Deformation structures formed in diamond grains during polycrystalline sample sintering at 7.7 GPa were studied using TEM. A number of deformation features were observed in diamond during sintering in the temperature range 700–2500 °C. Based on these data a sequence for the structure formation processes in diamond grains under P-T treatment was ascertained.

Thin-foil transmission electron microscopy and carbon replicas from fractures have been used in combination with x-ray analysis to identify the microstructural characteristics in the alpha --> beta transition in Si3N4 at 7.7 GPa and 1300-2300-degrees-C. Even in the initial stage, the transition is accompanied by the formation of perfect grains of the beta phase. An analogy is drawn with the microstructural development of phase transitions of normal type in metallic materials, from which it is concluded that the alpha --> beta transformation in Si3N4 at high pressure in the absence of impurities forming a second phase occurs in a solid-state reconstructive fashion and is controlled by vacancy diffusion relaxation.

The top of the "basaltic" layer may lie at 5 to 9 km below the surface in places and positions predictable geologically and radiochemically. A folded metamorphic complex (as old as 3.55 b.y.) is believed to exist at 3 to 4 km depth, underlain by ancient siallitic horizons which grade into the "basalt." The mean geothermal gradient in the Krivoy Rog Basin is 103 to 116 m/1°C, increasing slightly with depth. Not more than 100 to 120°C may be expected at depths of 10-12 km. — IGR Staff.