Do twin spectral peaks of olivine particles in the thermal infrared
  diagnose their sizes and porosities?
  A well-established constraint on the size of non-porous olivine grains or the
porosity of aggregates consisting of small olivine grains from prominent narrow
peaks in thermal infrared spectra characteristic of crystalline silicates is
reexamined. To thoroughly investigate thermal infrared peaks, we make
theoretical argument for the absorption and scattering of light by non-porous,
non-spherical olivine particles, which is followed by numerical verification.
Our study provides perfectly rational explanations of the physics behind the
small-particle effect of emission peaks in the framework of classical
electrodynamics and convincing evidence of small-particle's emission peaks in
the literature. While resonant absorption excited by surface roughness on the
order of submicrometer scales can be identified even for non-porous olivine
particles with a radius of $10~{\rm \mu m}$, it makes only a negligible
contribution to thermal infrared spectra of the particles. In contrast, the
porosity of non-spherical particles has a significant impact on the strength
and wavelength of the peaks, while the resonant absorption excited by an
ensemble of small grains takes place at a wavelength different than one expects
for surface roughness. We finally reaffirm that twin peaks of olivine in
thermal infrared spectra of dust particles in astronomical environments are the
intrinsic diagnostic characters of submicrometer-sized small grains and their
aggregate particles in fluffy and porous configurations.
