INTERCRYSTALLINE CATION PARTITIONING BETWEEN MINERALS OF THE TRIPLITE-ZWIESELITE-MAGNIOTRIPLITE AND THE TRIPHYLITE-LITHIOPHILITE SERIES IN GRANITIC PEGMATITES

Minerals of the triphylite-lithiophilite, Li(Fe, Mn)PO4, and the tripl ite-zwieselite-magniotriplite series, (Mn, Fe, Mg)(2)PO4F, occur in th e fate stage period of pegmatite evolution. Unfortunately, neither are the genetic relationships between these phosphates fully understood n or are thermodynamic data known. Consequently, phosphate associations and assemblages from 8 granitic pegmatites - Clementine II, Rubicon II and III, and Tsaobismund (Namibia); Hagendorf-Sud and Rabenstein (Ger many); Valmy (France); Viitaniemi (Finland)- have been tested for comp ositional zoning and intercrystalline partitioning of main elements by electron microprobe techniques. Although the selected pegmatites disp lay varying degrees of fractionation, and the intergrowth textures ind icate different genetic relationships between the phosphates, the plot s of mole fractions X(Fe) = Fe/(Fe + Mn + Mg + Ca), X(Mn) = Mn/ (Fe Mn + Mg + Ca), and X(Mg) = Mg/(Fe + Mn + Mg + Ca) can be fitted relati vely well with smooth curves in Roozeboom diagrams. Their deviations f rom symmetrical distribution curves are mainly dependent upon X(Mg) or X(Ca), and upon non-ideal solutions. Surprisingly small differences b etween the partition coefficients were detected for intergrowths of di fferent origin. However, the partitioning of shared components among c oexisting phases is clearly dependent upon the conditions of formation . Compositional zoning is observed only when both Fe-Mn phosphates are intergrown mutually or with other Fe-Mn-Mg mineral solid-solutions. T hus, the zoning does not seem to be due to continuous crystallization, but to later diffusion processes. The triplite structure has preferen ce for Mn, Mg, and Ca, while Fe prefers minerals of the triphylite ser ies. A quantification of main element fractionation between minerals o f the triphylite and the triplite series is possible in the cases wher e diffusion can be excluded. For the Fe/(Fe + Mn) ratios of core compo sitions an equation with a high correlation coefficient (R = 0.988) wa s determined: Fe/(Fe + Mn)(Tr) = [Fe/(Fe + Mn)(Li)]/{2.737 - (1.737)[F e/ (Fe + Mn)(Li)]} (Tr = triplite series, Li = triphylite series). Con sequently, the Fe/(Fe + Mn) ratio of the triplite series can now also be used in the interpretation of pegmatite evolution, just like that o f the triphylite series which has been successfully applied in the pas t.