Biagioni, Cristian; Capalbo, Carmen; Lezzerini, Marco; Pasero, Marco
European Journal of Mineralogy Volume 26 Number 1 (2014), p. 171 - 178
published: Feb 1, 2014
ArtNo. ESP147052601015, Price: 29.00 €
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 BaMn4+ 6Fe3+ 2O16. 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%): TiO2 3.06, MnO2 58.83, Mn2O3 8.64, Fe2O3 9.76, Al2O3 0.79, ZnO 0.03, SrO 2.77, BaO 14.63, Na2O 0.09, K2O 0.05, sum 98.65. The structural formula, based on 16 oxygen atoms and 8 octahedral cations per formula unit, is (Ba0.793Sr0.222Na0.024K0.009)Σ=1.048(Mn4+ 5.624Ti0.318Fe3+ 1.016Mn3+ 0.910Al0.129Zn0.003)Σ=8.000O16. 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) Å, β 90.713(2)°, V 563.56(2) Å3, space group P2/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 Å 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–H2O–Ba–H2O within the tunnels.