Garmite, CsLiMg2(Si4O10)F2, a New Mica-Group Mineral from “Quartz Blocks” of the Darai-Pioz Alkaline Massif, Tajikistan

Abstract

A new trioctahedral mica garmite, CsLiMg₂(Si₄O₁₀)F₂, a Cs analogue of tainiolite, was found in an substantially quartz rock in the Darai-Pioz alkaline massif, Rasht district (formerly Garm district), Central Tajikistan, and was named after the town Garm, an administrative center of the Rasht district. It is closely associated with Mn-bearing pectolite, aegirine, Sr-rich fluorite, quartz, datolite, and polylithionite. Garmite forms lamellae up to 0.2 mm across and up to 0.02 mm thick. The mineral is transparent, colorless, with perfect, mica-like cleavage on (001). The Moh’s hardness is 2.5; micro-indentation hardness VHN₂₀ = 90 kg/mm². D_meas = 3.34(2) and D_calc = 3.336 g/cm³. Garmite is optically biaxial (–), 2V_meas = –10(5)°, α = 1.582(2), β = 1.601(2), γ = 1.602(2). The chemical composition (electron microprobe, Li and H by SIMS, wt %): SiO₂ 47.39, Al₂O₃ 0.71, TiO₂ 0.71, Nb₂O₅ 0.12, FeO 2.12, MnO 0.85, MgO 9.01, ZnO 2.23, K₂O 0.16, Cs₂O 26.98, Li₂O 3.57, H₂O 0.08, F 7.23, –O=F₂ 3.04, total 99.90. The empirical formula calculated on the basis of 12 (O + F) apfu is (Cs_0.95K_0.02)_Σ0.97Li_1.21(Mg_1.37Zn_0.16Fe_0.15Al_0.07Mn_0.06Ti_0.04)_Σ1.85Si_3.99O_10.04(F_1.92OH_0.04)_Σ1.96. Garmite is monoclinic, space group C2/m, C2 or Cm; polytype 1M. The unit-cell parameters are a = 5.234(2), b = 9.042(4), c = 10.780(4) Å, β = 99.73(4)°; V = 502.8(6) ų; Z = 2. The strongest reflections of the powder X‑ray diffraction pattern (d Å, I, hkl) are 4.48, 35, 110; 3.70, 70, –112; 3.45, 44, 022; 2.608, 70, –201, 130; 2.580, 100, 200, –131; 2.241, 45, 220; 2.187, 80, –133. The holotype specimen is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow catalogue no. 95894.