Abstract. 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 {101¯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 Å
(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 P6/mmm, with a=7.2952(5) Å, c=3.7711(3) Å, V=173.81(2) Å3, and Z=1. The
crystal structure was solved and refined to a reliability factor of
R1[F2>4σ(F2)]=0.025 (wR2=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 Å) 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.