Temperature effects on the 15-85 mu m 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 em ission bands seen at wavelengths >20 mum are likely to be at temperatures b elow similar to 150 K. In order to investigate the effect of temperature on silicate spectra, we have obtained absorption spectra of powdered forsteri te and olivine, along with two orthoenstatites and diopside clinopyroxene, at 3.5 +/- 0.5 K and at room temperature (295 +/- 2 K). To determine the ch anges in the spectra the resolution must be increased from similar to1 to 0 .25 cm(-1) at both temperatures, because a reduction in temperature reduces the phonon density, thereby reducing the width of the infrared peaks. Seve ral 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 similar to 90 per cen t of their 295-K widths. However, in forsterite the 3.5-K widths of the 31- , 49- and 69-mum bands are, respectively, 90, 45 and 31 per cent of their 2 95-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 correspondin g 295-K peaks; the magnitude of the wavelength shift increases from similar to0-0.2 mum at 25 mum to similar to0.9 mum at 80 mum. In olivines and enst atites the wavelength shifts can be approximated by polynomials of the form ax + bx(2) where x = lambda (pk)(295 K) and the coefficients a and b diffe r between minerals; for diopside this formula gives a lower Limit to the sh ift. Changes in the relative absorbances of spectral peaks are also observe d. 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 abun dances inferred using room-temperature laboratory data. Spectral measuremen ts of a variety of minerals at a range of temperatures are required to quan tify these effects fully.