• journal_title：The Canadian Mineralogist
• Contributor：Yulia A. Uvarova ; Elena Sokolova ; Frank C. Hawthorne ; Atali A. Agakhanov ; Leonid A. Pautov
• Publisher：Mineralogical Association of Canada
• Date：2008-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.46.3.651
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：46
• issue：3
• firstpage：651
• section：Articles

The crystal structure of nalivkinite, ideally Li₂ Na Fe²⁺₇ Ti₂ (Si₈ O₂₄) O₂ (OH)₄ F, a new lithium member of the astrophyllite group, from the moraine of the Dara-i-Pioz glacier, the Alai mountain ridge, Tien-Shan Mountains, northern Tajikistan, triclinic, a 5.3745(6), b 11.9299(15), c 11.6509(14) Å, α 113.325(3), β 94.524(2), γ 103.080(2)°, V 656.2(2) ų, space group P1̅, Z = 1, D_calc. 3.293 g/cm³, has been refined to R₁ = 6.3% for 2041 unique (F₀ > 4σ F) reflections, collected on a Bruker single-crystal P4 diffractometer with a 4K CCD detector and MoKα X-radiation. Electron-microprobe analysis gives SiO₂ 36.11, Al₂O₃ 0.58, Na₂O 1.88, K₂O 2.68, Cs₂O 0.93, CaO 1.24, MgO 0.11, FeO 30.84, MnO 4.86, ZnO 0.23, TiO₂ 9.48, Nb₂O₅ 2.40, Ta₂O₅ 0.61, ZrO₂ 1.47, SnO 0.89, PbO 0.39, F 1.45, Li₂O_ICP–MS 1.30, (H₂O)_calc 2.75, sum 100.2 wt.% (H₂O determined from crystal-structure analysis). The empirical formula based on 31 (O + OH + F) anions is (Li_1.14 K_0.75 Cs_0.09 Na_0.02)_{∑ 2.00} (Na_0.78 Ca_0.22)_∑1.00 (Fe²⁺_5.64 Mn_0.90 Sn_0.09 Ca_0.07 Mg_0.04 Zn_0.04 Pb_0.02□_0.20) _∑7.00 (Ti_1.56 Nb_0.24 Zr_0.16 Ta_0.04)_∑2.00 [(Si_7.89 Al_0.15)_8.04O₂₄] O₂ (OH)₄ F. The crystal structure of nalivkinite is topologically identical to that of triclinic astrophyllite. For nalivkinite, a general formula for the astrophyllite-group minerals, A₂B C₇D₂ (T₈ O₂₄) O₂ (OH)₄X_0–1, ideally has A = Li; B = ^[10]Na; C = ^[6]Fe²⁺; D = ^[6]Ti; T = ^[4]Si and X = F. In the nalivkinite structure, four T sites are mainly occupied by Si with minor Al, with <T–O> = 1.623 Å. The M(1–4) sites are occupied by (Fe²⁺_1.71Ca_0.07Pb_0.02□_0.20), Fe²⁺_2.00, (Fe²⁺_1.10 Mn_0.90) and (Fe²⁺_0.83 Sn_0.09 Zn_0.04 Mg_0.04), with <M(1–4)–O> = 2.164, 2.146, 2.140 and 2.140 Å, respectively. The D site is occupied by (Ti_1.56 Nb_0.24 Zr_0.16Ta_0.04), with <D–O> = 1.959 Å. There are two interstitial sites, A and B, ideally occupied by Li and Na. The B site is occupied by (Na_0.78Ca_0.22), with <B–O> = 2.612 Å. The A site splits into two half-occupied sites, A(1a) and A(1b), with A(1a)–A(1b) = 1.28(4) Å. The ^[13]A(1a) site is occupied by K, Li, Cs and Na (K_0.75Li_0.14Cs_0.09Na_0.02apfu), with <A(1a)–O> = 3.310 Å. The ^[5]A(1b) site is occupied by Li (1.0 Li apfu), with <A(1b)–O> = 2.28 Å. The aggregate content of the A site is (Li_1.14 K_0.75 Cs_0.09 Na_0.02)_{∑ 2.00} or, ideally, Li₂. In the structure, an O sheet of Fe²⁺ octahedra and two heteropolyhedral (H) sheets of (Si₄O₁₂) ribbons and Ti octahedra form an HOH block of composition Fe²⁺₇ Ti₂ (Si₄ O₁₂)₂ O₂ (OH)₄ F. The HOH blocks link via common vertices of Ti octahedra (F atoms) along [001] and contain interstitial cations at the A and B sites. Nalivkinite is a Li analogue of astrophyllite, and the two structures are related by the isovalent substitution Li (nalivkinite) ↔K (astrophyllite) at the interstitial A site.