Occurrences of facetted re-entrants on rounded growth surfaces of natural diamonds

Abstract

Some unexpected growth-surface morphologies have been discovered as localised developments within the regions of non-facetted growth which are present (and can be dominant) in diamonds which have had epochs of mixed-habit growth during which crystallisation proceeded simulataneously both on normal octahedral faces and on non-crystallographic, hummocky surfaces having mean orientation {100}. These unexpected minor features are {111}-facetted re-entrants, not involving twinning. They have been observed in two crystallographic settings. In one case pairs of {111} facets develop to form re-entrant notches at the boundaries between adjacent sectors of non-facetted, mean-{100}-orientation growth. In this case growth can propagate on the pair of re-entrant facets, forming a column of octahedral growth inserted within the surrounding matrix of non-facetted growth material. In the second case, {111}-sided pyramidal pits (size range from about 10 μm to 120 μm) develop at points distributed over the area of non-facetted growth surface. There is evidence that they are initiated at particular growth horizons at which an episode of strong inhibition of growth on {111} commences, and that they are preferentially located at dislocation outcrops. In this case no detectable propagation of octahedral growth upon the facets occurs: they are directly overlain by renewed non-facetted growth.