Abstract
The correlation among the gohei twins, seagull and spearhead-type crystals, which are typical polycrystalline snow crystals of low temperature types, was discussed based on a number of microphotographs taken in the polar regions and obtained in laboratory experiments. The crystal structure of the spearhead type was determined, that is, the angle between the c-axes is 116° and the supplementary angle of intersection of two prism planes is 31°. The spearhead-type crystal is the twin whose twinning plane is {1013}, and corresponds to the gohei twin whose tip angle is 54°. It is one wing of the seagull-type crystals of the inside bend from the center. The other wing of the seagull-type crystals of the inside bend from the center grows from the boundary of the mother crystal of the spearhead-type crystal, and has the same crystal structure as the mother crystal. There is another snow crystal of the seagull-type, the seagull type of the outside bend from the center, which has an angle between two wings of about 130°. The structure of their wings is the same as the spearhead-type snow crystals. This may be a penetration twin of the spearhead-type snow crystals. The formation mechanisms of gohei twins and spearhead crystals are considered based on a cubic structure model and the rotation of an ice crystal lattice about <0001> axis on the basal face.
