  Dust grains of crystalline silicate, which are rarely presented in
interstellar space, were found in cometary nuclei. These crystalline silicates
are thought to have formed by annealing of amorphous silicate grains or direct
condensation of gaseous materials near the Sun in the solar nebula, and
incorporated into cometary nuclei in the cold comet-forming region after radial
transportation of grains in the solar nebula. Abundances of the crystalline
silicate dust grains were therefore expected to be smaller farther from the
Sun. We aim to better understand the formation mechanism of minerals
incorporated into comet 17P/Holmes based on its mineral abundances. To derive
the mineral composition of comet 17P/Holmes, we applied a thermal emission
model for cometary dust grains to mid-infrared spectra of comet 17P/Holmes
taken with the Cooled Mid-Infrared Camera and Spectrograph (COMICS) mounted on
the Subaru Telescope a few days later the great outburst in October 2007. The
resulting mass fraction of crystalline silicate, f_ cry, and an
olivine-to-pyroxene abundance ratio, f_OP, are f_cry = 0.31 +/- 0.03 and f_OP =
1.20 +0.16/-0.12, respectively. Based on a simple consideration of the mixing
of dust grains originating in both the interstellar medium and solar nebula,
the minerals of 17P/Holmes formed by non-equilibrium condensation. This result
is consistent with theoretical and experimental predictions for vaporization
and condensation of olivine in the solar nebula.
