Experimental magnesitization of calcite at 150℃ and partitioning behavior of REE between magnesite and hydrothermal solution

Abstract

Reagent grade calcite was reacted with MgCl₂ solutions doped with REE (Mg concentration of 0.089M-1.8M and REE concentration of about 0.3 ppm each) at 150℃ for 10 or 20 days. Solid products were identified by X-ray diffraction. Solid and liquid phases were analyzed for Mg, Ca and REE. The solid phases reacted with MgCl₂ solutions (0.89M and 1.8M of Mg²⁺) were almost magnesite (about 95%). The molar ratios of Mg/(Mg + Ca) in solutions of the run products were consistent with the extrapolation of the reported experimental equations in the high temperature range from 275℃ to 420℃ to lower temperatures. The values of pH at room temperature of the final solutions are ranging from 5-7, and they show a decreasing trend with increasing Mg concentration. This is probably caused by the precipitation reaction of brucite, which is more likely to occur at 150℃ than at 25℃. The REE partition coefficients between solid and liquid phases defined by K_d(REE)={X_REE/(X_Mg+X_Ca)}_solid/{[REE]/([Mg]+[Ca])}_liquid have been determined experimentally. The series variation of logK_d (REE) in the run in which the solid phases were almost magnesite, shows a decreasing trend in LREE and a convex tetrad effect.