Twinning in Natural Diamond. I. Contact Twins

Abstract

Growth histories of contact twins of natural diamond have been elucidated by nondestructive techniques of X-ray topography, using both conventional and synchrotron sources. Re¯ection 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.

Pink color in Type I diamonds: Is deformation twinning the cause?

“…Slip and twinning are two of the main deformation modes that allow a solid to change shape under the action of an applied stress. The classic definition of twinning states that the twin and parent lattices are related to each other by a reflection in some plane (see Yacoot et al 1998). In diamond this reflection or mirror plane is {111}.…”

“…They commonly occur as arrays of twins parallel to one another (i.e., polysynthetic twins). Twinning in diamond can occur during growth (e.g., Yacoot et al 1998;Machado et al 1998;Tomlinson et al 2011) or during deformation (Buerger 1945;Hirth and Lothe 1982;Christian and Mahajan 1995;Niewczas 2007). Early work using indentation (Phaal 1964) and high-pressure hightemperature (HPHT) experiments (de Vries 1975) produced deformation microtwins, which were also observed in natural samples (Varma 1970).…”

X-ray topography of a natural twinned diamond of unusual pseudo-tetrahedral morphology

“…Only X-ray topography can establish this conclusively in a non-destructive manner, as we wish to keep this rare sample intact. This method has already been successfully used on other twinned diamonds to understand their geometrical arrangement [12,13] and to explore defects in diamond (e.g. [14]).…”

“…The orientation of one half of the stone is derived from the other by a 1801 rotation about the [1 1 1] twin axis (see details in Ref. [12]). …”

Role of plastic deformation in tailoring ultrafine microstructure in nanotwinned diamond for enhanced hardness

“…Shuffle-set dislocations, on the other hand, are non-dissociated perfect dislocations and cannot create stacking faults and twins [26,28]. Twins are common in natural and synthetic diamonds as a type of planar defect [29][30][31][32], which can be produced via both phase transformation [33][34][35][36] and deformation [26,[37][38][39]. Both ng-and nt-diamond can be obtained from different carbon precursors through HPHT syntheses without resorting to catalysts [4,10].…”