Abstract
Isotopic and trace element variations within single diamond crystals are widely known from both natural stones and synthetic crystals. A number of processes can produce variations in carbon isotope composition and nitrogen abundance in the course of diamond crystallization. Here, we present evidence of carbon and nitrogen fractionation related to the growing surfaces of a diamond. We document that difference in the carbon isotope composition between cubic and octahedral growth sectors is solvent-dependent and varies from 0.7‰ in a carbonate system to 0.4‰ in a metal-carbon system. Ab initio calculations suggest up to 4‰ instantaneous 13C depletion of cubic faces in comparison to octahedral faces when grown simultaneously. Cubic growth sectors always have lower nitrogen abundance in comparison to octahedral sectors within synthetic diamond crystals in both carbonate and metal-carbon systems. The stability of any particular growth faces of a diamond crystal depends upon the degree of carbon association in the solution. Octahedron is the dominant form in a high-associated solution while the cube is the dominant form in a low-associated solution. Fine-scale data from natural crystals potentially can provide information on the form of carbon, which was present in the growth media.
Highlights
Compositional inhomogeneity of both natural and synthetic diamond crystals is a well-established phenomenon
A comparison of synthetic and natural crystals reveals some key features that can be obvious for synthetic crystals but which are extremely rare in natural diamonds
It is known that the growth of diamonds along the [100] direction during chemical vapour deposition (CVD) synthesis leads to lowering the impurities level in the crystal in comparison with material grown along [111] direction [6]
Summary
Compositional inhomogeneity of both natural and synthetic diamond crystals is a well-established phenomenon. Two models for explaining a heterogeneous distribution of carbon isotopes and key impurities within diamond crystals have been debated: changing of the carbon source and fractionation of a fluid over the course of diamond crystallization. Both of these models require marked changes in fluid composition as the driving factor for generating shifts in the carbon isotope ratio and/or the abundances of impurities, the most important of which being nitrogen. Crystals 2017, 7, 190 both in terms of concentration and isotopic composition has been observed in high temperature and high pressure (HTHP) synthetic diamonds [1,2,3,4,5]. It is known that the growth of diamonds along the [100] direction during chemical vapour deposition (CVD) synthesis leads to lowering the impurities level in the crystal in comparison with material grown along [111] direction [6]
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