The tetrahedral crystals which were present in emulsions that had been precipitated at high pAg values were investigated by means of electron microscopy and electron diffraction techniques. It was found that the defect structure of these crystals consisted of 2 non-parallel (111)-type twin planes which enclosed an angle of 70.5°. A secondary (441)-type twin plane was shown to be the contact plane between the two (111)-type twin variants. It was observed that, under the present precipitation conditions, the crystals were bounded by (111)-type planes. A growth model was presented which explained the morphology of both needle crystals and tetrahedral crystals in terms of the occurrence of particular twinning combinations during their growth. The re-entrant grooves which were present in the tetrahedral crystals were found to be non self-perpetuating; contrary to the cases of multiply-twinned tabular crystals or needle crystals. It was suggested that this implied that, when the re-entrant grooves had disappeared, tetrahedral crystals could grow further only by 2-dimensional nucleation. Their size therefore depended upon the time-scale over which a particular twinning combination occurred.

C.Goessens, D.Schryvers, J.Van Landuyt, R.De Keyzer: Journal of Crystal Growth, 1997, 172[3-4], 426-32