IRON ALTERATION MINERALS IN THE VISIBLE AND NEAR-INFRARED SPECTRA OF LOW-ALBEDO ASTEROIDS

Absorption features centered near 0.60-0.65 and 0.80-0.90 mum have been identified in the spectra of five low-albedo main-belt and outer-belt asteroids. These absorption features are attributed respectively to the 6A1 --> 4T1 (G) and 6A1 --> 4T1 (G) charge transfer transitions in minerals such as goethite, hematite, and jarosite that are products of the aqueous alteration of anhydrous silicates. A shoulder near 0.63 mum has also been identified in the absorption feature centered near 0.7 mum attributed to oxidized iron in phyllosilicates found predominantly in C- and G-class asteroid reflectance spectra. The coexistence of iron oxides with phyllosilicates in asteroids believed to have undergone aqueous alteration would be expected based upon analogy with terrestrial aqueous alteration and the observed mineralogy of carbonaceous chondrites. The number of low-albedo asteroids having only iron alteration absorption features compared to the number of low-albedo asteroids having spectral characteristics indicative of phyllosilicates is small. Either the conditions under which these asteroids formed are rare, or the iron alteration minerals could be formed in the interiors of objects where phyllosilicates dominate the surface mineralogy. (C) 1994 Academic Press, Inc.