ZINCOHOGBOMITE, A NEW MINERAL FROM EASTERN AEGEAN METABAUXITES

The new mineral zincohogbomite, ideally Zn2-2xTixAl4O8. the Zn-dominan t analogue of hogbomite, occurs as an accessory phase in diaspore- or corundum-bearing metabauxites from Samos Island (Greece) and the Mende res Massif (Turkey). It is associated with diaspore or corundum as wel l as chloritoid, muscovite, paragonite, margarite, titanian hematite, magnetite, ilmenite and gahnite. Zincohogbomite was formed at the expe nse of gahnite during prograde metamorphism. Single crystals of zincoh ogbomite initially grown on gahnite {111} faces attain diameters of 0. 3 mm. The colour of the mineral is deep brown to black. In thin sectio ns, zincohogbomite is transparent with strong pleochroism (O = dark br own, E = brown). It is uniaxial negative with n(o) = 1.878(2) and n(e) = 1.832(2) (589 nm), non-fluorescent. Micro-indentation hardness is V HN100 = 1200 to 1400 kg/mm(2) corresponding to about 7 on the Mohs-sca le. The calculated density is 4.36 g/cm(3). It is not soluble in HCl, H2SO4 or HNO3. Zincohogbomite is hexagonal, most probably P6(3)mc, wit h a = 5.708(4) Angstrom, c = 18.31(2) Angstrom, V = 516.7(7) Angstrom( 3), Z = 4. The strongest powder diffraction lines are (in Angstrom, in tensities estimated): 2.42 (100); 2.60 (80), 1.425 (80), 1.470 (70), 1 .592 (60). Crystal forms are {10 (1) over bar 4} and {0001}, twinning was not observed. The mean of 10 microprobe analyses of zincohogbomite from the type locality at Kerketefs Mountain/Western Samos is (in wt. %): TiO2 5.71, Al2O3 59.90, Cr2O3 0.12, FeO 13.85, MgO 0.48, CoO 0.20, NiO 1.01, ZnO 18.87, leading to the formula (based on 8 oxygens) n0.7 9Fe0.65Mg0.04Co0.01Ni0.05Cr0.01Ti0.24Al3.98O8. The maximum ZnO value e ncountered is 23.39 wt.%, corresponding to 0.97 Zn per formula unit. T he mineral and its name have been approved by the IMA Commission on Ne w Minerals and Mineral Names (No. 94-016).