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Franconite, NaNb2O5(OH)·3H2O: structure determination and the role of H bonding, with comments on the crystal chemistry of franconite-related minerals

  • M. M. M. Haring (a1) and A. M. McDonald (a1)

Abstract

The crystal structure of franconite, NaNb2O5(OH)·3H2O, has been characterized by single-crystal X-ray diffraction using material from Mont Saint-Hilaire, Québec, Canada. Results give a = 10.119(2), b = 6.436(1), c = 12.682(2) Å and β = 99.91(3)° and confirm the correct space group as P21/c. The crystal structure, refined to R = 4.63% and wR 2 =11.95%, contains one Na site, two distorted octahedral Nb sites and nine O sites. It consists of clusters of four edge-sharing Nb(O,OH)6 octahedra, linked through shared corners to adjacent clusters, forming layers of Nb(O,OH)6 octahedra. These alternate along [100] with layers composed of NaO(H2O)4 polyhedra, the two being linked together by well defined H bonding. The predominance of H bonding, essential to the mineral, results in a perfect {100} cleavage. Chemical analyses (n = 7) of four crystals give the empirical formula (Na0.73Ca0.13☐0.14)∑=1.00(Nb1.96Ti0.02Si0.02Al0.01)∑=2.01O5(OH)·3H2O (based on nine oxygens) or ideally NaNb2O5(OH)·3H2O. Franconite is crystallo-chemically related to SOMS [Sandia Octahedral Molecular Sieves; Na2Nb2−x MxO6−x (OH) x ·H2O with M = Ti, Zr, Hf], a group of synthetic compounds with strong ion-exchange capabilities. Both hochelagaite (CaNb4O11·nH2O) and ternovite (MgNb4O11·nH2O) have X-ray powder diffraction patterns and cation ratios similar to those of franconite indicating that these minerals probably have similar structures.

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