The previously unknown crystal structure of wycheproofite, NaAlZr(PO4)2(OH)2-H2O (space group P1̄, a= 5.263(1), b = 9.251(2), c = 9.480(2) Å, α = 109.49(3), β = 98.57(3), γ = 90.09(3)°, V= 429.60(15) Å3Z = 2) has been solved using single-crystal X-ray diffractometer data (CCD area detector, Mo-Kα) and refined to R1 =4.18% for 1731 ’observed’ reflections with F0 > 4σ(F0). The structure determination led to a completely revised unit cell. The atomic arrangement of wycheproofite is characterised by two different alternating layers parallel to (001): the first layer is composed of corner-linked ZrO6 octahedra and PO4 tetrahedra. The second layer is built from zigzag chains of edge-sharing AlO2(OH)4 octahedra along [100], and of Na1-xO3(OH)2(H2O)2-y (x ∼ 0.1, y ∼ 0.6) polyhedra. Adjacent layers are connected by oxygen ligands shared between PO4 tetrahedra and AlO2(OH)4 octahedra. Only very weak hydrogen bonding is necessary to reinforce the structural framework. The Al-, Zr- and P-O polyhedra are all fairly regular and average Al-O, Zr-O and P-O bond lengths are 1.898, 2.063 and 1.529 Å, respectively. The unique Na site is only partially occupied (∼ 88 %) and slightly disordered; it is surrounded by three O atoms, two OH groups and two H2O molecules with Na-O distances ranging from 2.247(14) to 2.725(4) Å. Two of its O ligands (Ow 11 and Ow12) are also only partially occupied, and split (Ow 11) or somewhat disordered (Ow 12), in agreement with bond-valence calculations. The originally given chemical formula, NaAlZr(PO4)2(OH)2-H2O, is therefore an idealised formula, although it is very close to the presently obtained formula.

Comparisons with the structures of the few other known natural and synthetic zirconium phosphates (e.g., kosnarite -KZr2(PO4)3; selwynite - NaK(Be,Al)Zr2(PO4)4·2H2O; mahlmoodite - FeZr(PO4)2-4H2O; synthetic α-Zr(HPO4)2-H2O, ZrKH(PO4)2 and Zr2(NaPO4)4·6H2O) demonstrates that a heteropolyhedral layer composed of corner-linked PO4 tetrahedra and ZrO6 octahedra is a common structural feature of several of these phosphates, and that it links them closely to the family of yavapaiite-related layered AM(XO4)2 compounds.

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