Spectral signature of oxidized pyroxenes: implications to remote sensing of terrestrial planets

Abstract

The Fe 2+ ions in pyroxenes occurring in basalts on surfaces of terrestrial planets are vulnerable to oxidation, particularly in magma in contact with the atmosphere; this produces Fe 3+ ‐bearing assemblages that may mask spectral features of remnant Fe 2+ in pyroxenes in telescopic reflectance spectra of oxidized planetary surfaces. To assess such interferences, reflectance spectra (0.3–2.6 urn) were obtained for a variety of pyroxenes that had been heated in air at elevated temperatures. Mossbauer spectral measurements enabled the relative proportions of Fe 3+ and Fe 2+ ions and Fe 3+ ‐bearing phases to be determined in the reaction products. Nanophase hematite having particle diameters smaller than 10 nm obscures the pyroxene Fe 2+ /M2 site crystal field (CF) bands of oxidized enstatite and pigeonite. Intensification of Fe 2+ → Fe 3+ intervalence charge transfer transitions in calcic clinopyroxenes mask pyroxene Fe 2+ /M1 site CF bands, as well as the Fe 2+ /M2 site CF bands of augites and subcalcic hedenbergites. The ferric iron derived from oxidation of Fe 2+ ions may remain as structural Fe 3+ ions in the oxidized calcic clinopyroxenes or exist as clusters of nanophase hematite particles having diameters of a few unit cells.