  We propose a new type of morphological instability in the diffusion-limited
growth of faceted crystals from the vapor phase that can explain the formation
of thin ice plates at temperatures near -15 C. The instability appears when the
attachment kinetics for facet growth depends strongly on the morphology of the
facet surface. In particular, we propose that the condensation coefficient for
growth of ice prism facets increases dramatically when the width of the facet
approaches atomic dimensions. This model reconciles several conflicting
measurements of ice crystal growth, and makes additional predictions for future
growth experiments. Other faceted crystalline materials may exhibit similar
morphological instabilities that promote the diffusion-limited growth of thin
plate-like or needle-like crystal structures.
