From structure topology to chemical composition. XXII. Titanium silicates : Revision of the crystal structure of jinshajiangite, NaBaFe2+4Ti2(Si2O7)2O2(OH)2F, A group-II TS-block mineral

The crystal structure of jinshajiangite from the Verkhnee Espe deposit, Kazakhstan, NaBaFe2+4Ti2(Si2O7)2O2(OH)2F, was refined from a twinned crystal to R1 = 3.13% on the basis of 6745 unique reflections [Fo > 4σFo], space group C1, Z = 8, a 10.7059(5), b 13.7992(7), c 20.760(1) Å , α 90.008(1), β 94.972(1), γ 89.984(1)°, V 3055.4(4) Å 3. The crystal used for the structure refinement was analyzed by electron microprobe. The empirical formula was calculated on 19 (O+F), with equation presented was calculated by analogy with jinshajiangite from Norra Karr, Sweden (Sokolova et al. 2009a) and H2O from the crystal-structure analysis. In the crystal structure, TS (Titanium Silicate) and I (Intermediate) blocks alternate along c. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The topology of the TS block is as in Group II of the TS-block minerals where Ti (+Nb)=2 apfu. In the O sheet, ten [6]MO sites are occupied mainly by °° Fe2+, with < MO-φ. = 2.181 Å. In the H sheet, four [6]MH sites are occupied mainly by Ti, with < MH-φ. = 1.954 Å, and ° eight [4]Si sites are occupied by Si, with < Si-O>. = 1.622 Å. The MH octahedra and Si2O7 groups constitute the H sheet. Fluorine atoms and OH groups are ordered at the XPM (H sheet) and XOA (O sheet) sites, respectively. The TS blocks link via common vertices of MH octahedra, i.e., MH-XPM- MH bridges. In the I block, Ba and K occur at the two AP sites, with Ba . K, and the two BP sites are occupied by Na and Ca, with Na > Ca. Jinshajiangite is isostructural with bobshannonite, Na2KBa(Nb,Ti)4Mn8(Si2O7)4O4(OH)4(O,F)2, Z = 4.