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 (Wo 45–50) 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 Fe 2+ 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 Fe 3+/Fe 2+ 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 Fe 2+ absorption bands in the 1-μm region of the spectra of Ca-rich pyroxenes. Only the band due to Fe 2+ 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.