Twinning in Natural Diamond. I. Contact Twins

Growth histories of contact twins of natural diamond have been elucidated by nondestructive techniques of X-ray topography, using both conventional and synchrotron sources. Reflection conditions for the simultaneous imaging of both members of a diamond, twinned on (111), are given. The common `triangular' contact twin, known as a macle in the diamond trade, results from {111}-faceted growth from a central nucleation site, sometimes marked by an inclusion. If this period of growth is followed by one of dissolution, then the twinned rhombic dodecahedron may result. The dissolution shape of a twinned octahedron is the same as the twin of the dissolution shape of the octahedron. A peritropic twin was found to consist of two macles fortuitously joined on their common (111) facets in only approximate twin orientation. A lozenge-shaped diamond was found to contain a twin component in the shape of an arrowhead. In all these variants, the composition `plane' can be far from planar, resulting from intergrowth of one twin component into the other.