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

Abstract

The crystal structure of faizievite, ideally K 2 Li 6 Na (Ca 6 Na) Ti 4 [Si 6 O 18 ] 2 [Si 12 O 30 ] F 2 , a 9.8156(9), b 9.8249(9), c 17.3087(16) Å, a 99.209(2), b 94.670(2), g 119.839(1)°, V 1403.7(4) Å 3 , space group P1, Z = 1, D calc. 2.846 g/cm 3 , has been solved and refined to an R 1 index of 7.5% based on 5044 unique (F 0 > 4sF) reflections collected on a Bruker single-crystal P4 diffractometer with a 4K CCD detector and MoKa X-radiation. Electron-microprobe analysis gave SiO 2 60.65, TiO 2 13.44, Nb 2 O 5 0.11, CaO 14.52, K 2 O 3.93, Na 2 O 1.99, SrO 0.72, Rb 2 O 0.13, F 1.30, Li 2 O 3.76, sum 100.24 wt.%. Concentrations of Li 2 O, Rb 2 O, 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 (Si 6 O 18) ring, as in baratovite, and a double (Si 12 O 30) ring, as in milaritegroup 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 (LiO 4) tetrahedra and (TiO 6) 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, t 1 and t 2 , with t 1 ≈ t 2 ≈ 9.8 Å and t 1  t 2 close to 120°. The composition of the four individual sheets within the planar cell t 1 ,t 2 is: (1) (Si 6 O 18), (2) (Si 12 O 30)Na, (3) KLi 3 Ti 2 , (4) (Ca 6 Na)F 2. The crystal structure of faizievite is a hybrid of the structures of baratovite, KLi 3 Ca 7 Ti 2 (Si 6 O 18) 2 F 2 , and berezanskite, KLi 3 Ti 2 (Si 12 O 30) (milarite group). The baratovite block of composition [KLi 3 (Ca 6 Na)Ti 2 (Si 6 O 18) 2 F 2 ] comprises sheets (3)(1)(4)(1). The berezanskite block of composition [KNaLi 3 Ti 2 (Si 12 O 30)] 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....