The arrojadite-dickinsonite series, KNa 4 Ca(Fe,Mn) (super 2+) 14 Al(OH) 2 (PO 4 ) 12 ; crystal structure and crystal chemistry

The complex crystal structure of the arrojadite-dickinsonite, KNa4Ca(Fe,Mn)f: Al(OH,F)z(P04)12, series was studied by single-crystal X-ray diffraction techniques. Three compositions were studied and one, dickinsonite from Branchville, Connecticut, is summarized here. The others are arrojadites from the Nancy Mine, North Groton, New Hampshire, and the Nickel Plate Mine, Keystone, South Dakota. Dickinsonite is monoclinic, a = 24.940(6), b = 10.131(4), c = 16.722(2)A, f3 = 105.60(2)°, space group A2ja, Z = 4. R = 0.078 for 7740 measured intensities. Of the 49 nonequivalent atoms in the asymmetric unit, fifteen are larger cations, whose coordination polyhedra include six symmetry-independent octahedra, one tetrahedron, one square pyramid, one seven-coordinated polyhedron, two distorted cubes, one non-cubic polyhedron of order eight, two of ten, and one of twelve-coordination. The arrojadite-dickinsonite structure type is related to that of wyllieite, Na2Fe~+Al(P04h, as seen from a cell with X2= 1/4 - Xl + Z.. Y2= 1/4 + Y.. Z2= - x.. projected down Z2,where (x.. Y.. ZI) are parameters for the reduced cell used in the refinement. Eliminating disordered Ca in dickinsonite gives the same ratio L(M + X): L(P) = 5: 3 as for wyllieite. Average bond distances in dickinsonite are