Spectral reflectance properties of nontronite exposed to Mars-like surface conditions and low-temperature heating (<300 °C)

Nathalie Turenne, Sahejpal Sidhu, Daniel M. Applin, Edward A. Cloutis, Z.U. Wolf, Stanley A. Mertzman, Elisabeth M. Hausrath, Teresa Fornaro, Adrian Brown

Available online 7 February 2023


“Spectral reflectance properties of nontronite [a phyllosilicate with an ideal formula of Na0.3Fe23+(Si,Al)4O10(OH)2·nH2O] were investigated under simulated Mars surface conditions of atmospheric pressure and composition, and after heating to 110°, 180°, and 300 °C. The data can be used to determine the conditions to which nontronites on the martian surface may have been subjected. Nontronite’s spectral features include Fe3+ associated absorption features below 1000 nm, H2O/OH features near 1400 nm, H2O features near 1900 nm, a characteristic metal-OH (Fe3+) feature in the 2280–2290 nm region, and an additional Fe-OH absorption feature near 2400 nm. Heating in a low-pressure CO2 environment leads to the loss of Fe-OH-associated features below 1000 nm at temperatures as low as 110 °C. Both OH and H2O are progressively lost upon heating to 300 °C, but small, spectrally-detectable amounts remain at 300 °C. The longer wavelength Fe-OH absorption bands in the 2280 and 2400 nm region persist up to these temperatures. Comparison to other smectites and Fe-rich phyllosilicates show that a strong 2280–2290 nm absorption feature and weak or absent ~2320 nm feature are unique to nontronite. A broader absorption feature in the 2300 nm region suggests the presence of nontronite and one or more Mg-bearing phyllosilicate(s). A Mg+Fe3+ phyllosilicate is implausible because of the difficulty in incorporating Mg2+ into the dioctahedral structure of nontronite. With increasing grain size, nontronite spectra become darker and progressively more blue-sloped beyond ~1500 nm with absorption bands showing increasing depth until saturation is reached for some of them beyond a few hundred-micron grain size. The spectral changes documented in this study, as well as a comparison to previous studies, suggest that the absorption features associated with nontronite can be used to place constraints on conditions that nontronite may have been exposed to in the past. This study demonstrates that dehydration as well as dehydroxylation can occur at temperatures <300 °C.”