Modeling Mid-IR Spectral Effects of Lattice Dislocations in Forsterite Crystals

The mid-infrared spectra (5–40 µm) of astrophysical objects containing silicates are complicated by a number of competing factors relating to the physical characteristics of the silicate mineral grains, such as grain size, shape, chemistry, and crystallinity. These factors leave distinct spectral characteristics in the crystalline resonance features within the 8–40 µm wavelength range, which potentially allow for determination of mineralogical and physical characteristics of the silicates. While clean retrievals of these factors from spectra remain elusive, a critical step toward accurate characterization of the silicate component includes combining lab and modeling studies that attempt to isolate the spectral effects of each factor. This spectral modeling work explores the effect of planar lattice dislocations and degree of crystallinity of small (1.0–3.0-µm) grains on the crystalline resonance features of forsterite in the 8–13 µm spectral region. The absorption efficiencies, Q_abs, for forsterite grains are modeled using the Discrete Dipole Approximation (DDA) code DDSCAT v7.3. This model preserves grain shape while simultaneously accounting for the three different crystallographic axes. Starting with a simple grain shape built with dipoles, the model introduces planar lattice dislocations with orientations perpendicularly to one of the three crystallographic axes. We investigated: (1) the number and depth of the dislocations, (2) grain shape, and (3) degree of crystallinity (by randomly adding regions of non-crystalline (amorphous) olivine). We find the introduction of planar dislocations each axis, independently, alters the crystalline resonances in spectral shape and wavelength position in spectrally diagnostic ways that are discernible in laboratory and mid-IR spectra of asteroids and comets. All effects increase with the number and depth of dislocations increases. Increasing amorphous material causes the absorption features to broaden and become smoother, improving fit with lab data and observational data of comet comae. This suggests a light to moderate degree of amorphization of the crystalline grains. These models were used to interpret the high-velocity shock experiments of Lederer et al. (in prep.; see Whizin et al. iPoster) corroborating findings from both lab and Stardust grains that c-planar dislocations originating from shock explain observed effects in both IR spectra and TEM imaging.