Modeling the Disordered Layer Structure of Vashegyite, Al5(PO4)4(OH)3(H2O)7· <i>n</i> H2O, <i>n</i> <b>∼</b> 9, from Chvaletice, Czech Republic

Abstract The crystal structure of the hydrated aluminum phosphate mineral vashegyite from Chvaletice, Czech Republic, was characterized using synchrotron single-crystal diffraction data. The data were indexed with an orthorhombic cell, a = 10.8424(3), b = 14.9140(4), c = 19.3408(5) Å. The c parameter is markedly shorter than that for the originally described mineral (22.67 Å) due to partial dehydration and a shrinking of the separation of the (001) layers of the layer structure. The dehydration results in disorder in the (001) layer stacking, manifested in strong streaking of diffraction spots along c*. A structure solution was obtained in P2an. The solution was unusual in comprising a fully ordered heteropolyhedral layer at z = ½ but a disordered layer of partially occupied sites at z = 0. It was possible to construct AlO4, PO4, and AlO6 polyhedra from the sites in the disordered layer. Groups of the polyhedra formed corner-sharing segments identical to those in the ordered layer. From the spatial relationships between the polyhedra in the ordered and disordered layers, a model was developed for the local layer ordering in space group P21/n. The layers are formed from intergrown Al2(PO4)2Φ7 and Al2(PO4)2Φ8 clusters that form ribbons along [100]. The ribbons are interconnected along [010] via corner-sharing of PO4 tetrahedra with AlO4 tetrahedra to give an open 2D framework of composition Al5(PO4)4(OH)3(H2O)7. The heteropolyhedral framework encompasses 10-sided cavities. H2O molecules occupy sites within the cavities and between the layers, giving an overall formula for partially dehydrated vashegyite of Al5(PO4)4(OH)3(H2O)7·8.7H2O.