The 506 nm absorption feature in pyroxene spectra: Nature and implications for spectroscopy-based studies of pyroxene-bearing targets

High-resolution (0.34 nm) reflectance spectra of a suite of terrestrial ortho- and clinopyroxenes were characterized in the 506-nm region. This region exhibits absorption bands attributed to spin-forbidden transitions in Fe2+ located in the M2, and possibly M1, crystallographic site(s). The most intense absorption bands (up to 3.8% deep in <45 mu m fractions) are present in low Ca-content orthopyroxene spectra. This region exhibits two (spectral Group I) or more (spectral Group II) absorption bands in the 500-515 nm interval. Group I spectra are associated with the lowest Ca-content samples. For orthopyroxenes, the number of constituent absorption bands and band depths vary as a function of Ca content; increasing Ca content results the appearance of more than two absorption bands and a general reduction in band depths, offsetting an expected increase in band depth with increasing Fe2+ content; band depths may also be reduced due to the long wavelength wing of ultraviolet region Fe-O charge transfer absorptions. Band depths and shapes in this region are also a function of grain size, with the strongest bands appearing for larger grain sizes - in the 90-250 mu m range. The number and position of constituent absorption bands can be used to constrain factors such as cooling rates, as expressed in the formation of Guinier-Preston zones versus coarser-grained augite exsolution lamellae. Band depths in the spectra of fine-grained (<45 mu m) clinopyroxenes do not exceed 1% and are generally lowest for spectral type A clinopyroxenes, where most of the Fe2+ is present in the M1 crystallographic site. The appearance of the 506 nm band in the spectra of pyroxene-bearing asteroids can be used to constrain pyroxene composition and structure. The results of this study suggest that detailed analysis of absorption features in the 506 nm region is a powerful tool for determining the composition and structure of pyroxenes. The spectral resolution of the VIR-MS spectrometer aboard the Dawn spacecraft - which will examine Asteroid 4 Vesta, a body possessing surficial pyroxenes - will be sufficient to provide some constraints on pyroxene composition. (C) 2009 Elsevier Inc. All rights reserved.