Abstract

Graftonite-beusite is relatively widespread in the LCT family of granitic pegmatites of the Bothnian basin and of the Uto-Mysingen field in Sweden. It commonly constitutes the matrix containing either coarse lamellae of sarcopside, which in turn hosts fine lamellae of triphylite, or abundant lamellae of triphylite with bimodal size-distribution. Deformed and recrystallized granular aggregates of graftonite with triphylite, wolfeite and sarcopside are rare. Graftonite-beusite is systematically Ca,Mn-rich and Mg-poor relative to sarcopside, which in turn is slightly Mn-rich and distinctly Mg-poor relative to the Li-concentrating triphylite. Bulk compositions of the lamellar aggregates, crystal-chemical and textural relationships among the individual phosphates, and cation distribution-coefficients indicate two patterns of exsolution that generated the lamellar intergrowths from disordered homogeneous precursors: Ca,Li-poor graftonite exsolved lithian sarcopside, which in turn broke down to sarcopside with triphylite lamellae, and Ca,Li(Mn)-rich graftonite-beusite exsolved triphylite directly. In regional zoning of pegmatite groups, Ca-bearing graftonite-beusite is either the first Fe,Mn(Li)-phosphate encountered in the beryl-columbite pegmatites, or it is found within the zone of beryl-columbite-triphylite pegmatites. Appreciable concentrations of Ca, Fe, Mn (and in part Mg), but low levels of F and Na, are required to stabilize graftonite-beusite. Good correlation of the Mn/(Mn+Fe) values of the phosphates with those of garnet suggests that the homogeneous graftonite-beusite precursor could have been generated by phosphorus-induced destabilization of garnet.

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