The new scandium-bearing variety of tusionite from the East Pamirs (Tadjikistan)

Mirakov M. A., Pautov L. A., Shodibekov M. A., Plechov P. Yu., Karpenko V. Yu.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2018. V. 147. N 4. P. 84-96

https://doi.org/10.30695/zrmo/2018.1474.06

Full text is available on eLIBRARY.RU

Language: Russian

Abstract

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 pleochroism is weak, O (orange-yellow) ≥ E (light yellow). Optically uniaxial, negative, = 1.870(5), = 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^–1. 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) Å. 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.

Key words: tusionite, nordenskiöldine, boron, tin, scandium, miarolitic granitic pegmatites, East Pamirs, Kukurt.