ИСТИНА

Experiments in the model system Si-Al-Na-Li-H-F-O at T=800°C and p=1kbar show possibility of equilibrium crystallization of fluorine minerals (cryolite, topaz, villiomite) or the origin of immiscible aluminofluoride melt (brine) in case of high fluorine concentration in the system. The area of coexistence of two immiscible silica and salt melts extends for both nepheline- and quartz-normative compositions of silica melts. In case of two melts coexistence tantalum and niobium are distributed into the silicate melt. By virtue of the different values of partition coefficients the ratios of tantalum to niobium in residual aluminosilicate melt increase. The experimentally determined phase relations are in general agreement with compositions of topaz and/or cryolite bearing granites from different regions. In natural systems, in the final stages of crystallization high amounts of incompatible elements can lead to the separation of salt melt. Brine appears to be an effective concentrator of significant ore elements such as REE, Y, Sc, U, Th, Mo, W etc. [1]. Occurrence of salt melt can also lead to increasing of Ta/Nb ratio in the most evolved varieties of rare metal granites observed in numerous massifs [2]. [1] Gramenitskii & Shchekina (2001) Geochem.Int. 43, 39-52. [2] Syritso at al. (2001) Petrology 9/3, 268-289