Chiral
symmetry breaking during the chiral crystallization from
a sodium chlorate (NaClO3) aqueous solution is an intriguing
phenomenon because it provides insights into the prebiotic process
of biohomochirality. However, a mechanism of the emergence and amplification
of chirality remains controversial, especially for crystallization
from highly supersaturated solution, and one of the hypotheses proposed
before is a transition toward the homochiral state during the early
stages of crystallization. In this contribution, we directly examined
the early stage of crystallization by in situ polarized-light microscopy.
The observation revealed that achiral crystals, which appear prior
to the formation of chiral crystals, transform to the chiral crystal
through two kinds of polymorphic transformations: (1) martensitic
transformation (MT) and (2) solution-mediated phase transition (SMPT).
The SMPT is remarkably facilitated by contact with a chiral crystal.
Notably, the resulting enantiomorph through contact-facilitated SMPT
is strongly directed by the contacting enantiomorph. In contrast,
the MT yields two enantiomorphs in equal probability. The emergence
and amplification of chirality has generally been considered to be
a result of direct nucleation of a chiral crystal and its fragmentation.
In contrast, our observations provide a possibility that the MT and
contact-facilitated SMPT play a role for the emergence and amplification
of chirality, respectively.