Intercrystalline cation partitioning between minerals of the triplite-zwieselite-magniotriplite and the triphylite-lithiophilite series in granitic pegmatites |
| |
| Authors: | Paul Keller François Fontan André-Mathicu Fransolet |
| |
| Institution: | (1) Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany;(2) Laboratoire de Minéralogie, URA 067, Université Paul-Sabatier de Toulouse, Allée Jules-Guesde 39, F-31400 Toulouse, France;(3) Institut de Minéralogie, Université de Liège, Bâtiment B 18, Sart Tilman, B-4000 Liege, Belgium |
| |
| Abstract: | Minerals of the triphylite-lithiophilite, Li(Fe, Mn)PO4, and the triplite-zwieselite-magniotriplite series, (Mn, Fe, Mg)2PO4F, occur in the late stage period of pegmatite evolution. Unfortunately, neither are the genetic relationships between these phosphates fully understood nor are thermodynamic data known. Consequently, phosphate associations and assemblages from 8 granitic pegmatites — Clementine II, Rubicon II and III, and Tsaobismund (Namibia); Hagendorf-Süd and Rabenstein (Germany); Valmy (France); Viitaniemi (Finland) — have been tested for compositional zoning and intercrystalline partitioning of main elements by electron microprobe techniques. Although the selected pegmatites display varying degrees of fractionation, and the intergrowth textures indicate different genetic relationships between the phosphates, the plots 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 relatively well with smooth curves in Roozeboom diagrams. Their deviations from symmetrical distribution curves are mainly dependent upon X
Mg or X
Ca, and upon non-ideal solutions. Surprisingly small differences between the partition coefficients were detected for intergrowths of different origin. However, the partitioning of shared components among coexisting 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-solutios. Thus, the zoning does not seem to be due to continuous crystallization, but to later diffusion processes. The triplite structure has preference for Mn, Mg, and Ca, while Fe prefers minerals of the triphylite series. A quantification of main element fractionation between minerals of the triphylite and the triplite series is possible in the cases where diffusion can be excluded. For the Fe/(Fe+Mn) ratios of core compositions an equation with a high correlation coefficient (R=0.988) was determined: Fe/(Fe+Mn)Tr=Fe/(Fe+Mn)Li]/{2.737-(1.737)Fe/(Fe+Mn)Li]} (Tr=triplite series, Li=triphylite series). Consequently, the Fe/(Fe+Mn) ratio of the triplite series can now also be used in the interpretation of pegmatite evolution, just like that of the triphylite series which has been successfully applied in the past. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
