The crystal structure of gordaite NaZn4(SO4)(OH)6Cl · 6H2O

Abstract

Abstract The new mineral gordaite [14] from the Sierra Gorda, Chile, has chemical composition NaZn4(SO4)(OH)6Cl · 6H2O and crystallizes trigonal with space group P[unk]. The lattice parameters are a = 8.3556(3) Å and c = 13.025(1) Å. The structure was determined using 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 [Zn6(OH)12O2]4− which are formed by edgeshared Zn(OH)6 octahedra. One seventh of the octahedral sites in the sheet is vacant. Above and below these sites [Zn(OH)3Cl]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 [Zn6 oct□oct(OH)6Zn2 tet(OH)6Cl2(SO4)2]2−. To compensate for the negative charge Na+ ions which are octahedrally coordinated by water molecules are incorporated between the layers. A complex hydrogen bonding system further links subsequent layers. The structure of gordaite is closely related to that of the synthetic material Zn4(OH)6SO4 · m H2O with m = 3,5 described by Bear et al. in 1986 and presumably the same material as the one investigated by MacEwan, Cruz Cumplido and Cano Ruiz (1966) who state an average chemical composition of Zn12(OH)15(SO4)3Cl3 · 5H2O. A comparison to these materials as well as to Zn5(OH)8Cl2 · H2O described by Nowacki and Silverman in [12] is drawn.