The Silicate Chemistry of the A-type Asteroids

R.J. Whiteley (University of Hawaii (IfA)), P.G Lucey, K. Keil (University of Hawaii (HIGAP))

Previously obtained high quality near-infrared spectra of asteroids revealed that a class of objects, the A-types, have spectra which are quite similar to the mineral olivine. We compared the spectra of asteroids 446 Aeternitas, 289 Nenettta, 246 Asporina, and 863 Benkoela with a series of powdered laboratory olivines of varying Fe content. We conclude that the 1.05 micron olivine feature in all four asteroids is identical to the same feature in very Fe-poor laboratory olivines. The presence of very low Fe olivine indicates that the objects' origin lies in a reduced parent body, unlike most common meteorite types. The only meteorite type with a monomineralic olivine silicate fraction and this unusual Fe-poor olivine chemistry is the main group of pallasites. Thus it seems likely that the well-known A-type asteroids are the parent bodies of the main group of pallasite meteorites, and are not metal-free olivine assemblages.