The Size and Shape of Diamond Crystals of Different Origin

The size and shape of diamond crystals of different origin are analyzed. Diamonds with a size of less than about 0.5 mm are classified as microcrystals. Diamonds found in meteorites typically show non-faceted anhedral crystals of various sizes. Only the Canyon Diablo iron meteorite has cubic microcrystals of unclear crystallogenesis. Nano, micro- and macro-sized crystals of diamond in meteorites are usually aggregate in nature. The release of diamond polyhedra in meteorites is limited by the too small size of its crystals in chondrites and by its solid-phase transformation from very fine-grained diamond and graphite in ureilites and octahedrites. The size and shape of diamond crystals found in meteorite impact craters are determined by the nature of the source carbon material. The process of solid-phase transformation of graphite or other carbon-bearing materials (e.g., coal, plant remains) to diamond in meteorite craters does not allow euhedral crystal to be formed. At the same time, in the case of diamonds formed from impacts, on the (0001) faces of impact apographitic diamonds, polyhedra of nano-microdiamonds crystallize from the gas phase. These crystals are often form autoepitaxially, because they crystallize in an oriented manner on the lonsdaleite -diamond matrix. Diamonds found in metamorphic rocks, ophiolites and modern volcanites show faceted microcrystals. A wide range of sizes, from 0.1 mm to 10 cm, is characteristic of faceted diamond crystals from kimberlites, lamproites and lamprophyres. Diamond crystals from different mantle rocks acquire a multifaceted shape after reaching certain embryo sizes — the most likely appearance of diamond polyhedra larger than 40-50 nm. Octahedra forms are dominant for natural diamond crystals of different sizes and origin.