STRUCTURE AND CHEMISTRY OF TERRESTRIAL AND SYNTHETIC ALLOPHANES AND THEIR IMPORTANCE FOR MARS

The nanospherical aluminosilicate allophane is formed by limited weathering of rocks and minerals, particularly volcanic tephra, or by precipitation from solutions. On Earth, allophane can be an important soil component in volcanically active regions. It lends unique properties to the soils it occurs in, affecting soil texture, water holding capacity, and nutrient availability. We precipitated synthetic allophanes and other nanominerals in order to study their structure and properties, and to collect spectral data for use in helping identify these materials via remote sensing on Mars.

The small size and lack of long-range crystalline order of allophane makes it challenging to analyze. Characterization of natural and synthetic samples using wet chemical methods, FTIR, SEM, NMR, XAS, and Mössbauer spectroscopy suggests that the synthetic samples are good compositional and structural analogs for naturally occurring allophanes, although the synthetic versions exhibit more variable particle sizes. Natural allophanes have Al:SI ratios ranging from 0.9 to 2, with the end-members forming in different geological environments. Our results suggest these endmembers may be distinguishable in remotely sensed spectra. Published studies have shown that allophane reacted with solutions can recrystallize to clay minerals over time periods of days to weeks, but our results show that aging alone of synthetic allophanes in dry or gel form does not result in crystallization over similar time periods. Allophane is known to strongly sorb phosphorus. This process irreversibly changes allophane structure and solubility via exchange of P for structural Si. If allophanes are present on Mars, they could be important in controlling the chemical cycling of nutrient elements like P, possibly affecting the stability and subsequent recrystallization pathway of the allophanes.

Nanoscale iron oxyhydroxides and aluminosilicates including allophane have been identified on Mars. These materials contain important clues to geologic history and surficial weathering conditions. A better understanding of terrestrial allophane will help with interpretation of these samples and the history of martian surface conditions.