TWINNING IN NATURAL DIAMOND - I - CONTACT TWINS

Growth histories of contact twins of natural diamond have been elucida ted by nondestructive techniques of X-ray topography, using both conve ntional and synchrotron sources. Reflection conditions for the simulta neous imaging of both members of a diamond, twinned on (111), are give n. The common 'triangular' contact twin, known as a made in the diamon d trade, results from {111}-faceted growth from a central nucleation s ite, sometimes marked by an inclusion. If this period of growth is fol lowed 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 commo n (111) facets in only approximate twin orientation. A lozenge-shaped diamond was found to contain a twin component in the shape of an arrow head. In all these variants, the composition 'plane' can be far from p lanar, resulting from intergrowth of one twin component into the other .