Crystal structure determination of orthorhombic variscite2O and its derivative AlPO4 structure at high temperature

Variscite [Al(PO4)·2H2O] 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 variscite1O and the “Messbach-type” as variscite2O, 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. 31P and 27Al 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 variscite2O. High-temperature XRD, thermal analyses, and NMR measurements were applied to study the nature of the transformation of variscite2O to a derivative AlPO4 structure above 473 K. Charge-flipping analysis showed that the crystal structure of the new anhydrous AlPO4 phase (AlPO4-var2O in analogy to its parent structure) can be described as a 3D framework of alternating AlO4 and PO4 tetrahedra linked by bridging O atoms. Thermogravimetric analyses revealed almost complete dehydration above ~450 K, and NMR results were consistent with tetrahedral Al and P atoms.