Reflectance Spectroscopy of Phyllosilicates from 0.4 to 1.0 MU M

In 1992, an absorption feature near 0.65 mu m in the spectra of a few low-albedo asteroids was identified (Vilas et al. 1994, Icarus 109, 247). Although the 0.7 mu m absorption feature, attributed to the existence of iron-bearing phyllosilicates, is well-known, this new 0.65 mu m feature is unexplained. Shock experiments have shown that dehydration can reduce the amount of H₂O in phyllosilicates, thereby changing their crystal structures, without changing their compositions (Weldon et al. 1982, JGR 87, 10102). It has been hypothesized that the 0.65 mu m feature indicates the dehydration of phyllosilicates on the surfaces of these asteroids due to bombardment by intra-solar system debris. We have set up a laboratory reflectance spectroscopy experiment to test this hypothesis. An HR-320 (Instruments SA, Inc.) Spectrograph with a Milton Roy 150 grooves/mm diffraction grating (blazed at 800 nm) fitted with an SBIG ST-6V CCD array detector was used to acquire the data. (The CCD has 375 x 242 array of pixels, the size of each being 23 x 27 mu m.) The resolution of this instrument is 0.67 Angstroms with a dispersion of 200 Angstroms/mm at 5000 Angstroms. Data have been recorded for three phyllosilicates containing a high iron content: antigorite, ripidolite, and clinochlore. The unshocked samples were ground and sieved to a fine particle size (<= 250 mu m). Initial analyses of these data are presented in addition to results from shocked samples. This work was supported in part by the J. Reid Anderson Research Fund, Denison University, and NASA Grant NAGW-1765.