Two Astrophyllite-Supergroup Minerals: Bulgakite, A New Mineral From the Darai-Pioz Alkaline Massif, Tajikistan and Revision of the Crystal Structure and Chemical Formula of Nalivkinite

Abstract Bulgakite, ideally Li 2 (Ca,Na)Fe 2+ 7 Ti 2 (Si 4 O 12 ) 2 O 2 (OH) 4 (O,F)(H 2 O) 2 , and nalivkinite, ideally Li 2 NaFe 2+ 7 Ti 2 (Si 4 O 12 ) 2 O 2 (OH) 4 F(H 2 O) 2 , are astrophyllite-supergroup minerals. Bulgakite is a new mineral from the Darai-Pioz alkaline massif in the upper reaches of the Darai-Pioz river, in the area of the joint Turkestansky, Zeravshansky, and Alaisky ridges, Tajikistan. Bulgakite was found in fenitized amphibole–quartz–feldspar rock with brannockite, sogdianite, bafertisite, albite, and titanite. Bulgakite is brownish orange, transparent in thin grains, and has a vitreous luster. Mohs hardness is 3, D meas. = 3.30(2) g/cm 3 , D calc. = 3.326 g/cm 3 . Bulgakite is biaxial (+) with refractive indices (λ = 589 nm) α = 1.695(3), β = 1.711(2), γ = 1.750(3); 2 V meas. = 70(5)°, 2 V calc. = 67°, strong dispersion: r > v . Cleavage is perfect parallel to {001} and moderate parallel to {010}. Chemical analysis by electron microprobe gave SiO 2 35.63, Al 2 O 3 0.95, Na 2 O 1.04, K 2 O 3.27, Cs 2 O 0.31, CaO 2.56, MgO 0.16, ZnO 0.15, FeO 29.24, MnO 7.14, TiO 2 11.07, Nb 2 O 5 0.49, ZrO 2 0.37, SnO 2 1.18, F 1.01, Li 2 O 1.36 (AAS), Rb 2 O 0.85 (AAS), (H 2 O) calc. 4.04, sum 100.38 wt.%, H 2 O was calculated from crystal-structure analysis. The empirical formula based on 31.94 (O + OH + F + H 2 O) pfu is (Li 0.94 K 0.91 Rb 0.12 Cs 0.03 ) Σ2 (Ca 0.60 Na 0.40 ) Σ1 (Fe 2+ 5.34 Mn 1.32 Li 0.25 Mg 0.05 Na 0.04 Zn 0.02 ) Σ7.02 (Ti 1.82 Sn 0.10 Nb 0.05 Zr 0.04 ) Σ2.01 [(Si 7.78 Al 0.24 ) Σ8.02 O 24 ]O 2 (OH) 4 (F 0.70 O 0.30 )[(H 2 O) 0.94 □ 1.06 ] Σ2 , Z = 1. Bulgakite is triclinic, space group P , a 5.374(1), b 11.965(2), c 11.65(3) A, α 113.457(8), β 94.533(8), γ 103.08(1)°, V 657.5(8) A 3 . The six strongest reflections in the X-ray powder diffraction data [ d (A), I, ( hkl )] are: 10.54, 100, (001); 3.50, 100, (003); 2.578, 100, (130); 2.783, 90, (1 2); 1.576, 68, (3 1, 2); 2.647, 55, ( 11). The crystal structure has been refined to R 1 = 2.6% for 3592 unique F o > 4σ F ) reflections. In the crystal structure of bulgakite, there are four [4] T sites, with T –O> = 1.626 A, occupied mainly by Si, with minor Al. The TO 4 tetrahedra constitute the T 4 O 12 astrophyllite ribbon. The [6] D site is occupied mainly by Ti, with D –φ> = 1.965 A (φ = O, F). The T 4 O 12 astrophyllite ribbons and D octahedra constitute the H (Heteropolyhedral) sheet. In the O (Octahedral) sheet, there are four Fe 2+ -dominant [6] M (1–4) sites, with = 2.159 A (φ = O, OH). Two H and the central O sheets form the HOH block, and adjacent HOH blocks link via a common anion (X P D ) of two D octahedra. In the I (Intermediate) block between adjacent HOH blocks, there are two interstitial cation sites, A and B , and a W site, partly occupied by H 2 O. The A site splits into two partly occupied sites, [13] A (1) and [6] A (2), with A(1)–A(2) = 1.16 A. The [6] A (2) site is occupied by Li with = 2.285 A (φ = O, F, H 2 O), and the [13] A (1) site is occupied by K, Rb, and Cs with = 3.298 A. The aggregate content of the A site is (Li 0.94 K 0.91 Rb 0.12 Cs 0.03 ) Σ2 , ideally Li 2 apfu . The [10] B site is occupied by (Ca 0.60 Na 0.40 ) with B –φ> = 2.593 A. The W site is occupied by [(H 2 O) 0.94 □ 1.06 ] Σ2 pfu . The mineral is named bulgakite after Lev Vasil9evich Bulgak (born 1955), Russian mineralogist, gemologist, and discoverer of several new minerals. The crystal structure of nalivkinite has been revised and refined to R 1 = 4.52% for 3546 unique ( F o > 4σ F ) reflections: space group P , a 5.374(3), b 11.948(5), c 11.676(5) A, α 113.360(6), β 94.538(8), γ 103.01(1)°, V 658.7(9) A 3 , Z = 1, D calc. = 3.347 g/cm 3 . The revised empirical formula of nalivkinite is based on 32.14 (O + OH + F + H 2 O) pfu : (Li 1.14 K 0.75 Cs 0.09 Pb 0.02 ) Σ2 (Na 0.71 Ca 0.29 ) Σ1 (Fe 2+ 5.62 Mn 0.90 Zr 0.08 Na 0.08 Mg 0.04 Zn 0.04 ) Σ6.76 (Ti 1.56 Nb 0.24 Sn 0.09 Zr 0.08 Ta 0.04 ) Σ2 [(Si 7.86 Al 0.15 ) Σ8.01 O 24 ]O 2 (OH) 4 F[(H 2 O) 1.14 □ 0.86 ] Σ2 . The presence of H 2 O groups in the bulgakite and nalivkinite structures was confirmed by infrared spectroscopy. Bulgakite is a Ca-analogue of nalivkinite. Bulgakite and nalivkinite are related by the following substitution: 0.3 B Ca 2+ + 0.3 X O 2– ↔ 0.3 B Na + + 0.3 X F – .

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