Etch pits and dislocations in ice crystals

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Abstract
Etching of ice crystals grown from both the vapour and the melt has revealed etch pits in concentrations of up to 106/cm2 which apparently mark the sites of emergent dislocations. Silvered Formvar replicas show the etch pits on the basal surface to be hexagonal pyramids on which spiral or closed terraces of steps are often visible. These features are interpreted in terms of the etching of helical dislocations. The etch pits tend to group themselves along low-angle grain boundaries and slip lines. When the crystal is mechanically stressed, etch channels appear on the basal plane along the ⟨1120⟩ directions which suggests that these are the preferred glide directions. Double rows of small etch pits radiating from impurity specks and local centres of high stress probably mark the sites of dislocation loops intersecting the surface. Pyramidal hillocks appearing in concentrations of 105/cm2 on the basal faces of crystals grown from the vapour may be growth features and not the result of etching.

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