Abstract

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), 2Vmeas. = 37(3)°, 2Vcalc. = 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) Å3, 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 (K0.76Rb0.18Na0.12Cs0.040.90)S2(Ca0.62Na0.38)S1(Fe2+3.51Mn2.94Mg0.28Zr0.02 Zn0.020.23)S7(Ti1.57Nb0.30Sn0.11 Zr0.02)S2(Si8.09Al0.03)S8.12O30.65H4.65F0.35, 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.

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