DIAMONDS AND CARBONADO OF THE PRIMORSKII-KRAI - MINERALOGY, CRYSTAL-CHEMISTRY, AND GENESIS

The diamonds and carbonade discovered in the Primorskii Krai gold-bear ing placers suggest this region is a part of the large north China dia mond-bearing province. The different diamond varieties found here were formed by different processes. The structural features of the diamond single crystals (twinning, growth patterns, structure defects) and th e composition of mineral inclusions in diamond (chromium-free omphacit e, Mn-bearing ilmenite) indicate that the diamonds formed at temperatu re and pressure of eclogite facies of metamorphism rather than at the parameters of their thermodynamic stability. The carbonade composition is dominated by native element minerals (diamond, aluminum), oxides ( rutile, anatase), silicates (zircon), phosphates (xenotime and monazit e), and hydroxyl-aluminophosphates (gorceixite, florencite). The struc tural relations point to the succession of mineral formation from diam ond and rutile to hydroxyl-aluminophosphates. From their porous textur e, grain and pore size, and the defects of the cubic structure of the diamond microcrystals, the carbonade polymineral aggregates were assum ed to form in the pneumatolytic process at low pressures, high tempera ture gradients, and variable gaseous phase composition. Such condition s are typical of the open magmatic systems of subvolcanic and volcanic environments. The occurrence of the regularly oriented hexagonal lons daleite layers in the diamond structure is attributed to the dislocati ons in the cubic lattice of carbon atoms. These anomalies are probably syngenetic and due to the kinetics of crystalline carbon synthesis. T he Late Jurassic ultrabasic volcanic rocks are suggested as the source for a diamond placer that contains diamond single crystals and polycr ystalline aggregates.