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

Abstract

Abstract The crystal structure of jinshajiangite from the Verkhnee Espe deposit, Kazakhstan, NaBaFe 2+ 4 Ti 2 (Si 2 O 7 ) 2 O 2 (OH) 2 F, was refined from a twinned crystal to R 1 = 3.13% on the basis of 6745 unique reflections [ F o > 4σ F o ], space group , Z = 8, a 10.7059(5), b 13.7992(7), c 20.760(1) A, α 90.008(1), β 94.972(1), γ 89.984(1)°, V 3055.4(4) A 3 . The crystal used for the structure refinement was analyzed by electron microprobe. The empirical formula was calculated on 19 (O + F), with (OH + F) = 3 pfu : (Na 0.77 Ca 0.23 ) Σ1 (Ba 0.60 K 0.36 ) Σ0.96 (Fe 2+ 2.33 Fe 3+ 0.26 Mn 1.26 Zr 0.04 Mg 0.02 Zn 0.01 ) Σ3.92 (Ti 1.79 Nb 0.18 Mg 0.02 Al 0.01 ) Σ4 (Si 2 O 7 ) 2 O 3.93 H 1.93 F 1.07 , Z = 8; Fe 2 O 3 was calculated by analogy with jinshajiangite from Norra Karr, Sweden (Sokolova et al . 2009a) and H 2 O 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] M O sites are occupied mainly by Fe 2+ , with O –φ> = 2.181 A. In the H sheet, four [6] M H sites are occupied mainly by Ti, with H –φ> = 1.954 A, and eight [4] Si sites are occupied by Si, with = 1.622 A. The M H octahedra and Si 2 O 7 groups constitute the H sheet. Fluorine atoms and OH groups are ordered at the X P M (H sheet) and X O A (O sheet) sites, respectively. The TS blocks link via common vertices of M H octahedra, i.e ., M H –X P M – M H bridges. In the I block, Ba and K occur at the two A P sites, with Ba > K, and the two B P sites are occupied by Na and Ca, with Na > Ca. Jinshajiangite is isostructural with bobshannonite, Na 2 KBa(Nb,Ti) 4 Mn 8 (Si 2 O 7 ) 4 O 4 (OH) 4 (O,F) 2 , Z = 4.