gems

E.J. Gübelin examined black inclusions in about a dozen slabs cut from octahedral diamonds from Ghana to determine their mineral composition and what it might reveal about the origin and cooling history of the host diamonds. The original crystals weighed about 3–4 carats each, and all the slabs strongly absorbed short-wave ultraviolet light, indicating type Ia diamond. The inclusions generally consisted of a central grain surrounded by dark-lined fissures extending along octahedral directions. Gübelin attributed these tension cracks to the difference in thermal expansion between the grain and the diamond and reported that a thin film of pyrrhotite accounted for most of the dark linings. The slabs were ground until the enclosed grains were exposed for electron microprobe analysis. Bright, colorless grains yielded magnesium and silicon, consistent with olivine, while most black grains contained several mineral phases, chiefly sulfides identified as pyrrhotite, pentlandite, and chalcopyrite. One apparently black grain proved to be colorless olivine once its dark coating was removed. In slab D‑1, X-ray maps showed iron and nickel concentrated in the center of a sulfide grain and copper concentrated mainly along its rim. The iron–nickel sulfide phases in two inclusions contained about 7.8% nickel by weight. Gübelin interpreted this unusually high nickel content for a pyrrhotite-like phase as evidence of limited exsolution and suggested that the host diamond had cooled rapidly. The copper-rich phase had the composition of chalcopyrite, though a high-pressure polymorph remained possible. A small silica phase in direct contact with the sulfides also remained uncertain. Overlapping X-ray diffraction patterns prevented a definitive identification, but comparison with published photomicrographs suggested that it might be quartz that had replaced coesite while retaining its form. In slab D‑4, a tiny yellow grain was identified as pyroxene, with its classification as clinopyroxene or orthopyroxene unresolved. On the combined evidence, particularly from slabs D‑1 and D‑4, Gübelin favored an eclogitic origin for the diamonds.