Glimpses into the growth history of natural diamonds

Abstract

The growth histories of diamonds may be classified crudely as “normal” or “abnormal”. In normal histories, growth continuously in octahedral habit is recorded by concentric, complete {111} growth layers. Very rarely is growth layering not clearly revealed in X-ray diffraction topographs. Dislocations may be many or few; but in diamonds of normal growth and free from inclusions most dislocations originate at the crystal's centre of growth and from thence radiate (often with remarkable straightness) out to the crystal faces. Rare occurences are stacking faults, bounded by Frank sessile partial dislocations. Coated diamonds show a transition, often abrupt, from normal facetted growth to a fibrous mode of growth, with simultaneous inclusion of micron-size foreign particles. A much rarer abnormal mode of growth comprises a combination of forms: the normal, facetted {111} form together with non-facetted, generally hummocky, surfaces of mean orientation {100}. Under the latter surfaces (i.e. in the “cuboid” growth sectors) a population of micron-sized X-ray diffraction-contrast-producing bodies may be found. The octahedral growth sectors are generally richer, sometimes much richer, in nitrogen impurity platelet precipitates than their adjacent cuboid growth sectors at corresponding epochs of growth.