Tewite: A K–Te–W new mineral species with a modified tungsten-bronze type structure, from the Panzhihua-Xichang region, southwest China

Tewite, with end-member formula (K_1.5□_0.5)_∑2(Te⁴⁺_1.25W_0.25□_0.5)_∑2W₅O₁₉ or K_1.5(Te_1.25W_0.25)_∑1.5W₅O₁₉, is a new mineral species (IMA2014-053), discovered in the vicinity of Nanyang village, Huaping County, located in the south of the Panzhihua–Xichang region, southwestern China. Tewite occurs in the Neoproterozoic Sinian light-weathered biotite–quartz monzonite, near the contact zone with gabbro. The associated minerals are alkali feldspar, biotite, clinoamphibole, ilmenite, zircon, zoisite, tourmaline, monazite-(Ce), allanite-(Ce), scheelite, tellurite and the new mineral wumuite (KAl_0.33W_2.67O₉, IMA2017-067a), in addition to an unidentified, potentially new mineral corresponding to WO₃. Tewite forms platy crystals that range from 0.08 × 0.1 × 0.1 to 0.1 × 0.2 × 0.5 mm in size. The crystals are greenish yellow, with a light-yellow to white streak, translucent to transparent, and with adamantine luster. Mohs hardness is 3½–4, and the tenacity is brittle. It has perfect {100}, {001}, and {010} cleavages, and the calculated density is 6.903 g/cm³. The mineral is biaxial positive with 2V = 70°. The Gladstone–Dale relationship predicts an average index of refraction of 2.04. The chemical composition determined by wavelength-dispersive electron-microprobe analysis is (wt%, average of n = 16): Na₂O 0.13, K₂O 5.08, WO₃ 83.34, TeO₂ 11.32, total 99.86. The empirical formula, based on 19 O, is (K_1.61Na_0.06□_0.33)_∑2.00(Te_1.06W_0.35□_0.59)_∑2.00W₅O₁₉. The eight strongest X-ray powder-diffraction lines [d in Å (I) (hkl)] are: 6.486 (50) (040), 3.8333 (100) (001), 3.6212 (30) (200), 3.1983 (65) (080), 2.4541 (50) (081), 1.8442 (30) (390), 1.6365 (25) (401), and 1.5743 (55) (480). Refinement of the crystal structure (R₁ = 0.075) gave the following data: orthorhombic, space group Pban, a = 7.2585(4), b = 25.8099(15), c = 3.8177(2) Å, Z = 2. Tewite has a new tungsten-bronze (TB)-type derivative structure. Distorted TeO₆ octahedra break TB slabs into ribbons which are displaced by ½ a relative to their neighbors, while K partly occupies two mutually exclusive sites in hexagonal channels || c. Short-range order and displacement of K ions are likely responsible for an observed incommensurate modulation of the average structure indicated by weak satellite reflections along c*, which were not considered in the structure refinement. The formation of the mineral may be related to the nearby quartz-vein-type Au mineralization; the mineral is formed by the metasomatic reaction of a high-temperature fluid rich in W and Te with the potassium feldspar in the monzonite. Tewite is a mineralogically unique K–Te–W oxide. The mineral is classified as a composite oxide, New Dana Classification class 07.04.03, of stoichiometry A₂B₂C₅O₁₉.