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Variscite [Al(PO 4 )·2H 2 O] is an uncommon secondary phosphate mineral but is important in a variety of environmental and technological applications. It exists in at least one monoclinic (metavariscite) and two orthorhombic polymorphs (“Lucin-type” and “Messbach-type”), but the fine-grained nature of the “Messbach-type” variscite has hampered the determination of its crystal structure. The crystal structure of the latter from Tooele County, Utah, was solved and refined using laboratory powder X‑ray diffraction (XRD) data, charge-flipping, and the Rietveld method. Both variscite modifications belong to the family of framework 3D MT structures in which octahedra (M) and tetrahedra (T) are linked by bridging O atoms. Topological analysis reveals that the two structures are polytypes. Based on our results and our structural interpretations, we refer to “Lucin-type” variscite as variscite 1O  and the “Messbach-type” as variscite 2O , to be consistent with modern polytype terminology. The similarity of these two structures suggests that  1O - 2O  interstratifications may exist in nature, which is consistent with observed broadening of diffraction peaks of the Tooele material.  31 P and  27 Al MAS/NMR measurements are consistent with the XRD-determined crystal structure, and they show distinct signals for each of the two independent P and Al positions in variscite 2O .

Crystal structure determination of orthorhombic variscite 2O and its derivative AlPO 4 structure at high temperature