Pittongite, a new tungstate with a mixed-layer, pyrochlore - Hexagonal tungsten bronze structure, from Victoria, Australia

Pittongite, (Na,H₂O)_x[(W,Fe)(O,OH)₃], x ≈ 0.7 (IMA 2005-034), is a new mineral species from a tungsten deposit at Pittong, near Ballarat, in Victoria, Australia. The name recalls the discovery site, in turn derived from an Australian aboriginal word for father. The mineral occurs as glistening, creamy yellow encrustations of very thin (0.3-0.5 μm) platy crystals on etched blades of ferberite up to 4 cm long, enclosed in massive white reef quartz. It has formed by alteration of ferberite in a supergene environment in the presence of oxidizing, acidic solutions containing Na. The ferberite occurs in several hydrothermal quartz reefs, along with small amounts of bismuth, gold, bismuth_inite, and other secondary mngstates and molybdates such as koechlinite and elsmoreite. Pittongite is hexagonal, space group P6m2, with a 7.286(1), c 50.49(1) Å, V 2321.2 ų, refined from synchrotron X-ray powder-diffraction data. The density of the mineral could not be measured, but D_calc is 5.715 g/cm³. Pittongite has a cream streak, estimated Mohs hardness of 2-3, and is non-fluorescent. Owing to the nature of the crystals, only limited optical data could be obtained; pittongite has an average index of refraction of 2.085, and is uniaxial negative. The strongest seven lines in the X-ray powder-diffraction parem [d_obs in A(I)(hkl)] are: 3.153(100)(0016,201), 3.111(91)(202,203), 1.823(76)(220), 1.578(64)(2216), 3.306(62)(116,1013), 2.450(59)(2013) and 5.956(52)(102,103). The average result of seven spot electron-microprobe analyses gave (wt%) Na₂O 2.97, K₂O 0.06, CaO 0.39, Fe₂O₃5.66, A1₂O₃0.51 and WO₃ 84.15; H₂O determined by CHN analyzer 4.73%, for a total of 98.47 wt%. The empirical formula (on the basis of W + Fe³⁺ + Al = 1) is: (Na_0.22H₂ O_0.44Ca_0.02 K_0.003)_Σ0.683(W_0.82 Fe³⁺ _0.16Al_0.02)_Σ1.00 [O_2.70 (OH)_0.30]_Σ3.00. The crystal structure of pittongite is closely related to that of pyrochlore and can be derived from it by periodic, unit-cell-scale twinning parallel to (111)_pcl. This gives a stacking along cof pyrochlore blocks of two different widths, 6 and 12 Å, separated by pairs of hexagonal tungsten bronze (HTB) layers. TEM studies showed that disorder in the stacking sequence is common. The tungsten, together with minor iron and aluminum, occupies the octahedral sites in both the HTB and pyrochlore blocks. The sodium atoms and H₂O molecules occupy the A-atom positions of the pyrochlore blocks. Pittongite has structural and chemical similarities to phyllotungstite, CaFe³⁺ ₃ H(WO₄)₆·₁₀H₂O.