Abstract

In order to assess the stability of Fe-rich alluaudites in pegmatites, we performed hydrothermal experiments between 400 and 700 °C (1 kbar), in the Na–Fe2+–Fe3+ (+PO4) ternary system. These experiments produced several new phosphate minerals, among which was Na2Fe3+(HPO4)(PO4)·H2O [a 15.5004(8), b 7.1465(5), c 29.239(2)Å, V 3238.9(3)Å3, Pnma], which shows a crystal structure based on chains of corner-sharing FeO6 octahedra, similar to those occurring in the jahnsite-group minerals. Alluaudite-type phosphate minerals occupy a large stability field in the center of the Na–Fe2+–Fe3+ (+PO4) system; this stability field covers between 5.8 (500 °C) and 21.1% (700 °C) of the diagram surface. A comparison of the chemical composition of natural Fe-rich alluaudite-type phosphate minerals with the experimental data obtained herein indicates that these minerals crystallized below ca. 450 °C in pegmatites. This temperature value is in good agreement with the secondary Na-metasomatic origin of these phosphate minerals. Moreover, a structural classification of phosphates occurring in the Na–Fe2+–Fe3+ (+PO4) diagram has been established, taking into account the connectivity between FeO6 octahedra occurring in the crystal structures of the synthesized phases. This classification indicates a variation of the structural complexity of phosphates, characterized by an increasing dilution of FeO6 octahedra in the structure as the Na content increases.

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