• journal_title：The Canadian Mineralogist
• Contributor：Alexander V. Stepanov ; Galiya K. Bekenova ; Vladimir L. Levin ; Elena Sokolova ; Frank C. Hawthorne ; Elena A. Dobrovol’skaya
• Publisher：Mineralogical Association of Canada
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.50.1.159
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：50
• issue：1
• firstpage：159
• section：Articles

Tarbagataite, (K,□)₂(Ca,Na)(Fe²⁺,Mn)₇Ti₂(Si₄O₁₂)₂O₂(OH)₄(OH,F), is a new Ca- and Fe²⁺-dominant astrophyllite-group mineral discovered in a pegmatite in the Verkhnee Espe deposit, Akjailyautas Mountains, Kazakhstan. Tarbagataite occurs as intimate intergrowths of tarbagataite and astrophyllite flakes. Dimensions of these intergrowths range up to 10 × 3 × 0.2 mm. Flakes are elastic, brown or pale golden brown, with a colorless to very pale yellow streak and a vitreous to pearly lustre. Tarbagataite is opaque in large grains, transparent in thin flakes, with a Mohs hardness of 3, and does not fluoresce under cathode or ultraviolet light. Cleavage is perfect parallel to {001} and moderate parallel to {010}; no parting was observed. Its calculated density is 3.263 g/cm³. The mineral is biaxial positive with α 1.710, β 1.715, γ 1.745 (all ± 0.003, λ 589 nm), 2V_meas. = 37(3)°, 2V_calc. = 45°. It is pleochroic according to the scheme X < Z < Y, where X = yellow brown, Y = orange red, Z = yellow orange. Tarbagataite is triclinic, space group P1̅, a 5.3863(3), b 11.9141(6), c 11.7171(6) Å, α 112.978(2), β 94.641(2), γ 103.189(2)°, V 661.84(9) ų, Z = 1. The strongest lines in the X-ray powder-diffraction pattern [d(Å)(I)(hkl)] are: 3.258(100)(1̅1̅3), 4.095(80) (021), 2.858(80)(014), 2.761(70)(14̅2,1̅3̅1), 3.497(50)(030), 2.560(50)(130,14̅3). 3.735(30)(02̅3), 2.646(30)(2̅11,004), 3.005(20) (013). Chemical analysis by electron microprobe gave Nb₂O₅ 2.98, SnO₂ 1.20, ZrO₂ 0.32, TiO₂ 9.29, SiO₂ 36.11, Al₂O₃ 0.12, ZnO 0.12, FeO 18.71, MnO 15.48, CaO 2.58, MgO 0.83, Cs₂O 0.38, Rb₂O 1.28, K₂O 2.67, Na₂O 1.14, F 0.49, H₂O 3.11, O=F −0.21, sum 96.60 wt.%; H₂O was determined from structure refinement, OH + F = 5 apfu. The empirical formula, calculated on 31 anions (O + F) pfu, is (K_0.76Rb_0.18Na_0.12Cs_0.04□_0.90)_S2(Ca_0.62Na_0.38)_S1(Fe²⁺_3.51Mn_2.94Mg_0.28Zr_0.02 Zn_0.02□_0.23)_S7(Ti_1.57Nb_0.30Sn_0.11 Zr_0.02)_S2(Si_8.09Al_0.03)_S8.12O_30.65H_4.65F_0.35, Z =1. The simplified and endmember formulae are (K, □)₂(Ca,Na)(Fe²⁺,Mn)₇Ti₂ (Si₄O₁₂)₂O₂(OH)₄(OH,F) and (K□)CaFe²⁺₇Ti₂(Si₄O₁₂)₂O₂(OH)₄(OH), respectively. The infrared spectrum of the mineral contains the following absorption bands: 455, 531, 570, 656, 698, 950 with shoulders at 1078 and 1064, 1637, 3656 to 3277 cm⁻¹. The crystal structure of tarbagataite was refined to an R₁ index of 5.82 %. Tarbagataite is isostructural with astrophyllite, ideally K₂NaFe²⁺₇Ti₂(Si₄O₁₂)₂ O₂(OH)₄F. Tarbagataite differs from astrophyllite in the composition of the interstitial A and B sites and the X site: (K,□)₂ (A), (Ca,Na) (B), and (OH,F) (X). Tarbagataite and astrophyllite are related by the substitution ^A□ + ^BCa²⁺ + ^X(OH)⁻ ↔ ^AK⁺ + ^BNa⁺ + ^XF⁻. The name is for the locality where the mineral was discovered: the Verkhnee Espe deposit is located in the northern part of the Tarbagatai mountain range in the Akjailyautas Mountains of Kazakhstan.