Titanium and iron poor zincohogbomite-16H, Zn-14(Al,Fe3+,Ti,Mg)(8)Al24O62(OH)(2), from Nezilovo, Macedonia: occurrence and crystal structure of a newpolysome

The new polysome zincohogbomite-16H, Zn13.03Mg0.49Fe2.123+Ti1.15Al29.21O62( OH)(2), (space group P6(3)mc, Z = 1, a = 5.729(2), c = 37.097(5) Angstrom; d(calc) = 4.52 g/cm(3)) occurring as small plates in pink marbles at Nezilo vo, Macedonia, is a mineral of the hogbomite group. The hexagonal plates ex hibit parallel intergrowth or occur in aggregates up to 2 mm in size. The t ransparent orange mineral shows a pale yellow streak and has adamantine lus ter. Zincohogbomite-16H is optically uniaxial negative (omega(red) =1.850 a nd w(yellow) = 1.845, epsilon = not determined), pleochroic: omega - brown- yellow, epsilon - pale greenish-yellow, Mobs hardness 6-7, neither cleavage nor parting were observed. Zincohogbomite-16H is a metamorphic reaction pr oduct in the high grade metamorphic assemblage of the marbles at Nezilovo ( Macedonia): it coexists with gahnite and zinc-rich phlogopite. The crystal structure of zincohogbomite-16H was solved and refined from X-r ay single-crystal data to R = 2.42% with 79 Variables and 518 observations > 4 sigma(F). The new polysome has a close packed oxygen array composed of 16 oxygen layers stacked along the c axis in the sequence ABCABCBACBACBABC. The oxygen array consists of cubic close packed subunits (complete gahnite -spinel blocks of six layers) with intercalated units of the hexagonal clos e packed type. Out of the 15 cation sites seven are tetrahedrally (T) and e ight are octahedrally (M) coordinated; taking into account their multiplici ties the formula T14M32O62(OH)(2) can be derived. The tetrahedra are mainly occupied by Zn, whereas Al, Fe, and Ti have octahedral coordination.