Orientational Dependences of Diamonds Grown in the NiMnCo–Silicate–H2O–C System under HPHT Conditions and Implications to Natural Diamonds

: The detailed phase compositions in the NiMnCo- C system with Mg2Si3O8middot5H2O added and its effects on {111} and {100} diamonds grown at 5.5 GPa and 1355 degrees C are reported. The results reveal that Mg2Si3O8middot5H2O reacts with catalysts and produces tephroite, forsterite, coesite, and H2O. With its content in the carbon source increasing from 0.0 to 5.0 wt %, the morphology of diamonds evolves from a bulky cubic octahedron/ truncated cube to a {111}/{100}skeleton with {110} dendrites and finally to {100} antiskeletal dendrites. The concentration of nitrogen impurity (Ntotal) in diamonds exhibits a Ntotal{111} > Ntotal{100} pattern and decreases from similar to 117 and similar to 100 to similar to 36 and similar to 21 ppm, respectively. Except liquid water inclusion, hydrogen and oxygen are also found to enter the surfaces, grain boundaries, inclusions, or cracks of the growing diamond in the form of -CH3, -CH2, and R1-CH(R2)-OH groups and compete with nitrogen atoms entering the diamond lattice, thus resulting in the decreased Ntotal. Besides, the termination of diamond structure by C-H and C-OH bonds is also controlled by the oriented density of step edge and causes hydrocarbon group content C{100} > C{111} and crystal growth rate V{100} > V{111}. The competition mechanism and the oriented density of step edge may be used to constrain the formation environment and interpret the lower nitrogen content in natural cubic and fibrous diamonds than in octahedral diamonds