Fe(II)2(PO4)(OH), a synthetic analogue of wolfeite.

This paper reports the hydrothermal synthesis and crystal structure refinement of diiron(II) phosphate hydroxide, Fe(II)2(PO4)(OH), obtained at 1063 K and 2.5 GPa. This phosphate is the synthetic analogue of the mineral wolfeite, and has a crystal structure topologically identical to those of minerals of the triplite-triploidite group. The complex framework contains edge- and corner-sharing FeO4(OH) and FeO4(OH)2 polyhedra, linked via corner-sharing to the PO4 tetrahedra (average P-O distances are between 1.537 and 1.544 A). Four five-coordinated Fe sites are at the centers of distorted trigonal bipyramids (average Fe-O distances are between 2.070 and 2.105 A), whereas the coordination environments of the remaining Fe sites are distorted octahedra (average Fe-O distances are between 2.146 and 2.180 A). The Fe-O distances are similar to those observed in natural Mg-rich wolfeite, except for two Fe-O bond distances, which are significantly longer in synthetic Fe(2+)2(PO4)(OH).