Morphological Features and Genesis of the Interpenetration Twins of Cubic Diamond Crystals

Abstract

The paper presents analysis of the crystal morphology of two similarly shaped interpenetration twins of cubic crystals: the Chinese Lantern diamond (Arkhangelsk diamond mining province, provided for this study by courtesy of the ALROSA company) and a diamond crystal from Brazil, which has been studied in much detail by A.E. Fersman. Comparison of data on these diamonds, which were obtained using original methodological techniques for interpreting morphological indications of crystal growth and dissolution, revealed a number of crystal morphological features of crystals characteristic of regular accretions of this type. In addition to tetragonal pits characteristic of the cuboids, ribbon-shaped, jagged, and drop-shaped relief features typical of surface dissolution, a new type of symmetrical sawtooth microrelief associated with etching of octahedral layers of cuboid growth was discovered on the diamond from the Arkhangelsk province. In accordance with the height and direction of the steps of the relief pattern, the development sequence of the aforementioned types of microrelief is determined according to the increase in the depth of dissolution and the degree of change in the curvature of the surface of the tetrahexahedron on twin cuboids. The genesis of the characteristic equatorial sharp ridge of octahedron faces encircling the twinning plane was revealed. It is demonstrated to has been formed in relation to an early stage of the systematic coalescence of fibrous diamond cuboids and to be a determining condition of its development, first as a interpenetration twin of flat-faced octahedral crystals. Crystal morphological similarities between indications of dissolution on curved tetrahexahedra from kimberlites of the Arkhangelsk pipes and placer sources in Brazil and the north of the Yakutian diamond-bearing province is discussed.