Effects of MgSiO₃ on the crystal growth and characteristics of type-Ib gem quality diamond in Fe-Ni-C system

We report the effects of MgSiO₃ addition on the crystal growth and characteristics of type-Ib diamonds synthesized in Fe-Ni-C system. The experiments were carried out with pressure at 5.5 GPa, temperature at 1385 ^°C-1405 ^°C, and duration of 23.1 h. As MgSiO₃ increases from 0.0 wt% to 3.0 wt%, the diamond growth temperature increases from 1385 ^°C to 1405 ^°C, the addition of MgSiO₃ and the movement of P-T diagram toward the higher temperature direction result in a series of effects to the Fe-Ni-C system and crystal growth. Firstly, it increases the content of metastable recrystallized graphite and accelerates the competition with the carbon source needed for diamond growth, thus causing the decreased crystal growth rate. Diamond crystals exhibit the combination form of 111, 100, 113, and 110 sectors, the decreased 100 and 113 sectors, dominated 111 sector are all attributed to the higher growth rate in 100 direction caused by the synergy of MgSiO₃ and the movement of P-T diagram. The higher growth rate in 100 direction also increases the metal catalyst and graphite inclusions and leads to the increase of residual tensile stress on the crystal surface. Accompanying with the high growth rate, a higher dissolution rate along 100 and 113 directions than 111 direction occurs at the microstructure and forms the significantly developed (111) stepped growth layer. In addition to the movement of P-T diagram, the addition of MgSiO₃ poisons the catalyst and increases the nitrogen content of diamond from 120 ppm to 227 ppm.