THE CRYSTAL-STRUCTURE OF GORDAITE NAZN4(SO4)(OH)(6)CL-CENTER-DOT-6H(2)O

The new mineral gordaite [14] from the Sierra Gorda, Chile, has chemic al composition NaZn4(SO4)(OH)(6)Cl . 6 H2O and crystallizes trigonal w ith space group P (3) over bar. The lattice parameters are a = 8.3556( 3) Angstrom and c = 13.025(1) Angstrom. The structure was determined u sing direct methods and difference Fourier maps. Structure refinement yielded a final R(F)-value of 8.19%. The structure is characterized by brucite-like sheets of composition [Zn-6(OH)(12)O-2](4-) which are fo rmed by edge-shared Zn(OH)(6) octahedra. One seventh of the octahedral sites in the sheet is vacant. Above and below these sites [Zn(OH)(3)C l](2-) tetrahedra are located sharing their basal hydroxides with six octahedra. Sulfate groups are connected to the sheets on both sides by corner sharing. The composition for this modified layer is uare(oct)( OH)(6)Zn-2(tet)(OH)(6)Cl-2(SO4)(2)](2-). To compensate for the negativ e charge Na+ ions which are octahedrally coordinated by water molecule s are incorporated between the layers. A complex hydrogen bonding syst em further links subsequent layers. The structure of gordaite is close ly related to that of the synthetic material Zn-4(OH)(6)SO4 . m H2O wi th m = 3, 5 described by Bear et al. in 1986 and presumably the same m aterial as the one investigated by MacEwan, Cruz Cumplido and Cane Rui z (1966) who slate an average chemical composition of Zn-12(OH)(15)(SO 4)(3)Cl-3 . 5 H2O. A comparison to these materials as well as to Zn-5( OH)(8)Cl-2 . H2O described by Nowacki and Silverman in [12] is drawn.