Diamonds of new alkaline carbonate-graphite HP syntheses: SEM morphology, CCL-SEM and CL spectroscopy studies

Colorless octahedral diamonds up to 150 mu m in size were spontaneously crystallized from carbon solutions in alkaline-carbonate melts in the Na2Mg(CO3)(2)-graphite and NaKMg(CO3)(2)-graphite systems at pressures of 8-10 GPa and temperatures of 1700-1800 degrees C, Seeded growth of carbonate-carbon (CC) diamond layers was realized on both octahedral {111} and cubic {100} faces of natural and synthetic "metal-carbon" (MC) diamond single crystals 0.5-0.7 mm in size. Scanning electron microscopy (SEM) morphology studies clearly demonstrate that a preferable mechanism of diamond growth from alkaline CC melts is the deposition of newly formed layers in parallel with octahedral faces, in much the same way as in the case of natural diamonds. A color cathodoluminescence (CL) SEM study shows that the specific feature of the CC diamonds is the lack of surface color CL as for natural diamonds of type II with lower nitrogen concentration. The CL spectra of the CC diamonds consist of three-band system H3, 575 nm, and a weak blue A-band. The structure of the H3 band closely resembles that of natural diamonds of type IIa. (C) 1999 Elsevier Science S.A. All rights reserved.