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THE CRYSTAL CHEMISTRY OF FAIZIEVITE, K₂ Li₆ Na (Ca₆Na) Ti₄ [Si₆O₁₈]₂ [Si₁₂O₃₀] F₂, A NOVEL STRUCTURE BASED ON INTERCALATED BLOCKS OF THE BARATOVITE AND BEREZANSKITE STRUCTURES

Yulia A. Uvarova^1,, Elena Sokolova¹, Frank C. Hawthorne¹, Atali A. Agakhanov² and Leonid A. Pautov²

¹ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
² Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow 117071, Russia

E-mail address: umuvarov{at}cc.umanitoba.ca

The crystal structure of faizievite, ideally K₂ Li₆ Na (Ca₆Na) Ti₄ [Si₆O₁₈]₂ [Si₁₂O₃₀] F₂, a 9.8156(9), b 9.8249(9), c 17.3087(16) Å, 99.209(2), β 94.670(2), 119.839(1)°, V 1403.7(4) ų, space group P, Z = 1, D_calc. 2.846 g/cm³, has been solved and refined to an R₁ index of 7.5% based on 5044 unique (F₀ > 4F) reflections collected on a Bruker single-crystal P4 diffractometer with a 4K CCD detector and MoK X-radiation. Electron-microprobe analysis gave SiO₂ 60.65, TiO₂ 13.44, Nb₂O₅ 0.11, CaO 14.52, K₂O 3.93, Na₂O 1.99, SrO 0.72, Rb₂O 0.13, F 1.30, Li₂O 3.76, sum 100.24 wt.%. Concentrations of Li₂O, Rb₂O, SrO and BaO were determined by ICP–OES. There are fifteen tetrahedrally coordinated sites: twelve sites are occupied by Si, with a grand <Si–O> of 1.613 Å; three sites are occupied by Li, with a grand <Li–O> distance of 1.928 Å. There are six octahedrally coordinated sites. The two Ti(1) and Ti(2) sites are occupied by Ti_2.00 with <Ti–O> = 1.937 and 1.934 Å, respectively. The four M sites are occupied by Ca with minor Sr and Na: the M(1) site (= Ca_2.00), with <M(1)–O> = 2.441 Å, the M(2) site [= (Ca_1.87 Sr_0.13)], with <M(2)–O> = 2.424 Å, the M(3) site [= (Ca_1.56 Na_0.40 Sr_0.04)], with <M(3)–O> = 2.415 Å, and the M(4) site [= (Ca_0.73 Na_0.27)], with <M(4)–O> = 2.418 Å. There are two interstitial A sites: the A(1) site is [12]-coordinated and is occupied by (K_1.93 Ba_0.04 Rb_0.03), with <A(1)–O> = 3.092 Å; the A(2) site is [9]-coordinated and is occupied by (Na_{0.86 0.14}), with <A(2)–O> = 2.718 Å. The crystal structure of faizievite consists of four types of (001) sheets. The Si tetrahedra form two types of six-membered rings: a single (Si₆O₁₈) ring, as in baratovite, and a double (Si₁₂O₃₀) ring, as in milarite-group minerals, in the ratio 2:1. Each type of ring forms a distinct sheet (Na atoms occur in a sheet of milarite double rings). Two other sheets are: a sheet of corner-sharing (LiO₄) tetrahedra and (TiO₆) octahedra with K atoms in large voids, and a sheet of edge-sharing M octahedra (M = Ca >> Na). Each sheet is characterized by a planar cell based on translation vectors, t₁ and t₂, with t₁ t₂ 9.8 Å and t₁ t₂ close to 120°. The composition of the four individual sheets within the planar cell t₁,t₂ is: (1) (Si₆O₁₈), (2) (Si₁₂O₃₀)Na, (3) KLi₃Ti₂, (4) (Ca₆Na)F₂. The crystal structure of faizievite is a hybrid of the structures of baratovite, KLi₃Ca₇Ti₂(Si₆O₁₈)₂F₂, and berezanskite, KLi₃Ti₂(Si₁₂O₃₀) (milarite group). The baratovite block of composition [KLi₃(Ca₆Na)Ti₂(Si₆O₁₈)₂F₂] comprises sheets (3)(1)(4)(1). The berezanskite block of composition [KNaLi₃Ti₂(Si₁₂O₃₀)] comprises sheets (3)(2). In the faizievite structure, baratovite and berezanskite blocks alternate along [001] and sum to the following sequence: (3)(1)(4)(1)(3)(2). There are minor differences between the chemical compositions of baratovite and berezanskite and analogous blocks in the crystal structure of faizievite. Baratovite, K Li₃ Ca₇ Ti₂ (Si₆O₁₈)₂ F₂, and the baratovite block in faizievite, KLi₃(Ca₆Na)Ti₂(Si₆O₁₈)₂F₂, are related by the substitution: ^[6]Ca₂₊ ^[6]Na⁺. Berezanskite, K^[9] Li₃ Ti₂ (Si₁₂O₃₀), and the berezanskite block in faizievite, K^[9]NaLi₃Ti₂(Si₁₂O₃₀), are related by the substitution: ^{[9] [9]}Na⁺. Assembly of faizievite from baratovite and berezanskite components is accompanied by the following substitution: ^[6]Ca²⁺ + ^{[9] [6]}Na⁺ + ^[9]Na⁺.

Keywords: faizievite, new mineral species, crystal-structure refinement, baratovite, berezanskite, intercalated framework.