UV-Vis-NIR absorption features of heated phyllosilicates as remote-sensing clues of thermal histories of primitive asteroids

Powder samples of five kinds of phyllosilicates (antigorite, lizardite, clinochrysotile, chlorite, and saponite) have been heated in vacuum at 200,300,400,500,and 600℃, and their UV-Vis-NIR reflectance spectra have been measured to test the hypothesis that the empirical correlation between the UV and 3-μm absorption strengths of primitive asteroids and CI/CM chondrites are due to the presence of phyllosilicates (T. HIROI et al., Meteorit. Planet. Sci., 31,321,1996). The chlorite sample showed virtually no change in absorption strengths. Only the saponite sample showed a clear positive correlation between the UV and 3-μm absorption strengths, whose reason being yet to be studied. Heating experiments with coexisting carbon (graphite rod) at 600℃ have given no systematic trend. Although an apparent conclusion would be that saponite is the cause of the correlation between the UV and 3-μm absorption strengths, it may not be true because saponite is not as abundant as serpentines in CM chondrites (M. E. ZOLENSKY et al., Geochim. Cosmochim. Acta, 57,3123,1993).