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

Abstract

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 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 LucinAmerican Mineralogist 2 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 confirm 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 ~450K, and NMR results were consistent with tetrahedral Al and P atoms.