The crystal structure of walentaite, originally reported as a calcium iron arsenate phosphate, has been determined and the As has been found to be predominantly in the trivalent state, as AsO3 trigonal pyramids and as As3O6 trimeric clusters. The mineral has body-centred orthorhombic symmetry with a = 26.188(5), b = 7.360(2), c = 10.367(2) Å. The structure was solved in space group Imma using synchrotron single-crystal diffraction data and refined to R1 = 0.054 for 1454 observed reflections. The walentaite structure is based on the stacking of heteropolyhedral layers along [100] with the layers held together by hydrogen bonds to interlayer hydrated cations M(H2O)6, M = Mn2+, Fe2+, Ca2+, Na+. The layers, of composition Fe33+(P0.84As0.16O4)2(O,OH)6, have the same topology of corner-connected octahedra and tetrahedra as in the alunite-supergroup mineral kintoreite-1c. Unit-cell-scale twinning of the kintoreite-like layers gives a saw-tooth aspect to the layers. The AsO3, As3O6 and (Ca,Na)O6 groups are attached to the surface of the layers. Although the kintoreite-like heteropolyhedral layers are fully ordered in Imma, the surface-attached groups and the interlayer hydrated cations are highly disordered, so the Imma model represents the average structure. Models for the local ordering have been developed from interpretation of the partial site occupancies and splitting of the disordered atoms. The crystal structure results have been combined with electron microprobe analyses to obtain a new formula for walentaite, Fe33+(P0.84As0.16O4)2(O,OH)6As2.563+Ca0.42Na0.28Mn0.352+Fe0.302+O6.1(OH)0.9(H2O)5.2, in which 89% of the As is present as As3+.

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