THE CRYSTAL STRUCTURE OF ERCITITE, Na2(H2O)4[Mn3+2(OH)2(PO4)2], AND ITS RELATION TO BERMANITE, Mn2+(H2O)4[Mn3+2(OH)2(PO4)2]

The crystal structure of ercitite, ideally Na-2(H2O)(4)[Mn-2(3+)(OH)(2)(PO4)(2)], orthorhombic, a 6.2499(6), b 8.7479(9), c 19.9554(17) angstrom, V 1091.0 angstrom(3), space group Cmca, Z = 4, D-calc = 2.77 g.cm(-3), from the Tanco granitic pegmatite, Bernic Lake, southeastern Manitoba, Canada, has been solved by direct methods and refined to R-1 = 4.0% on the basis of 431 unique observed reflections vertical bar F-o >= 4 sigma F vertical bar collected on a Bruker P4 diffractometer equipped with a CCD 1K Smart detector and MoK alpha radiation. There is one distinct P site occupied by P and coordinated by four O atoms in an tetrahedral arrangement, with a < P-O > distance of 1.541 angstrom. There are two octahedrally coordinated M sites: M(1) is occupied by (Mn0.533+Fe0.463+Al0.01) with a <M(1)-O> distance of 2.028 angstrom, and M(2) is occupied by (square Mn-0.93(0.07)2+) with a <M(2)-O> distance of 2.252 angstrom. The M(1) octahedron shows four short meridional bonds (similar to 1.95 angstrom) and two long apical bonds (2.176 angstrom) characteristic of Jahn-Teller-distorted octahedrally coordinated Mn3+. There is one distinct Na site occupied by (Na0.89Ca0.04 square(0.07)) and coordinated by six anions with a < Na-O > distance of 2.504 angstrom. The structural unit in ercitite, [Mn-2(3+)(OH)(2)(PO4)(2)], is chemically and topologically identical to the structural unit in bermanite, Mn2+(H2O)(4)[Mn-2(3+)(OH)(2)(PO4)(2)]. However, the stacking of these sheet-like units differs between the two minerals; the sheets in ercitite link through corner-sharing of [7]-coordinated Na polyhedra that share edges with the polyhedra of the structural units, whereas the sheets in bermanite link by sharing polyhedron corners with interstitial {Mn2+O2(H2O)(4)} octahedra.