A new type of cubic-stacked layer structure in anthoinite, AlW[O.sub.3][(OH).sub.3]

Anthoinite, AlW[O.sub.3][(OH).sub.3], from the Mt. Misobo Mine, Democratic Republic of the Congo, has triclinic symmetry with cell parameters a = 8.196(1)[Angstrom], b = 9.187(1) [Angstrom], c = 11.316(1) [Angstrom], [alpha] = 92.82(1)[degrees], [beta] = 94.08(1)[degrees], [gamma] = 90.23(1)[degrees], space group [bar.I1], Z = 8. The structure was solved by applying ab initio structure solution methods (Reverse Monte Carlo/Simulated Annealing) to both X-ray and neutron powder diffraction data and was refined using the Rietveld method. The structure is built up of two types of [M.sub.4][(O,OH).sub.16] planar tetrameric clusters of edge-sharing octahedra, one containing predominantly Al and the other predominantly W. The Al-rich and W-rich clusters interconnect via corner sharing to form stepped layers parallel to (001). The layers are held together by strong hydrogen bonding. The structure can be described as a rocksalt derivative structure, with the close-packed anion layers parallel to (012), and with Al and W atoms ordered into one third of the octahedral sites within the cubic close-packed anion lattice. The structure is complicated by partial disorder between Al and W in the tetrameric clusters and associated disorder in the H atom sites. Infrared and [sup.27]Al MAS NMR results are also presented for anthoinite. Keywords: Structure of anthoinite, ab initio structure determination, new layer structure in AlW[O.sub.3][(OH).sub.3], layer structure of anthoinite DOI: 10.2138/am.2010.3377