Cryolite as a Reference Mineral of Rare Metal Mineralization: An Experimental Study

Phase relations and the distributions of rare earth elements (REE), Sc, Y, and Li between aluminofluoride and aluminosilicate melts in the model granite system Si–Al–Na–K–Li–F–O–H were experimentally studied at 700°C, 1 and 2 kbar, and water contents of 3 to 50 wt %. Our original and available literature experimental data on phase relations in the granite system saturated with water and fluorine and containing trace elements are compared with the mineral assemblages of rare-metal cryolite-bearing granites from the Zashikhinsky, Katugin, and Ulug-Tanzek deposits in eastern Siberia. Liquid immiscibility between granite and salt aluminofluoride melts, which occurs at high contents of fluorine and lithium in the system, is proved to facilitate the accumulation of rare elements in salt cryolite-like melts. At a temperature of 700°C and pressures of 1 and 2 kbar, aluminofluoride melt in the granite system crystallizes and forms cryolite. Fluorine-bearing minerals of trace and rare earth elements, such as pyrochlore and gagarinite, occur at these deposits in association with cryolite and lithium micas. Comparison of experimental data and natural observations provides arguments in support of the hypothesis that liquid immiscibility should play an important role in the formation of cryolite. Cryolite is thought to be able to serve as a reference mineral for rare metal-rare earth mineralization in granites with high lithium and fluorine content.