Stability properties and phase relations of Fe4-x3+Fe3x2+ (PO4)3(OH)3-3xO3x in the quaternary system FeO-Fe2O3-P2O5-H2O

The phase relations in the quaternary system FeO-Fe2O3-P2O5-H2O were investigated under hydrothermal conditions at 386 and 586 degreesC at a pressure of 0.3 GPa. Under these conditions a restricted solid solution series coexists with the H2O bearing fluid phase between the Fe-2(PO4)O and Fe-4(PO4)(3)(OH)(3) compositions. The members of this solid solution series were found to be isotypic to tetragonal beta -Fe-2(PO4)O. At 586 degreesC the composition of this series covers the range between 0.18 less than or equal to x less than or equal to 0.60 according to a stoichiometry of Fe4-x3+Fe3x2+(PO4)(3)(OH)(3-3x)O-3x. With decreasing temperature this compositional range is reduced to compositions between 0.52 less than or equal to x less than or equal to 0.58. At 586 degreesC the Fe-2(PO4)O-ss solid solution series was found to coexist with hematite (Fe2O3), sarcopside (Fe-3(PO4)(2)), Fe-7(PO4)(6) and monoclinic Fe-4(PO4)(3)(OH)(3) whereas at 386 degreesC they coexist with alpha -Fe2O3 (hematite), Fe-3(PO4)(2) (sarcopside), Fe-7(PO4)(6) and Fe-3(PO4)(2)(OH)(2) (barbosalite). The synthesis runs with the same compositions and P-T conditions but different starting compounds yielded a nearly phase pure member of the Fe-2(PO4)O-ss solid solution series. Therefore these results support the assumption that the Fe-2(PO4)O-ss solid solution is a stable product under the applied hydrothermal conditions.