The stability of primary alluaudites in granitic pegmatites: an experimental investigation of the [Na.sub.2]([Mn.sub.2-2x][Fe.sub.1+2x])[(P[O.sub.4]).sub.3] system

In order to assess the geothermometric potential of the [Na.sub.2]([Mn.sub.2-2x][Fe.sub.1+2x]) [(P[O.sub.4]).sub.3] system (x = 0-1), which represents the compositions of natural weakly oxidized alluaudites, we performed hydrothermal experiments between 400 and 800[degrees]C, at 1 kbar, under an oxygen fugacity (f([O.sub.2])) controlled by the Ni-NiO (NNO), [Fe.sub2][O.sub.3]-[Fe.sub.3][O.sub.4] (HM), [Cu.sub.2]O-CuO (CT), and Fe-[Fe.sub.3][O.sub.4] (MI) buffers. When f([O.sub.2]) is controlled by NNO, single-phase alluaudites crystallize at 400 and 500[degrees]C, whereas the association alluaudite + maricite appears between 500 and 700[degrees]C. The limit between these two fields corresponds to the maximum temperature that can be reached by alluaudites in granitic pegmatites, because maricite has never been observed in these geological environments. Because alluaudites are very sensitive to variations of oxygen fugacity, the field of hagendorfite, [Na.sub.2]Mn[Fe.sup.2+][Fe.sup.3+] [(P[O.sub.4]).sub.3], has been positioned in the f([O.sub.2])-T diagram, and provides a tool that can be used to estimate the oxygen fugacity conditions that prevailed in granitic pegmatites during the crystallization of this phosphate. Keywords Primary alluaudites * Na-Mn-[Fe.sup.2+]-[Fe.sup.3+] phosphates * Phase relations * Petrological implications * Pegmatites