Hexagonal Microstructure of Ice Crystals Grown from the Melt

Abstract

Electron microscopy of etched and nascent surfaces of single ice crystals shows a characteristic microstructure of elongated, hexagonal prisms. The small hexagonal units vary greatly in size with an expected width of the order of 3 microns and a length of 6 microns. The coordination among microcrystalline units characterizes a particular single crystal, the size of which may be determined by processes of freezing and subsequent history, but the sizes of the microcrystalline units of which they are composed appear to be independent of the rates of freezing. The etched surfaces show that the center of the hexagonal unit etches to a greater depth than does the outer area, indicating a higher degree of crystal disorder along the axis of symmetry. Ice surfaces in process of growing from the melt also appear to consist of hexagonal pyramidal pits which are built up by concentric steps around the C‐axis. This structure appears to be consistent with that of the etched surfaces. Observations of growing‐ice surfaces formed from cesium fluoride solutions reveal that the fluoride impurity tends to reduce the prominence of the pyramidal microstructure in a manner dependent upon concentrations such that the structure disappears for concentrations greater than 10⁻³ M.