The growth histories of the contact twins in natural diamond were clarified by using non-destructive techniques of X-ray topography, involving both conventional and synchrotron sources. The reflection conditions for the simultaneous imaging of both members of a diamond which was twinned on (111) were given. It was noted that the common so-called triangular contact twin resulted from {111}-faceted growth from a central nucleation site; sometimes marked by an inclusion. If this period of growth was followed by one of dissolution, a twinned rhombic dodecahedron could result. The dissolution shape of a twinned octahedron was the same as the twin of the dissolution shape of the octahedron. A peritropic twin was found to consist of 2 triangular contact twins which fortuitously joined on their common (111) facets in an only approximate twin orientation. A lozenge-shaped diamond was found to contain a twin component in the shape of an arrow-head. In all of the variants, the composition-plane could be far from planar, as a result of the intergrowth of one twin component into the other.

Twinning in Natural Diamond - I: Contact Twins. A.Yacoot, M.Moore, W.G.Machado: Journal of Applied Crystallography, 1998, 31[5], 767-76