High Pressure Synthesis and Stability Field of Cobalt Garnet (Co<sub>3</sub>Al<sub>2</sub>Si<sub>3</sub>O<sub>12</sub>)

Abstract

Glass of 3CoO⋅Al2O3⋅3SiO2 composition was treated at temperatures ranging from 600° to 1900°C at pressures of 20 to 40kb using a girdle type high pressure apparatus. The glass specimen was placed in a platinum tube heater 3.0mm in diameter and 9.0mm long. The heat treatment temperature was measured by a platinum-alumel thermocouple inserted in the specimen chamber or was estimated from the temperature-heating power input relations at several pressures. The temperature distribution and the pressure gradient in the specimen chamber were measured. Minerals in the treated specimens were identified by the X-ray diffraction method and the microscopic observations. Specimens used for the identification were taken from a layer of 0.5mm thick at the central part of the specimen chamber, in order to minimize the variation of temperature and pressure of the treatment. The maximum deviation of the measured temperatures was ±40°C at 1700°C at 40kb. It decreased with decreases in temperature and pressure to ±20°C at temperatures lower than 1300°C at 20kb. The error in the pressure measurement was estimated to be ±2kb at the maximum.Cobalt garnet is stable at pressures above 23kb at 1200°C and above 24kb at 1500°C. Below these pressures at the respective temperatures, mixtures of cobalt spinel (CoO⋅Al2O3), cobalt olivine (2CoO⋅SiO2) and quartz were obtained from the glass. Garnet produced at 1400°C at 25kb broke down to spinel, olivine and quartz when it was held at 18 kb at the same temperature. Cobalt garnet melts congruently at 1670°C and 25kb. It seems that the garnet melts incongruently to spinel and liquid at temperatures from 1500°C to 1670°C at pressures lower than 23kb. The slope of melting curve of the garnet is 12°C/kb. At 900°C the glass devitrifies to a mixture of spinet, olivine and quartz even at 25kb, and at 800°C the same mixture was obtained at 30kb.The lattice constant a0 of the cobalt garnet (Co3Al2Si3O12) is 14.473±0.008 A and the refractive index nD is 1.82±0.01 (25°C). The color of the garnet is bright pink.