SAMFOWLERITE, A NEW CA MN ZN BERYLLOSILICATE MINERAL FROM FRANKLIN, NEW-JERSEY - ITS CHARACTERIZATION AND CRYSTAL-STRUCTURE

Samfowlerite, ideally Ca14Mn3Zn2(Zn,Be)2Be6(SiO4)6(Si2O7)4(OH,F)6, is monoclinic, P2(1)/c, with a 9.068(2), b 17.992(2), c 14.586(2) angstro m, beta 104.86(1)-degrees, and Z = 2. It occurs as colorless 0.05-mm-d iameter crystals in andradite-lined vugs in franklinite - willemite or e from Franklin, Sussex County, New Jersey. Electron- and ion-micropro be analyses yielded CaO 34.1, MgO 0.2, MnO 9.3, ZnO 9.5, BeO 5.6, SiO2 36.9, F 1.0, H2O (by difference) 3.8, less O=F 0.4, total 100 wt%. Op tically, it is biaxial negative, 2V 29.0(1)-degrees, with alpha 1.674( 3), beta 1.680(3), and gamma 1.681(3). The measured and calculated den sities are 3.28 +/- 0.05 and 3.31 g/cm3, respectively. The strongest p owder X-ray-diffraction lines are [d in angstrom (I)(hkl)]: 2.863(100) (321BAR,053), 2,653(50)(251), 2.388(50)(334BAR,172BAR), 2.771(40)(125B AR), 2.272(30)(172,234,402BAR,155BAR), 1.832(30)(374BAR), 1.860(20)(32 7BAR,406BAR,066), and 1.803(20)(381BAR,0.10.0,382BAR). The crystal str ucture has been solved and refined to an unweighted residual of 0.043 for 2495 observed reflections. Although samfowlerite is, strictly spea king, a mixed-anion (SiO4 and Si2O7) silicate, the structure may be vi ewed as being composed of layers of vertex-sharing TO4 tetrahedra (T = Si,Be,Zn) alternating with layers of vertex- and edge-sharing CaO8 bi capped trigonal prisms and MnO6 octahedra, the layers being parallel t o (102). The TO4 groups and CaO8-MnO6 groups share vertices with one a nother across the layer boundaries to form the three-dimensional struc ture. Within the layers of tetrahedra, the TO4 groups form a network o f 4-, 5-, and 8-membered rings. The apparent substitution of Be for Zn in the ratio 5/8 to 3/8 on one of the T sites suggests that the space group P2(1)/c is only that of an average structure, and that the true structure is of lower symmetry, with ordering of Zn and Be on separat e sites.