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 ~ { } \mu m are likely to be at temperatures below \sim 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 \pm 0.5 K and at room temperature ( 295 \pm 2 K ) .
To determine the changes in the spectra the resolution must be increased from \sim 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 \sim 90 \% of their 295 K-widths .
However , in forsterite the 3.5-K–widths of the 31- , 49- and 69- \mu m 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 \sim 0 - 0.2 ~ { } \mu m at 25 \mu m to \sim 0.9 ~ { } \mu m at 80 \mu m. In olivines and enstatites 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 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 \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 .
Spectral measurements of a variety of minerals at a range of temperatures are required to fully quantify these effects .