Abstract

The phase relations in the quaternary system FeO–Fe₂O₃–P₂O₅–H₂O were investigated under hydrothermal conditions at 386 and 586 °C at a pressure of 0.3 GPa. Under these conditions a restricted solid solution series coexists with the H₂O bearing fluid phase between the Fe₂(PO₄)O and Fe₄(PO₄)₃(OH)₃ compositions. The members of this solid solution series were found to be isotypic to tetragonal β-Fe₂(PO₄)O. At 586 °C the composition of this series covers the range between 0.18 ≤ x ≤ 0.60 according to a stoichiometry of Fe³⁺_4 − xFe²⁺_3x(PO₄)₃(OH)_3 − 3xO_3x. With decreasing temperature this compositional range is reduced to compositions between 0.52 ≤ x ≤ 0.58. At 586 °C the Fe₂(PO₄)O_ss solid solution series was found to coexist with hematite (Fe₂O₃), sarcopside (Fe₃(PO₄)₂), Fe₇(PO₄)₆ and monoclinic Fe₄(PO₄)₃(OH)₃ whereas at 386 °C they coexist with α-Fe₂O₃ (hematite), Fe₃(PO₄)₂ (sarcopside), Fe₇(PO₄)₆ and Fe₃(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 Fe₂(PO₄)O_ss solid solution series. Therefore these results support the assumption that the Fe₂(PO₄)O_ss solid solution is a stable product under the applied hydrothermal conditions.