Mineralogical Studies of Pyropes in Kimberlites from China  

作　　者：董振信 

出　　处：《Chinese Journal Of Geochemistry》1998年第1期29-43,共15页中国地球化学学报

摘　　要：Based on the measurements of refractive index, specific gravity, unit cell parame-ter, and mineral chemistry and infrared absorption spectrum analyses of pyropes in kimberlitesfrom China, systematic studies of the physical properties and compositional variations of py-ropes of different colors and diverse paragenetic types, within and between kimberlite provinceshave been undertaken. The origin of pyropes in the kimberlites and the depth of their forma-tion have been discussed.Pyropes of the purple series are different from those of the orange series in physical andchemical properties, for example, pyropes of the purple series are higher in α0, RI, SG,Cr2O3, MgO, Cr/(Cr + Al), Mg/(Mg + Fe), and Mg/(Mg + Ca), and lower in Al2O3,Fe2O3 + FeO than those of the orange series. The classification of garnets in kimberiites fromChina by the Dawson and Stephens’ method (1975) has been undertaken and clearly demon-strates that pyropes of diamond-rich kimberlites contain much more groups than those of dia-mond-Poor, especially diamond-free kimberlites. The higher in α0, RI, SG, Cr2O3,, Cr/(Cr+ Al), knorringite and Cr-component the pyropes are, the richer in diamond the kimberliteswill be. The infrared absorption spectrum patterns of pyropes change with their chemical com-position regularly, as reflected in the shape and position of infrared absorption peaks. Two ab-sorption bands at 862 - 901 cm(-1) will grade into degeneration from Splitting and the absorptionband positions of pyropes shift toward lower frequency with increasing Cr2O3 content and Cr/(Cr + Al) ratio of pyropes. LREE contents of orange pyrope megacrysts are similar to those ofpurple pyrope macrocrysts, but the former is higher in HREE than the latter, showing theirdifferent chondrite-normalized patterns. The formation pressures of pyropes calculated by Cr-component, Ca-component, knorringite molecules of pyropes show that some pyropes of thepurple series in diamondiferou kimberlites fall into the diamond stability field, but all pyropesof diamond-free kimBased on the measurements of refractive index, specific gravity, unit cell parame-ter, and mineral chemistry and infrared absorption spectrum analyses of pyropes in kimberlitesfrom China, systematic studies of the physical properties and compositional variations of py-ropes of different colors and diverse paragenetic types, within and between kimberlite provinceshave been undertaken. The origin of pyropes in the kimberlites and the depth of their forma-tion have been discussed.Pyropes of the purple series are different from those of the orange series in physical andchemical properties, for example, pyropes of the purple series are higher in α0, RI, SG,Cr2O3, MgO, Cr/(Cr + Al), Mg/(Mg + Fe), and Mg/(Mg + Ca), and lower in Al2O3,Fe2O3 + FeO than those of the orange series. The classification of garnets in kimberiites fromChina by the Dawson and Stephens' method (1975) has been undertaken and clearly demon-strates that pyropes of diamond-rich kimberlites contain much more groups than those of dia-mond-Poor, especially diamond-free kimberlites. The higher in α0, RI, SG, Cr2O3,, Cr/(Cr+ Al), knorringite and Cr-component the pyropes are, the richer in diamond the kimberliteswill be. The infrared absorption spectrum patterns of pyropes change with their chemical com-position regularly, as reflected in the shape and position of infrared absorption peaks. Two ab-sorption bands at 862 - 901 cm(-1) will grade into degeneration from Splitting and the absorptionband positions of pyropes shift toward lower frequency with increasing Cr2O3 content and Cr/(Cr + Al) ratio of pyropes. LREE contents of orange pyrope megacrysts are similar to those ofpurple pyrope macrocrysts, but the former is higher in HREE than the latter, showing theirdifferent chondrite-normalized patterns. The formation pressures of pyropes calculated by Cr-component, Ca-component, knorringite molecules of pyropes show that some pyropes of thepurple series in diamondiferou kimberlites fall into the diamond stability field, but all pyropesof diamond-free kimbe

关 键 词：榴子石 金伯利岩 中国 矿物学 金刚石 

分 类 号：P578.947[天文地球—矿物学]