The crystal structure of ganophyllite; monoclinic subcell.

Abstract

The crystal structure of a monoclinic subcell (a' 5.534, b' 13.565, c' 25.09 A, beta ' 93.96o, I2/a) of ganophyllite has been determined and refined to R = 0.078. The structure of the true cell (a = 3a' = 16.60, b = 2b' = 27.13, c = 2c' = 50.18 A, beta = beta ' = 93.96o, A2/a) can be explained by this average subcell structure. The mineral is a modified layer structure as suggested by Smith & Frondel (M.M. 36-893). The octahedral layer is wavy like a sine curve along the b-axis and is sandwiched between two tetrahedral layers, each composed of triple chains. The upper and lower chains sandwiching the octahedral layer are always at the outside of the curvature and the small tetrahedral layer is inside. Thus the layer misfit is elegantly minimized in this structure. The interlayer cation is coordinated by four oxygens or hydroxyls at the edges of the chains and by four water molecules. Ordering of the interlayer cations and water molecules, site preference of Al in the tetrahedra, and resultant slight changes in polyhedra of the subcell structure, explain why ganophyllite has a large cell. The structural formula of ganophyllite determined from X-ray, microprobe and structural analyses is (K,Na,Ca)xMn8(Si,Al)12(O,OH)32(OH)4.nH2O, with x 1-1.5, and n = or > 4; Z = 24 in the true large cell. The average monoclinic subcell structure can also explain the structure of the triclinic polytype recently found by Jefferson (M.A. 79-157).-T.Y.