Abstract

The crystal structures of green (brown) variants of manitobaite, ideally (Na16□)Mn2+25Al8(PO4)30, monoclinic, Pc, Z = 2: green: a 13.4517(7), b 12.5266(7), c 26.6765(13) Å, β 101.582(1)°, V 4403.6(7) Å3, Dcalc. 3.642 g/cm3; brown: 13.4499(6), b 12.5046(5), c 26.6148(11) Å, β 101.221(1)°, V 4390.7(5)(3) Å3, Dcalc. 3.621 g/cm3, from pegmatite #22 at Cross Lake, Manitoba, Canada, have been solved by direct methods and refined to R1 = 5.0 (6.0)% for 22,580 (25,613) unique (Fo > 4σF) reflections collected on a Bruker single-crystal P4 diffractometer equipped with a 4K CCD detector and MoKα X-radiation. Chemical analysis by electron microprobe plus Fe3+ determination by Mössbauer spectroscopy gave: green: P2O5 44.19, Al2O3 6.91, Fe2O3 1.73, FeO 6.23, MnO 27.57, ZnO 0.54, MgO 0.73, CaO 1.71, Na2O 9.97, sum 99.58 wt%; brown: P2O5 44.42, Al2O3 6.96, Fe2O3 3.54, FeO 4.66, MnO 27.86, ZnO 0.53, MgO 0.81, CaO 1.59, Na2O 8.94, sum 99.32 wt%. The resulting empirical formulae are as follows: green: Na15.55Ca1.47Mg0.88Fe2+4.19Mn2+18.78Zn0.32Al6.54Fe3+1.05P30.08O120; brown: Na13.90Ca1.37Mg0.97Fe2+3.12Mn2+18.92 Zn0.31Al6.58Fe3+2.09P30.15O120. The general formula of manitobaite is (Na16□)(Mn2+,Fe2+,Mg,Zn,Ca)25(Al,Fe3+)8(PO4)30, and the end-member formula is (Na16□)Mn2+25Al8(PO4)30. There are 80 cation sites and 120 anion sites in the structure. There are 30 tetrahedrally coordinated sites occupied by P, 33 octahedrally coordinated sites occupied by Mn2+, Fe2+, Al, Fe3+ plus minor Mg and Zn, and 17 sites occupied predominantly by Na with coordination numbers from [5] to [8]. Manitobaite is an ordered superstructure of the alluaudite structure. In particular, there are eight octahedrally coordinated sites occupied predominantly by Al, emphasizing the key role of Al in producing superstructures in these minerals.

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