NUMERICAL SIMULATION OF THE T-P-FSIO2 STABILITY BOUNDARIES OF SPODUMENE CONTROLLING ITS ORIGIN IN RARE-METAL GRANITIC PEGMATITES

Abstract

Phase equilibria at temperatures up to 800degreesC and pressures up to 12 kbar were numerically simulated in the SiO2-LiAlSiO4-H2O singular system that includes quartz, petalite, alpha-spodumene, beta-spodumene, and eucrypite. The thermodynamic properties of the minerals were compiled from [I], and a T-P diagram of the state of this system was constructed. It was determined that the stability fields of two-mineral assemblages in the solidus region clearly depend on the pressure. The reactions Pet + 2Ecr = 3Spd and Pet = Spd + 2Qtz divide the diagram into three fields, with the Qtz + Pet and Pet + Ecr mineral assemblages stable within the left-hand field, Qtz + Pet, Pet + Spd, and Spd + Ecr stable in the middle field, and Qtz + Spd and Spd + Ecr stable in the right-hand field. The accuracy of the thermodynamic properties of the minerals is discussed. Stability fields are calculated for spodumene, petalite, and eucrypite in a system including solutions undersaturated with respect to quartz in isothermal sections at T = 400 and 600degreesC. The optimum parameters that control the crystallization of monomineralic spodumene during the metasomatic replacement of rare-metal granites are assayed at T = 400-600degreesC, P = 1-2 kbar, logf(SiO2) from -0.25 to -1.43, pH above the neutral point, and an F/Li ratio of the solution no higher than 13. Spodumene is unstable in quartz-bearing assemblages with Li-F micas.