Abstract

The new mineral zincohogbomite, ideally Zn (sub 2-2x) Ti x Al 4O8 , the Zn-dominant analogue of hogbomite, occurs as an accessory phase in diaspore- or corundum-bearing metabauxites from Samos Island (Greece) and the Menderes Massif (Turkey). It is associated with diaspore or corundum as well as chloritoid, muscovite, paragonite, margarite, titanian hematite, magnetite, ilmenite and gahnite. Zincohogbomite was formed at the expense of gahnite during prograde metamorphism. Single crystals of zincohogbomite initially grown on gahnite {111} faces attain diameters of 0.3 mm. The colour of the mineral is deep brown to black. In thin sections, zincohogbomite is transparent with strong pleochroism (O = dark brown, 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 VHN 100 = 1200 to 1400 kg/mm 2 corresponding to about 7 on the Mohs-scale. The calculated density is 4.36 g/cm 3 . It is not soluble in HCl, H 2 SO 4 or HNO 3 . Zincohogbomite is hexagonal, most probably P6 3 mc, with a = 5.708(4) Aa, c = 18.31(2) Aa, V = 516.7(7) Aa 3 , Z = 4. The strongest powder diffraction lines are (in Aa, intensities estimated): 2.42 (100), 2.60 (80), 1.425 (80), 1.470 (70), 1.592 (60). Crystal forms are {1014} 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.%): TiO 2 5.71, Al 2 O 3 59.90, Cr 2 O 3 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) Zn (sub 0.79) Fe (sub 0.65) Mg (sub 0.04) Co (sub 0.01) Ni (sub 0.05) Cr (sub 0.01) Ti (sub 0.24) Al (sub 3.98O8) . The maximum ZnO value encountered is 23.39 wt.%, corresponding to 0.97 Zn per formula unit. The mineral and its name have been approved by the IMA Commission on New Minerals and Mineral Names (No. 94-016).

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