From Structure Topology To Chemical Composition. XIX. Titanium Silicates: Revision of the Crystal Structure and Chemical Formula of Bafertisite, Ba2Fe2+4Ti2(Si2O7)2O2(OH)2F2, A Group-II TS-Block Mineral

Abstract

Abstract The crystal structure and chemical formula of bafertisite, Ba 2 Fe 2+ 4 Ti 2 (Si 2 O 7 ) 2 O 2 (OH) 2 F 2 , have been revised. Three samples of bafertisite were studied using electron-microprobe analysis, Mossbauer spectroscopy, IR and Raman spectroscopy, and single-crystal X-ray diffraction. These samples are from (1) the Bayan Obo REE deposit, Inner Mongolia, China (holotype); (2) the Gremyakha-Vyrmes alkaline complex, Kola Peninsula, Russia; and (3) the Darai-Pioz alkaline massif, Tajikistan. Bafertisite is a TS-block mineral of Group II, Ti = 2 apfu (atoms per formula unit) per (Si 2 O 7 ) 2 (Sokolova 2006). Bafertisite is triclinic, C , a 10.677(6), b 13.767(7), c 11.737(5) A, α 90.12(1), β 112.28(4), γ 90.02(1)°, V 1596(3) A 3 (unit-cell parameters are for bafertisite from Kola, sample 2). Chemical analysis was done by electron microprobe, the H 2 O content was calculated from the crystal-structure solution and refinement, and the occurrence of Fe 2+ and lack of Fe 3+ were confirmed by Mossbauer spectroscopy. The empirical formulae were calculated on the basis of 20 (O + F) anions, with (OH + F) = 4 apfu, and they are of the form A P 2 M O 4 M H 2 (Si 2 O 7 ) 2 (X O ) 4 (X P ) 2 , Z = 4: (1) (Ba 1.89 K 0.03 ) Σ1.92 (Fe 2+ 3.33 Mn 0.47 Mg 0.11 ) Σ3.91 (Ti 1.86 Nb 0.07 Zr 0.02 Mg 0.05 ) Σ2 (Si 2.05 O 7 ) 2 O 2 [(OH) 1.82 F 0.18 ] Σ2 F 2 ; (2) (Ba 1.82 Sr 0.02 K 0.02 ) Σ1.86 (Fe 2+ 3.24 Mn 0.57 Al 0.06 Mg 0.03 Ca 0.01 Zr 0.01 Zn 0.01 Na 0.02 ) Σ3.95 (Ti 1.96 Nb 0.03 Zr 0.01 ) Σ2 (Si 2.05 O 7 ) 2 O 2 [(OH) 1.59 F 0.41 ] Σ2 F 2 ; and (3) (Ba 1.90 K 0.02 ) Σ1.92 (Fe 2+ 2.23 Mn 1.61 Mg 0.02 Zr 0.04 Zn 0.03 ) Σ3.93 (Ti 1.90 Nb 0.09 Zr 0.01 ) Σ2 (Si 2.03 O 7 ) 2 O 2 [(OH) 1.75 F 0.25 ] Σ2 F 2 . The crystal structures of (1), (2), and (3) were solved and refined from twinned crystals to R 1 = 2.90, 2.46, and 2.74% on the basis of 4538, 4685, and 4692 unique reflections (| F | > 4σ| F |) and can be described as a combination of a TS (Titanium Silicate) block and an I (Intermediate) block. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The topology of the TS block is as in Group II of the Ti disilicates: Ti + Nb = 2 apfu per (Si 2 O 7 ) 2 (as defined by Sokolova 2006). Here we report structure-refinement results for bafertisite from Kola, sample (2), with the lowest value of R 1 = 2.46%. In the O sheet, five [6] M O sites are occupied mainly by Fe 2+ , less Mn, and minor Mg, Al, Zr, Zn, and Ca, with O –φ> = 2.179 A (φ = O,OH), and they ideally give Fe 2+ 4 apfu . In the H sheet, two [6] M H sites are occupied mainly by Ti, with H –φ> = 1.963 A (φ = O,F), and they ideally give Ti 2 apfu ; four [4] Si sites are occupied by Si, with = 1.626 A. The M H octahedra and Si 2 O 7 groups constitute the H sheet. The two [12] Ba-dominant A P (1,2) sites, with P –φ> = 2.979 A (φ = O, F), ideally give Ba 2 apfu . Two X O M (1,2) and two X O A (1,2) sites are occupied by O atoms and OH groups with minor F, respectively, ideally giving (X O ) 4 = (X O M ) 2 + (X O A ) 2 = O 2 (OH) 2 pfu . Two X P M (1,2) sites are occupied by F, giving F 2 apfu . TS blocks link via a layer of Ba atoms which constitute the I block. Simplified and endmember formulae of bafertisite are Ba 2 (Fe 2+ ,Mn) 4 Ti 2 (Si 2 O 7 ) 2 O 2 (OH,F) 2 F 2 and Ba 2 Fe 2+ 4 Ti 2 (Si 2 O 7 ) 2 O 2 (OH) 2 F 2 , Z = 4.