Oriented graphite single-crystal inclusions in diamond

A crystallographic study of 13 specimens of diamonds with euhedral single-crystalline graphite inclusions in their centres is presented. All inclusions belong to the hexagonal graphite modification (space group P6 ₃/mmc; a₀ ≈ 2.46 Å, c₀ ≈ 6.70 Å) and are up to 300 μm in diameter. Comparison of the measured c-lattice parameters of the graphite crystals with lattice parameters of graphite at elevated isostatic pressures indicates remnant pressures of up to 2.6 GPa acting on the inclusions. All samples exhibit distinct orientation relations between graphite and diamond. In 12 samples the direction [001] of graphite (G[001]) approximately parallels one of the directions [111] of cubic diamond (D[111]). The largest deviation is about 4°. A further division of these 12 samples is: (a) The three G[100] directions have angles of about 4° with three D[110] directions. This orientation relation is observed in five samples, (b) The angle between the same directions is about 34° in six samples, (c) In one sample this angle is about 16°. In a 13th specimen G[001] approximately parallels one of the three D[100]. The deviation is of about 6°, and one of the G[100] directions nearly parallels one of the D[110]. These orientation relations are analysed with a simplified application of the "coincidence site lattice (CSL)" concept. A 14th specimen is different to all others, as it exhibits a large (about 300 μm edge length) region with a sharp hexagonal borderline filled with a "patchwork" of tiny graphite "islands". Several parallel lamellae of up to 5 μm thickness result in a hexagonal pyramidal form. This graphite inclusion is suggested to be protogenetic with respect to the diamond.