THE NEW SCANDIUM-BEARING VARIETY OF TUSIONITE FROM THE EAST PAMIRS (TADJIKISTAN)
The new Sc-bearing variety of tusionite has been found in the East Pamirs, in the near-miarolic complex of the Dorozhny granite pegmatite in the Kukurt pegmatite field (the bank of the Kukurt River, Gorno- Badakhshansky autonomous region, Tadjikistan). Miarolitic pegmatites are related to alpine leucocratic granites of Shatputsky intrusive complex and occur in rocks of Sarydzhilginskaya suite (PR3?) metamorphosed under amphibolite and epidote-amphibolite facies. Tusionite occurs there 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), boron-reach gadolinite-(Y), ixiolite, manganocolumbite, pyrochlore, chernovite-(Y), cassiterite, varlamoffite, lepidolite, Hf-reach zircon (up to 17.0 wt % HfO2). Tusionite forms thin lamellae (0.05-0.25 mm) and fan-like aggregates. It is yellow, transparent with vitreous luster. Its pleochro- ism is weak, O (orange-yellow) > E (light yellow). Optically uniaxial, negative, ω = 1.870(5), ω = = 1.760(3). The Raman spectra is reported for tusionite for the first time: with excitation 532 nm, wavenumbers of main bands: 228, 305, 379, 466, 661, 733, 750, 942, 1218, 1458 cm к X-ray powder diffraction pattern is close to tusionite from the Southwest Pamirs. Hexagonal, unit cell parameters: a = = 4.772 (3), c = 15.28(3) A. Chemical composition (electron microprobe, wt %, average of 14 analyses): Ta2O5 0.06; SnO2 50.28; TiO2 0.02; Sc2O3 1.02 (0.62-1.53); CaO 0.03; FeO 0.21; MnO 24.06; B2O3 24.51 (calc. 23.90); total 99.62. Substitution scheme involving Sc is not unambiguous. Substitution of Sn+4 for Sc+3 with replacement of part of Mn+2 for Mn+3 is possible, however Sc is in negative correlation with not only Sn but also with (Fe + Mn) that can be a result of Sc location in both octahedral positions: with Sn+4 and Mn+2