Temperature effects on the 15-85-micron spectra of olivines and pyroxenes

Abstract

Far-infrared spectra of laboratory silicates are normally obtained at room temperature even though the grains responsible for astronomical silicate emission bands seen at wavelengths >20 micron are likely to be at temperatures below ~150 K. In order to investigate the effect of temperature on silicate spectra, we have obtained absorption spectra of powdered forsterite and olivine, along with two orthoenstatites and diopside clinopyroxene, at 3.5+-0.5 K and at room temperature (295+-2K). To determine the changes in the spectra the resolution must be increased from 1 to 0.25 cm^-1 at both temperatures since a reduction in temperature reduces the phonon density, thereby reducing the width of the infrared peaks. Several bands observed at 295 K split at 3.5 K. At 3.5 K the widths of isolated single bands in olivine, enstatites and diopside are ~ 90% of their 295 K-widths. However, in forsterite the 3.5-K-widths of the 31-, 49- and 69-micron bands are, respectively, 90%, 45% and 31% of their 295 K widths. Due to an increase in phonon energy as the lattice contracts, 3.5-K-singlet peaks occur at shorter wavelengths than do the corresponding 295-K peaks; the magnitude of the wavelength shift increases from \~ 0-0.2 micron at 25 micron to ~0.9 micron at 80 micron. Changes in the relative absorbances of spectral peaks are also observed. The temperature dependence of lambda_pk and bandwidth shows promise as a means to deduce characteristic temperatures of mineralogically distinct grain populations. In addition, the observed changes in band strength with temperature will affect estimates of grain masses and relative mineral abundances inferred using room-temperature laboratory data.