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Titanium and iron poor zincohoegbomite-16H, Zn14(Al, Fe3+, Ti, Mg)8Al24O62(OH)2, from Nezilovo, Macedonia: occurrence and crystal structure of a new polysome

The new polysome zincohoegbomite-16H, Zn14(Al, Fe3+, Ti, Mg)8Al24O62(OH)2, (space group P63mc, Z=1, a=5.729(2), c=37.097(5) A ; dcalc=4.52 g/cm3) occuring as smallplates in pink marbles at Nežilovo, Macedonia, is a mineral of the hoegbomite group. The hexagonal plates exhibit parallel intergrowth or occur in aggregates up to 2mm in size. The transparent orange mineral shows a pale yellow streak and has adamantine luster. Zincohoegbomite-16H is optically uniaxial negative (ored=1.850 and oyellow=1.845, e=not determined), pleochroitic: o - brown-yellow, e - pale greenish-yellow, Mohs hardness 6-7, neither cleavage nor parthing were observed. Zincohoegbomite-16H is a metamorphic reaction product in the high grade mrtsamorphic assemblage of the marbles at Nežilovo (Macedonia): it coexists with gahnite and zinc-rich phlogopite. The crystal structure of zincohoegbomite-16H was solved and refined from X-ray single-crystaldata to R=2.42% with 79 variables and 518 observations >4sigma(F). The new poysome 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 close packed type. Out of 15 cation sites seven are tetrahedrally (T) and eight are octahedrally (M) coordinated ; taking into account their multiplicities the formula T14M32O62(OH)2 can be derived. The tetrahedra are mainly occupied by Zn, whereas Al, Fe, and Ti have octahedral coordination.

zincohoegbomite-16H; crystal chemistry; X-ray single-crystal structure refinement; Nežilovo; Macedonia

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