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Hydrothermal synthesis and crystal structure of Na(Na,Mn)₇Mn₂₂(PO₄)₁₈·0.5H₂O, a new compound of fillowite structure type

¹ Institut für Mineralogie und Kristallchemie, Universität Stuttgart, D-70550 Stuttgart, Germany
² Laboratoire de Minéralogie, Université de Liège, B-4000 Liège, Belgium
³ Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany
⁴ Institut für Anorganische Chemie, Universität Stuttgart, D-70550 Stuttgart, Germany

^* Corresponding author, e-mail: paul.keller{at}mineralogie.uni-stuttgart.de

The chemical compound Na(Na,Mn)₇Mn₂₂(PO₄)₁₈·0.5H₂O was synthesized with a Tuttle pressure vessel at 800°C and 1 kbar during experimental investigations on the conditions of fillowite formation. The crystal structure of a single-crystal, space group R (No. 148), a = 15.2741(9), c = 43.334(3) Å, Z = 6, was determined from X-ray intensity data and refined up to R₁ = 0.0728 and R_1{FO>4(FO)} = 0.0546. The crystal structure of Na(Na,Mn)₇Mn₂₂(PO₄)₁₈·0.5H₂O is similar to the structure of fillowite, Na₆Ca₃(Mn,Fe)₂₁(PO₄)_18., but a significant difference between the synthetic H₂O containing compound and fillowite is the replacement of (Na12)O₈- by Mn(H₂O)₂O₅- polyhedra, which form characteristic six-member rings. The coordination polyhedra around Na12 and Mn12 are so different that simple isomorphous substitution of Na12 by Mn12 is very unlikely, but whole six-member rings either of (Na12)O₈- or (Mn12)(H₂O)₂O₅ polyhedra seem to be distributed statistically over the crystal structure. No indication for ordering, e.g. sheet by sheet, has been observed. Na(Na,Mn)₇Mn₂₂(PO₄)₁₈·0.5H₂O is the first fillowite-like compound for which water is located in the structure from single-crystal study. This result is proved by infrared spectral data and indicates that water can play an essential role in the fillowite structure-type. Structural features of the synthetic compounds Na(Na,Mn)₇Mn₂₂(PO₄)₁₈·0.5H₂O, Na₄Ca₄Mg₂₁(PO₄)₁₈ and fillowite are compared.

Key-words: Na(Na,Mn)₇Mn₂₂(PO₄)₁₈·0.5H₂O, crystal structure, fillowite, phosphate minerals, pegmatites.