Gorbunovite CsLi₂(Ti,Fe⁺³)(Si₄O₁₀)(F,OH,O)₂ – a new mica supergroup mineral from the Darai-Pioz alkaline massif, Tajikistan

Abstract Gorbunovite, CsLi 2 (Ti,Fe +3 )(Si 4 O 10 )(F,OH,O) 2 , a new caesium trioctahedral mica, was discovered in the Darai-Pioz alkaline massif, Tajikistan. The mineral is named after the Russian chemist, leader of the Tajik-Pamir Expedition, Academician Nikolai Petrovich Gorbunov (1892–1938). Gorbunovite occurs as lamellar grains or flakes from 2 μm to 0.2 mm in a quartz–pectolite aggregate and is associated with quartz, fluorite, pectolite, baratovite, aegirine, leucosphenite, neptunite, reedmergnerite, orlovite, sokolovaite, mendeleevite-(Ce), odigitriaite, pekovite, zeravshanite, kirchhoffite and garmite. The mineral is colourless, transparent, with vitreous lustre. Mohs hardness is 2½. D meas. is 3.28 (2), D calc is 3.302 g/cm 3 . Gorbunovite is optically biaxial (–), α = 1.609(2), β = 1.621(2), γ = 1.623(2), 2V meas. 30(5) and 2V calc. 44. Gorbunovite is monoclinic, space group C 2/ m, C 2 or Cm (polytype 1 M ), a = 5.236(2), b = 9.054(4), c = 10.767(4) Å, β = 99.61(4)°, V = 503.3(6) Å 3 and Z = 2. The strongest lines in the powder X-ray diffraction pattern [ d , Å, ( I )] are: 4.49 (25), 3.94 (20), 3.69 (46), 3.57 (23), 3.45(34), 2.991 (42), 2.608 (77), 2.581 (100), 2.240 (33), 2.188 (62), 2.020 (24), 1.722 (27) and 1.511 (23). Chemical composition (microprobe analysis; H 2 O, Li 2 O – SIMS) is: SiO 2 47.44, TiO 2 9.40, Al 2 O 3 0.66, MgO 0.63, Fe 2 O 3 3.64, ZnO 1.01, K 2 O 0.39, Cs 2 O 26.64, Li 2 O 5.83, H 2 O 0.89, F 4.48, –O=F 1.89, total 99.17. The empirical formula is (Cs 0.96 K 0.04 ) Σ1.00 Li 1.98 (Ti 0.60 Fe +3 0.23 Mg 0.08 Al 0.07 Zn 0.06 ) Σ1.04 Si 4.00 O 10 (F 1.19 OH 0.50 O 0.31 ) Σ2 .