The growth of ice crystals on freshly cleaved covellite surfaces

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Abstract
Oriented ice crystals growing as thin plates on freshly cleaved surfaces of covellite, when examined by reflected light, show interference colours which give a measure of the crystal thickness. Some growing crystals remain constant in colour, failing to thicken in a direction normal to the basal plane. Growth can be initiated on the basal plane by contact with a thicker crystal, by increase of the excess vapour density, or by causing mechanical damage to the surface. When two crystals of different thickness touch, layers grow from the point of contact across the thinner crystal, with velocity inversely proportional to the thickness, for thickness 200 to 1000 Å. This implies that the layers are growing primarily by surface diffusion of adsorbed molecules to their edges. This surface diffusion is characterized by a distance x, from which a growing layer collects molecules; measurements indicate that x has a maximum value of 7·5 μ at -6°c. The shape of layers and crystals are circular, polygonal, or dendritic depending on their thickness, and the excess vapour density of the environment. The relevance of the results to the variation of ice crystal habit with temperature is examined.

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