"A model of HPHT color enhancement mechanism in natural gray diamond"
A model of HPHT color enhancement mechanism in natural gray diamonds V.G. Vins, O.V. Kononov 신소재공학부 4학년 오민호 Abstract (개요) HPHT-processing of natural gray diamonds is studied and a new model of the color enhancement mechanism is proposed. It is proposed that there occurs a polymorphous transformation of graphite inclusions into diamond during HPHT-processing. 1. Introduction After nitrogen, hydrogen is the most abundant impurity in natural diamond. Gray diamonds are assumed to grow under supersaturation conditions created when the temperature falls gradually as the cooling kimberlite pipe moves towards the surfaces. 2. Experimental procedure The absorption spectra before and after HPHT-processing were recorded on the entire aperture of the samples. T=1800±50℃, P≒6.0 Gpa T=1950±50℃, P≒6.5 Gpa T=2100±50℃, P≒7.0 Gpa 3. Results and discussion HPHT-processing changed gray coloration of diamonds to yellow. In order to understand the mechanism of HPHT induced color enhancement in gray diamonds let us consider the origin of gray color and the influence of hydrogen on diamond color. 3.1 The origin of gray color in diamond The initial gray color is attributed to uniform absorption and reflection of light within the entire range of the optical spectrum. The uniform absorption and reflection of light is due to numerous microscopic graphite inclusions. 3.2 Source of hydrogen …, diamond crystallization involves hydrocarbons, mainly methane, products of its dissociation, hydrogen, hydrogen oxides as well as nitrogen hydrides, inert and other gases. During the formation of carbon crystal phases, hydrogen could form CH radicals and get trapped at diamond defects and diamond crystal surface and at the edge faces of graphite crystals. 3.3 Color enhancement mechanism ... transformation of graphite into diamond accompanied by transition of hydrogen from graphite to diamond. Under HPHT pressures and temperatures exceeding the diamond-graphite equilibrium line (over 1800℃) C-H bonds in graphite get spilt. As a result of this, hydrogen atoms become mobile and diffuse forming a kind of a cloud.. 3.3 Color enhancement mechanism These atoms get localized at the nearest structural defects... Hydrogen having been lost from the surface of graphite inclusions, the interatomic distance grows larger and weaker in each hexagonal structure and in the end, edge carbon atoms spilt away from the graphite matrix. 3.3 Color enhancement mechanism ..free carbon atoms lose their bonds with graphite, diffuse and get trapped at vacancies, dislocations, internal surfaces and external surfaces of diamond crystals. Microscopic graphite inclusions disappear completely, thus, removing the gray component and making diamond crystals more transparent (colorless).