  We construct a theoretical model for low-temperature crystallization of
amorphous silicate grains induced by exothermic chemical reactions. As a first
step, the model is applied to the annealing experiments, in which the samples
are (1) amorphous silicate grains and (2) amorphous silicate grains covered
with an amorphous carbon layer. We derive the activation energies of
crystallization for amorphous silicate and amorphous carbon from the analysis
of the experiments. Furthermore, we apply the model to the experiment of
low-temperature crystallization of amorphous silicate core covered with an
amorphous carbon layer containing reactive molecules. We clarify the conditions
of low-temperature crystallization due to exothermic chemical reactions. Next,
we formulate the crystallization conditions so as to be applicable to
astrophysical environments. We show that the present crystallization mechanism
is characterized by two quantities: the stored energy density Q in a grain and
the duration of the chemical reactions \tau . The crystallization conditions
are given by Q > Q_{min} and \tau < \tau _{cool} regardless of details of the
reactions and grain structure, where \tau _{cool} is the cooling timescale of
the grains heated by exothermic reactions, and Q_{min} is minimum stored energy
density determined by the activation energy of crystallization. Our results
suggest that silicate crystallization occurs in wider astrophysical conditions
than hitherto considered.
