Effects of MgSiO3 on the crystal growth and characteristics of type-Ib gem quality diamond in Fe–Ni–C system* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51772120, 51872112, and 11804305), the China Postdoctoral Science Foundation (Grant No. 2017M622360), and the Project of Jilin Science and Technology Development Plan (Grant No. 20180201079GX).

Abstract

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.