The influence of temperature on the spectra of the A-asteroids and implications for their silicate chemistry

Previously obtained high-quality near-infrared spectra of asteroids revealed a class of objects, the A-asteroids, with reflectance Spectra similar to the mineral olivine. We compare the asteroid spectra to a compositional series of powdered olivines varying from Fe-rich to Fe-poor. With continua removed and absorption band strengths normalized, the A-asteroid spectra are most similar, but not identical, to the spectrum of a very low-Fe olivine suggesting the asteroidal olivine to be Fe-poor. The spectra of the laboratory samples differ systematically from those of the A-asteroids. The spectra of the A-asteroids are much more similar to the spectra of low-Fe olivines obtained at low temperatures reasonable for the heliocentric distance of the main asteroid belt. We conclude that the spectral differences between ambient temperature laboratory measurements of olivines and the spectra of A-asteroids are mainly due to temperature-dependent spectral effects. This is the first detection of these effects in the spectra of asteroids. We further conclude that the A-asteroid silicate fraction is olivine with a composition less than Fa(10+/-5). The low iron content of these asteroid silicates indicates the objects' origin in reduced parent bodies with compositions unlike the ordinary chondrites. The combination of monomineralic olivine silicate fraction and very low iron olivines is solely represented in the meteorite collection by the main group of pallasite meteorites. The silicate fraction of other olivine-rich meteorites is too Fe-rich to be represented on the A-asteroids. This work strengthens the link between the pallasite meteorites and the A-type asteroids.