A method to determine silicate abundances from reflectance spectra with applications to asteroid 29 amphitrite associating it with primitive achondrite meteorites

Abstract

Theoretical studies of spectral reflectances (0.25–2.55 μm) of olivine and pyroxene and their mixtures have been carried out to obtain better understanding of the mineral assemblages of asteroidal surfaces. A model consisting of layers of equal-sized grains is found to be sufficient for converting reflectance spectra of mineral powder into absorption coefficient spectra of the crystal. Reflectance and transmittance spectra of six samples of pyroxene and olivine were measured and converted into absorption coefficient spectra by this model. Additional parameters were experimentally determined to apply to asteroidal surfaces whose transmittance spectra cannot be measured. The absorption bands in absorption coefficient spectra obtained by this model were assigned to the transitions between d-electron splitting energy states of Fe 2+ ions by crystal-field theory calculations, which employed the refined crystal structures and effective charges of pyroxenes and olivines. Because the absorption peak areas due to Fe 2+ were found to be nearly proportional to the Fe concentration within each of orthopyroxene and olivine, Fe concentration can be estimated from absorption coefficient spectra which is obtained from reflectance spectra by this model. By combining this model and crystal-field theory calculations, a general method was formulated to deconvolve reflectance spectra of pyroxene-olivine mixtures and to estimate the Fe concentration in each mineral. This method was applied to the surface mineral assemblage of 29 Amphitrite which belongs to the S-type asteroid class. The mineral assemblage of Amphitrite can best be represented by those of primitive achondrites including lodranite, winonaite, and silicate inclusions in the IAB and IIICD irons.