Microinclusions in dark gray diamond crystals of octahedral habit from Yakutian kimberlites

Micrometer-sized inclusions in dark gray diamond crystals of octahedral habit from Yakutian kimberlites were studied by analytical scanning electron microscopy and analytical transmission electron microscopy. In addition to peridotite-suite silicate inclusions (enstatite and olivine), which were previously studied in detail as macroinclusions in perfect diamond crystals, there are abundant inclusions of native metals (Fe-Cr and Fe-Cr-Ti intermetallic compounds and metallic Fe), sulfides (Fe-Ni sulfide, polydymite, and chalcopyrite), and carbonates (calcite). Rare grains of ilmenite, fluorite, apatite, zircon, phlogopite, and some other minerals were also found. The gray and black colors of diamonds were previously attributed to the presence of graphite. Although this phase was identified in the electron diffraction patterns of all the samples, its content was very low. Microinclusions of induced octahedral morphology composed of intimate enstatite-calcite intergrowths were also found. Possible deep processes responsible for the formation of diamonds with such a peculiar inclusion association are discussed in light of new data on experimental synthesis.