Polytypism of AlPO4•2H2O variscite

Variscite (orthorhombic), a hydrated aluminophosphate with formula Al(PO4)•2H2O, and metavariscite (the monoclinic polymorph) are uncommon secondary minerals that are relevant in environmental applications and particularly in technological applications when synthesized in a dehydrated form. Although two orthorhombic modifications are known (so-called "Lucin-type" and "Messbach-type"), the fine-grained nature of the "Messbach-type" variscite has hampered determination of its crystal structure.In this contribution, the crystal structure of the latter (a natural specimen from Tooele County, Utah) has been solved and refined using X-ray powder diffraction data via ab initio charge-flipping methods and the Rietveld method. Our results, structural interpretations, and topological analysis demonstrate that the two orthorhombic structural modifications are polytypes, and we refer to them as variscite1O ("Lucin-type") and as variscite2O ("Messbach-type"), to be consistent with modern polytype terminology. The structure of variscite2O is similar to that of the 1O polymorph, with a doubling of the b unit-cell parameter. The variscite2O crystal structure contains two crystallographically independent Al3+ cations coordinated by two H2O molecules and four oxygen atoms of the PO4 groups. Two crystallographically independent PO4 tetrahedra share their corners with four adjacent AlO4(OH2)2 octahedra. Both orthorhombic polymorphs belong to the family of framework 3D MT structures in which octahedra (M) and tetrahedra (T) are linked by bridging O atoms, and topological analysis suggests that these two structures may be considered polytypes. Similarities between these polytypes, along with observed broadening of diffraction peaks of the Tooele material, suggest that interstratifications of the two forms may exist in nature. Besides the long-range characterization of the crystal structure by X-ray diffraction, information on the short-range structural properties of this mineral have been gained through 31P and 27Al MAS/NMR measurements. Results from NMR corroborate the crystal structure determination and they show distinct signals for each of the two independent P and Al positions in variscite2O. In addition, high-temperature XRD, thermal analyses, and NMR measurements clarified the nature of the transformation of variscite2O to the derivative AlPO4 structure. The crystal structure of this new anhydrous AlPO4 phase (AlPO4-variscite2O 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 Al and P atoms located at tetrahedral sites.