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The broad interstellar absorption features at 9.7$\mu$m and 18$\mu$m, attributed to silicate grains, appear in polarisation toward the BN object in Orion and toward GL2591 (Aitken et al. 1989). The profiles of the features, their relative strengths, and the wavelength separation of the polarisation and absorption peaks are all sensitive to the presence of mantles on the silicate grains and also to the presence of impurities within the silicates.
The effects of grain mantles upon the shapes and strengths of the silicate absorption features are investigated. Grains are modeled as Rayleigh-limit spheroids with various axial ratios and mantle volumes. Highly-absorbing mantles (those with a large value of $k$, the imaginary part of the refractive index, e.g. amorphous carbon) are found to obliterate the absorption features for only moderate mantle volumes. Mantles with a lower $k$ (such as amorphous ice or organic refractory) can introduce a polarisation reversal on the short wavelength side of the 10$\mu$m feature, which is not observed. These effects place constraints on the volume of material present as mantles on interstellar silicate grains. The limits are dependent upon the assumed grain shape but the observed wavelengths of the features restrict the shape somewhat.
Comparison with observations of the silicate features both in absorption and in polarisation leads us to conclude that a) the band strength of the 18$\mu$m feature is stronger in interstellar grains than was assumed by Draine \& Lee (1985) and b) that some additional impurity, which is absorbing (high $k$) at 6$\mu$m is required in interstellar silicate grains, either as a thin coating or mixed into the grains, in order to explain the relatively high polarisation observed in the short wavelength wing of the 10$\mu$m feature.
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