Near-infrared reflectance spectroscopy of Ca-rich clinopyroxenes and prospects for remote spectral characterization of planetary surfaces.

Abstract

We present results of laboratory near-infrared reflectance studies of a set of calcic pyroxenes with comparable calcium contents (Wo45-50) but variable iron content and oxidation states. This new dataset complements earlier published data (Cloutis and Gaffey, 1991, J. Geophys. Res. 96, 22809-28826, and references therein). In particular, our new spectra extend the scarce available spectral data on chemically analyzed Fe-rich high-Ca clinopyroxenes. We attempted to interpret the spectral behavior of our samples in terms of chemistry and coordination site occupancies. Tentatively, we conclude that Fe-rich calcic pyroxenes have very low contents of Fe2+ in the M2 sites and belong to the spectral type A lacking the 2-μm band. This may be due to high Ca and Mn contents in these pyroxenes. Fe-poor high-Ca pyroxenes are more spectrally variable. In general, they tend to belong to the spectral type B with two major bands near 1 and 2 μm, unless the samples have high Fe3+/Fe2+ ratios or are rich in Mn and Ca. Some of them (including unusual meteorite Angra dos Reis) are of type B despite very high Ca contents. We applied the Modified Gaussian Model (MGM) to characterize three major Fe2+ absorption bands in the 1-μm region of the spectra of Ca-rich pyroxenes. Only the band due to Fe2+ in the M1 coordination site near 1.15 μm may be potentially useful to estimate the Fe content in calcic pyroxenes on remotely-sensed surfaces of Solar System bodies. The spectral variability of basaltic meteorites (angrites that are rich in calcic pyroxenes is also discussed. The presence of spectral type A calcic: pyroxenes in these meteorites complicates unambiguous identification of olivine in asteroid spectra.