THE CRYSTAL STRUCTURE AND CRYSTAL CHEMISTRY OF MANITOBAITE, IDEALLY (Na16 )Mn2+ 25Al8(PO4)30, FROM CROSS LAKE, MANITOBA

Abstract

The crystal structures of green (brown) variants of manitobaite, ideally (Na 16 □)Mn 2+ 25 Al 8 (PO 4 ) 30 , monoclinic, Pc , Z = 2: green: a 13.4517(7), b 12.5266(7), c 26.6765(13) A, β 101.582(1)°, V 4403.6(7) A 3 , D calc. 3.642 g/cm 3 ; brown: 13.4499(6), b 12.5046(5), c 26.6148(11) A, β 101.221(1)°, V 4390.7(5)(3) A 3 , D calc. 3.621 g/cm 3 , from pegmatite #22 at Cross Lake, Manitoba, Canada, have been solved by direct methods and refined to R 1 = 5.0 (6.0)% for 22,580 (25,613) unique ( F o > 4σ F ) reflections collected on a Bruker single-crystal P 4 diffractometer equipped with a 4K CCD detector and Mo K α X-radiation. Chemical analysis by electron microprobe plus Fe 3+ determination by Mossbauer spectroscopy gave: green: P 2 O 5 44.19, Al 2 O 3 6.91, Fe 2 O 3 1.73, FeO 6.23, MnO 27.57, ZnO 0.54, MgO 0.73, CaO 1.71, Na 2 O 9.97, sum 99.58 wt%; brown: P 2 O 5 44.42, Al 2 O 3 6.96, Fe 2 O 3 3.54, FeO 4.66, MnO 27.86, ZnO 0.53, MgO 0.81, CaO 1.59, Na 2 O 8.94, sum 99.32 wt%. The resulting empirical formulae are as follows: green: Na 15.55 Ca 1.47 Mg 0.88 Fe 2+ 4.19 Mn 2+ 18.78 Zn 0.32 Al 6.54 Fe 3+ 1.05 P 30.08 O 120 ; brown: Na 13.90 Ca 1.37 Mg 0.97 Fe 2+ 3.12 Mn 2+ 18.92 Zn 0.31 Al 6.58 Fe 3+ 2.09 P 30.15 O 120 . The general formula of manitobaite is (Na 16 □)(Mn 2+ ,Fe 2+ ,Mg,Zn,Ca) 25 (Al,Fe 3+ ) 8 (PO 4 ) 30 , and the end-member formula is (Na 16 □)Mn 2+ 25 Al 8 (PO 4 ) 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 Mn 2+ , Fe 2+ , Al, Fe 3+ 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.