Morphologies of diamond crystals

Abstract

Morphological characteristics (external morphologies, surface microtopographies and internal morphologies) of natural diamond crystals, synthetic diamond crystals grown in diamond stable region, and those grown under metastable, high temperature and low pressure conditions are critically analysed and are confronted with the theoretically analysed morphologies of diamond crystals. Theoretical analyses are based on the recent understandings of crystal growth mechanisms and crystal morphology, particularly on the growth rate vs. chemical potential difference relations and interface roughness, which predict the mutual relations among dendritic, hopper and polyhedral morphologies. Theoretical morphologies of polyhedral crystals and surface microtopographies of crystal faces are analysed on the basis of interface roughness and periodic bond chains.
Natural diamonds are classified into single crystalline, poly c rystalline, intermediate and combined types. Although natural crystals have invariably experienced dissolution, original as-grown crystal morphologies are evaluated based on the surface microtopographic and internal morphological observations. Natural diamond crystals exhibit the closest morphology to the theoretical morphology. Synthetic crystals grown under diamond stable conditions show some deviation from the theoretical morphology. This deviation is interpreted as due to the larger growth unit in synthetic solution system, in contrast to the smaller growth units of atomic size in natural diamond growth. Those grown under meta, stable conditions exhibit thegreatest deviation, and in many respects, show reverse characteristics in relation to the theoretical morphology.
Judged from the morphological analyses, it is concluded that natural diamond crystals grow by the spiral growth mechanism from low supersaturated solution phase. Geological implications are also discussed.