Wumuite (KAl0.33W2.67O9) – a new mineral with an HTB-type structure from the Panzhihua–Xichang region in China

Wumuite, ideally KAl0.33W2.67O9 , is a new mineral species found in the Neoproterozoic Sinian light-weathered biotite–quartz monzonite in the southern part of the Panzhihua–Xichang region (Nanyang village: 26 ∘ 46 ′ 8.21 ′′ N, 101 ∘ 27 ′ 13.86 ′′ E), China. It is associated with quartz, orthoclase, albite, biotite, hornblende, kaolinite, ilmenite, goethite, hematite, zircon, zoisite, tourmaline, monazite-(Ce), allanite-(Ce), scheelite, tellurite, tewite, and an unidentified, potentially new mineral corresponding to WO3 . Wumuite occurs as light green hexagonal tabular crystals, is up to 0.3 mm in diameter, and has a vitreous to adamantine luster and a white streak; i.e., it is transparent. The mineral is brittle with good cleavage parallel to { 10 1 ¯ 0 } and {0001} . It has a Mohs hardness of about 5–6 and a calculated density of 6.52 g cm−3 . Electron microprobe analyses yielded the following (in wt % – average of 10 spot analyses of one sample): K2O 5.55, WO3 91.16, TeO2 0.59, and Al2O3 2.52, with a total of 99.82. The empirical formula for wumuite calculated on the basis of Oapfu=9 is K0.80(W2.68Al0.34Te0.03)∑3.05O9 , ideally K(W2.67Al0.33)∑3O9 or KAl0.33W2.67O9 . The strongest four diffraction lines [ d A ( I ) (hkl)] are 6.261(36)(010), 3.727(30)(001), 3.161(100)(020), and 2.413(40)(021). Wumuite is hexagonal, in space group P 6/mmm, with a=7.2952(5) A, c=3.7711(3) A, V=173.81(2) A 3 , and Z=1 . The crystal structure was solved and refined to a reliability factor of R1 [ F 2 > 4 σ ( F 2 ) ] = 0.025 (wR 2=0.072 ) based on 164 unique reflections (777 measured reflections, R(int)=0.011 ). Wumuite has a hexagonal tungsten bronze (HTB)-type structure. The layers of corner-sharing [(W,Al)O]6 octahedra, with the layers oriented normal to the short (3.7713 A) c repeat and along with the W–O–W links, connect to form a hexagonal ring channel (tunnel). K is distributed in the hexagonal channel. An associated new mineral, tewite, which was discovered in the same area, also has a new tungsten bronze (TB)-type-related structure and has a genetic connection with wumuite in both back-scattered electron (BSE) images and synthetic experiments. The formation of wumuite is likely related to the nearby quartz-vein-type Au mineralization. The mineral was formed by a metasomatic reaction between W-rich hydrothermal fluids and the potassium feldspar in the monzonite.