Ferrihollandite, BaMn4+

Abstract

The recent revisitation of the nomenclature of tunnel oxides belonging to the hollandite supergroup led to the definition of a new end-member, ferrihollandite, ideally BaMn 4+ 6 Fe 3+ 2 O 16 . In particular, the type hollandite from Kajlidongri, Jhabua District, Madhya Pradesh, India, proved to be ferrihollandite. A new occurrence of ferrihollandite is here described from the manganese hydrothermal ores from Vagli, Apuan Alps, Tuscany, Italy, on the basis of chemical and X-ray diffraction data. Electron-microprobe data point to the following composition (in wt%): TiO 2 3.06, MnO 2 58.83, Mn 2 O 3 8.64, Fe 2 O 3 9.76, Al 2 O 3 0.79, ZnO 0.03, SrO 2.77, BaO 14.63, Na 2 O 0.09, K 2 O 0.05, sum 98.65. The structural formula, based on 16 oxygen atoms and 8 octahedral cations per formula unit, is (Ba 0.793 Sr 0.222 Na 0.024 K 0.009 ) ∑=1.048 (Mn 4+ 5.624 Ti 0.318 Fe 3+ 1.016 Mn 3+ 0.910 Al 0.129 Zn 0.003 ) ∑=8.000 O 16 . The TG-DSC data show three main endothermic processes, related to the loss of water and the release of oxygen upon heating. The total mass loss is ~ 9.7 wt% in the temperature range between 25 and 1200 °C; the mass loss related to the release of water is ~ 1.5 wt%. Single-crystal X-ray diffraction study gives a monoclinic unit cell, a 10.0001(7), b 5.7465(4), c 9.8076(8) A, β 90.713(2)°, V 563.56(2) A 3 , space group P 2/ n . The crystal structure has been solved and refined on the basis of 1675 observed reflections, with a final R 1 = 0.037. The basic structural features of ferrihollandite are the same as the 2.9 A hollandite-type compounds, but with long-range ordering of the tunnel cations. The occurrence of superstructure reflections could be related to the ordered sequence Ba–H 2 O–Ba–H 2 O within the tunnels.