Mid-infrared spectral evolution of amorphous magnesium silicate smokes annealed in vacuum: Comparison to cometary spectra

Amorphous magnesium silicate smokes were prepared by vapor phase condensation and annealed in vacuum. The samples were monitored by IR spectroscopy as a function of annealing time and temperature, focusing on the development of the 10 mu m silicate feature, The IR spectrum of the initial condensate displayed a broad band at 9.3 mu m. After annealing for 10.5 h at 1027 K, a dual maximum was established at 9.8 and 11.0 mu m, Following this period of evolution no significant changes were observed in the 10 mu m silicate feature during the next 37.5 fi of annealing, Thereafter, individual features sharpened as the sample became more ordered. There appears to be a natural pause in the spectral evolution of these samples, midway between the initially chaotic condensate and the more ordered glass, The duration of the spectral stall was investigated from 1000 to 1048 It and was highly dependent on the annealing temperature, The evolving 10 mu m feature was deconvolved by fitting the observed bands with symmetric Gaussian peaks positioned at 8.35, 9.00, 9.77, 10.87, 11.30, 11.87, and 12.22 pm. Preliminary results of annealing studies on iron silicate and mixed-metal, magnesium-iron silicate smokes revealed distinctly different rates of sample evolution. The "olivine-rich" comets Halley, Bradfield, Levy, Mueller, and Hale-Bopp all display emission features with dual maxima at 9.8 and 11.2 mu m which closely resemble the magnesium silicate smoke stall spectra. (C) 1998 Academic Press.