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

• Published in: Geology of ore deposits 2019-12, Vol.61 (8), p.809-817
• Main Authors: Mirakov, M. A., Pautov, L. A., Shodibekov, M. A., Plechov, P. Yu, Karpenko, V. Yu
• Format: Article
• Language: English
• Subjects: Amphibolite facies; Amphibolites; Bearing; Boron oxides; Cassiterite; Chemical composition; Diffraction patterns; Earth and Environmental Science; Earth Sciences; Electron microprobe; Electron probes; Feldspars; Fluorapatite; Fluorite; Hafnium oxide; Lamellae; Lasers; Magnetite; Manganese; Mineral Resources; Minerals and Mineral Parageneses; New varieties; Organic chemistry; Pegmatite; Position (location); Raman spectroscopy; Rivers; Scandium; Scandium oxides; Substitutes; Tantalum; Tantalum oxides; Tin dioxide; Titanite; Titanium dioxide; Unit cell; X-ray diffraction; Zircon
• ISSN: 1075-7015; 1555-6476
• DOI: 10.1134/S1075701519080087

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 (PR 3 ?) 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 % Sc 2 O 3 ) 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 % HfO 2 ). 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) Ta 2 O 5 ; 50.28 (49.43–51.03) SnO 2 ; 0.02 (0.00–0.19) TiO 2 ; 1.02 (0.62–1.53) Sc 2 O 3 ; 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) B 2 O 3 ; 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 .