Physical and chemical features of a large coated natural diamond crystal

Abstract

A large (390.7 carat, 78.14 g) natural diamond crystal, composed of a near colorless gem-quality core and a translucent yellowish-green overgrowth coating approximately 0.5 mm in thickness, was investigated to better understand the relationship between the two portions of this crystal. The coating is translucent as the result of numerous tiny inclusions of trapped magmatic melt/fluid that originated in the Earth's mantle. The nitrogen impurity is dominantly in the A form. Despite its ‘dirty’ outer appearance, the coating contains fewer defects and impurities in comparison to the core. The trapped melts are rich in SiO 2 (63.7 wt.%), Na 2O (7.1 wt.%), K 2O (7.0 wt.%) and H 2O (5.9 wt.%). In terms of trace elements, significant amounts of Ba, Sr, Zr, and the light rare earth elements are also present. No CO 2 or carbonate component was detected; this is in contrast with analytical results on other diamonds reported previously. Quartz is the only mineral that could be identified in the trapped melt. As indicated by the quartz peak shifts in the middle infrared region, these melt inclusions are confined at a pressure of ∼1.7 GPa. All these results indicate that the coating is a secondary growth over a preexisting diamond. Growth of the coating very likely occurred at a diamond stable condition instead of meta-stable crystallization. The trapped melt is chemically different in terms of major and minor element compositions from kimberlitic magma, which transported most diamonds to the Earth's surface.