Near-infrared spectroscopy of Ca-rich clinopyroxenes revisited: A new interpretation of anomalous type-B spectra and implications for remote sensing of inner solar system bodies

Abstract

Four Ca-rich clinopyroxene samples (3 terrestrial clinopyroxenes and a druse clinopyroxene from the D’Orbigny meteorite) have been studied by near-infrared (NIR) spectroscopy and X-ray structure refinement in order to address the issue of anomalous type-B NIR spectra seen in some terrestrial and extraterrestrial Ca-rich clinopyroxenes. The D’Orbigny druse clinopyroxene shows a typical type-B NIR spectrum, with two absorption bands located at ~ 1.1 and 2.3 μm. Based on previous Mössbauer and X-ray structure refinement results on synthetic clinopyroxenes, the 1.1 μm band is assigned to Fe2+ at the M1 site in diopside and high-pigeonite (i.e. pigeonite with C2/c symmetry), and the 2.3 μm band to Fe2+ at the M2 site in high-pigeonite. A comparison of NIR spectral type and X-ray structure refinement of the studied samples indicates that the average structure refinement of all type-B clinopyroxenes show a residual electron density peak in the difference-Fourier synthesis, corresponding to the split M2’ site which is mainly occupied by Fe2+. The results present a new interpretation for type-B NIR spectra of Ca-rich clinopyroxenes.