We compute the absorption efficiency (Qabs) of forsterite using the discrete
dipole approximation (DDA) in order to identify and describe what
characteristics of crystal grain shape and size are important to the shape,
peak location, and relative strength of spectral features in the 8-40 {\mu}m
wavelength range. Using the DDSCAT code, we compute Qabs for non-spherical
polyhedral grain shapes with a_eff = 0.1 {\mu}m. The shape characteristics
identified are: 1) elongation/reduction along one of three crystallographic
axes; 2) asymmetry, such that all three crystallographic axes are of different
lengths; and 3) the presence of crystalline faces that are not parallel to a
specific crystallographic axis, e.g., non-rectangular prisms and (di)pyramids.
Elongation/reduction dominates the locations and shapes of spectral features
near 10, 11, 16, 23.5, 27, and 33.5 {\mu}m, while asymmetry and tips are
secondary shape effects. Increasing grain sizes (0.1-1.0 {\mu}m) shifts the 10,
11 {\mu}m features systematically towards longer wavelengths and relative to
the 11 {\mu}m feature increases the strengths and slightly broadens the longer
wavelength features. Seven spectral shape classes are established for
crystallographic a-, b-, and c-axes and include columnar and platelet shapes
plus non-elongated or equant grain shapes. The spectral shape classes and the
effects of grain size have practical application in identifying or excluding
columnar, platelet or equant forsterite grain shapes in astrophysical environs.
Identification of the shape characteristics of forsterite from 8-40 {\mu}m
spectra provides a potential means to probe the temperatures at which
forsterite formed.Comment: 55 pages, 15 figure
