)1.3-2•(H2O)2.5-3), as well as low-temperature water release consistent with allophane. The chemical data from Gale Crater suggest that other silicate phases similar to allophane, such as Fe-substituted allophane (here, approximately (Fe2O3)0.01(Al2O3)0.99(SiO2)2•3H2O) and hisingerite (approximately Fe3+2Si2O5(OH)4•(H2O)), may also be present. In order to investigate the properties of these poorly crystalline components of the martian soil, we synthesized allophane, Fe-substituted allophane, and hisingerite; characterized the synthetic materials by infrared spectroscopy, electron microscopy, X-ray diffraction, and evolved gas analysis; and performed dissolution experiments at acidic, near-neutral, and alkaline conditions in order to determine dissolution kinetics and alteration phases for these poorly crystalline materials. Our analyses demonstrate that allophane, Fe-allophane, and hisingerite are appropriate analogs for silicate phases in the martian amorphous soil component. These poorly crystalline materials dissolve rapidly at all experimental pH conditions, indicating that similar materials on Mars must have had limited interaction with liquid water since their formation. For allophane, logrdiss = -11.05 – 0.088 × pH; for Fe-allophane, logrdiss = -11.09 – 0.091 × pH; and for hisingerite, logrdiss = -11.49 – 0.032 × pH. Additionally, incipient phyllosilicate phases form in hisingerite and allophane under high pH conditions, but are much more sparse at low pH, which, combined with the enrichment of Fe expected from weathering, may be a useful tool for examining returned samples of martian soils for evidence of past aqueous alteration