Abstract

Melonjosephite, Ca2[(Fe2+0.5FeO3+0.5)4(OH)2(PO4)4], Z = 2, orthorhombic, a = 9.542(1), b = 10.834(1), c = 6.374(1)Å, space group Pbam, has chains of edge-sharing Fe–O octahedra oriented parallel to the c axis. R(hkl) = 0.043 for 728 independent reflections. Dimers of edge-sharing Fe–O octahedra share vertices with octahedra of the chains to form sheets parallel to {010} with the formula [Fe2(OH)(Op)7]2. The (PO4) tetrahedra link within and between sheets to form a framework structure. The CaO7 polyhedron occurs in pockets between the sheets.

Although two independent Fe atom sites occur in the structure, the mixed valence pair (Fe2+, Fe3+) is evenly distributed between them. Average bond distances are Ca–0 = 2.47Å, Fe(1)–0 = 2.08Å, Fe(2)–O = 2.06Å, P(l)–O = 1.54Å, P(2)–O = 1.54Å.

Regions of the melonjosephite crystal structure can be directly related to the octahedral tetramer found in leucophosphite, K2[Fe3+4(OH)2(H2O)2(PO4)4].2H2O, and regions of the structure of olmsteadite, K2Fe2+2[Fe2+2Nb5+2O4(H2O)4(PO4)4] and related compounds.

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