Dissolution of synthetic diamonds to produce morphologies similar to natural diamonds: an experimental study

The results on dissolution of flat-faced synthetic diamond crystals of octahedral habit in an Fe-Ni-S melt at 4.0 GPa and 1 400°C are presented. It has been established that the resulting diamond morphology is similar to some natural kimberlitic diamonds and follows the particular sequence: flat-faced octahedron – laminar octahedron – trisoctahedroid with parallel striations in the <110> direction (“O1-D1”). Comparing the obtained results with earlier experimental works it is concluded that oxidisation of octahedral diamonds by means of ditrigonal etching layers and formation of tetrahexahedroid form is a result of diamond interaction with the fluidised kimberlite magma. We suggest that formation of octahedral diamonds with trigonal etching layers does not occur in kimberlite magma, and that diamonds of the O1-D1 morphological series avoided natural oxidation in kimberlite magma, but, like flat-faced octahedrons, were enclosed within xenoliths. Most probably, this dissolution process took place in the mantle prior to their capture by kimberlite. The results support an idea that metal-sulphide melts could be considered not only as a growth place for the world’s largest Cullinan-like diamonds found in South Africa (CLIPPIR type), but also as a mantle agent producing crystal morphologies typical for common kimberlitic diamonds: it depends upon carbon content in metal-sulphide melt – with supersaturation of the melt, the growth of diamond occurs while at the undersaturation conditions the dissolution begins.