Effects of oxidation on pyroxene visible-near infrared and mid-infrared spectra

Pyroxene spectral features in the visible near-infrared (VNIR) and mid-infrared (MIR) wavelengths are affected by oxidation resulting from traditional metamorphic processes as well as impact metamorphism. The observed effects are due to modifications in the crystal arising from changes in crystallization temperature or pressure or from substituting Fe3+ for Fe2+. Highly oxidized pyroxenes from terrestrial mantle xenoliths and shock experiments indicate that the spectral effects of oxidation are greater in clinopyroxene than orthopyroxene because clinopyroxene can accommodate more Fe3+ structurally. Changes in clinopyroxene VNIR related to increasing oxidation include a shift in the 0.8 mu m absorption band to shorter wavelengths and a strengthening of the Fe2+<-> Fe3+ intervalence charge transfer (IVCT) band, which reduces the band depth of the 1.0 mu m feature by similar to 20%. Although shocked clinopyroxenes are oxidized to similar levels to that seen in the mantle xenoliths, the effects of shock overprint those of oxidation in the VNIR. These include a decrease of similar to 76% intensity of the 2.35 mu m feature and a decrease of similar to 70% intensity of the 1.0 mu m feature. In the MIR, the effects of oxidation and shock are minimal, resulting in a 5% overall decrease in band depth. These shifts and changes can be interpreted as a result of changes in the polyhedra surrounding the Fe cations which reduce crystal field splitting and the order of the crystal structure. Determination of planetary surface composition through VNIR remote sensing methods requires careful consideration of potential changes induced via shock and/or oxidation processes.