The behavior of gas inclusions in diamond generated by temperature changes

Abstract

Gas inclusions are sometimes sealed into diamond crystals during crystal growth at high temperatures and pressures. The pressure in these inclusions changes as the diamond is lowered to room temperature with additional pressure changes occurring during subsequent reheating and cooling cycles. Liquid-filled inclusions may form from gas inclusions if the critical temperature of the gas exceeds room temperature. Gases that react with carbon follow pressure-temperature curves that are different from those of inert gases. Internal stresses are generated in the diamond which are proportional to the difference between the pressure in the inclusion and the external pressure around the diamond. These stresses increase as the cube of the inclusion diameter and fall off as the cube of the distance from the inclusion. Such stresses can cause plastic flow around the inclusion and residual elastic stresses in the diamond far from the inclusion. When the pressure in the inclusion falls below the diamond-graphite equilibrium line, graphitization can occur on the walls of the inclusion. Because graphite has a molar volume 50% greater than diamond, such graphitization reduces the volume of the inclusion and increases the pressure in the inclusion. Multiple inclusions generate stresses which are proportional to the inclusion concentration and which depend on their spatial distribution in the crystal.