ALBERT

Description: 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–Fe 2+ –Fe 3+ (+PO 4 ) ternary system. These experiments produced several new phosphate minerals, among which was Na 2 Fe 3+ (HPO 4 )(PO 4 )·H 2 O [ a 15.5004(8), b 7.1465(5), c 29.239(2)Å, V 3238.9(3)Å 3 , Pnma ], which shows a crystal structure based on chains of corner-sharing FeO 6 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–Fe 2+ –Fe 3+ (+PO 4 ) 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–Fe 2+ –Fe 3+ (+PO 4 ) diagram has been established, taking into account the connectivity between FeO 6 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 FeO 6 octahedra in the structure as the Na content increases.