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

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 >20μm are likely to be at temperatures below ~150K. 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+/-0.5K and at room temperature (295+/-2K). To determine the changes in the spectra the resolution must be increased from ~1 to 0.25cm^-1 at both temperatures, because a reduction in temperature reduces the phonon density, thereby reducing the width of the infrared peaks. Several bands observed at 295K split at 3.5K. At 3.5K the widths of isolated single bands in olivine, enstatites and diopside are ~90 per cent of their 295-K widths. However, in forsterite the 3.5-K widths of the 31-, 49- and 69-μm bands are, respectively, 90, 45 and 31 per cent of their 295-K widths. Owing 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 ~0-0.2μm at 25μm to ~0.9μm at 80μm. In olivines and enstatites the wavelength shifts can be approximated by polynomials of the form ax+bx² where x=λ_pk(295K) and the coefficients a and b differ between minerals; for diopside this formula gives a lower limit to the shift. Changes in the relative absorbances of spectral peaks are also observed. The temperature dependence of λ_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. Spectral measurements of a variety of minerals at a range of temperatures are required to quantify these effects fully.