A New Scandium-Bearing Variety of Tusionite from the Eastern Pamirs (Tajikistan)

A new Sc-bearing variety of tusionite has been found in the Eastern Pamirs, in the near-miarolitic Dorozhny granite pegmatite complex at the Kukurt pegmatite field (left bank of the Kukurt River, 45 km east of the settlement of Murghab, Gorno-Badakhshan Autonomous Oblast, Tajikistan). Miarolitic pegmatites are related to the alpine leucocratic granites of the Shatput intrusive complex and occur in rocks of the Sarydzhilga Formation (PR₃?) metamorphosed under amphibolite and epidote–amphibolite facies. Tusionite occurs in association with quartz, K-feldspar, albite, elbaite, Sc-bearing spessartine (up to 0.2 wt % Sc₂O₃) and accessory fluorapatite, Y-bearing fluorite, Sn-bearing titanite, magnetite, polycrase-(Y), B-rich gadolinite-(Y), ixiolite, manganocolumbite, pyrochlore, chernovite-(Y), cassiterite, varlamoffite, lepidolite, and Hf-rich zircon (up to 17.0 wt % HfO₂). Tusionite forms thin lamellae (0.05–0.25 mm) and fanlike aggregates. It is yellow and transparent with a vitreous luster. It has weak pleochroism, N_o (orange–yellow) > N_e (light yellow). It is a uniaxial negative mineral, n_o = 1.870(5), n_e = 1.760(3). The Raman spectrum has been reported for tusionite for the first time (laser excitation 532 nm); main bands: 228, 305, 379, 466, 661, 733, 750, 942, 1218, and 1458 cm^–1. Its X-ray diffraction pattern is similar to tusionite from the Southwestern Pamirs. Hexagonal unit cell parameters: a = 4.772(3), c = 15.28(3) Å. Chemical composition (electron microprobe, wt %, average of 14 analyses): 0.06 (0.00–0.31) Ta₂O₅; 50.28 (49.43–51.03) SnO₂; 0.02 (0.00–0.19) TiO₂; 1.02 (0.62–1.53) Sc₂O₃; 0.03 (0.00–0.19) CaO; 0.21 (0.16–0.46) FeO; 24.06 (23.59–24.34) MnO; 24.51 (calc. 23.90) B₂O₃; total 99.62 (99.08–100.79). The substitution mechanism involving Sc is not clear. Substitution of Sc⁺³ for Sn⁺⁴ with partial replacement of Mn⁺² by Mn⁺³ is possible; however, Sc negatively correlates not only with Sn, but also with (Fe + Mn), which may result from the location of Sc in both octahedral positions with Sn⁺⁴ and Mn⁺².