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From structure topology to chemical composition. XX. Titanium silicates: the crystal structure of hejtmanite, Ba 2 Mn 4 Ti 2 (Si 2 O 7 ) 2 O 2 (OH) 2 F 2 , a Group-II TS-block mineral

2016; Cambridge University Press; Volume: 80; Issue: 5 Linguagem: Inglês

10.1180/minmag.2016.080.026

1471-8022

Elena Sokolova, Fernando Cámara, F. C. Hawthorne, Leonid A. Pautov,

Inorganic Fluorides and Related Compounds

Abstract The crystal structure of hejtmanite, Ba 2 Mn 4 Ti 2 (Si 2 O 7 ) 2 O 2 (OH) 2 F 2 , from Mbolve Hill, Mkushi River area, Central Province, Zambia (holotype material) has been refined on a twinned crystal to R 1 = 1.88% on the basis of 4539 [|F| > 4|F|] reflections. Hejtmanite is triclinic, C 1̅, a = 10.716(2), b = 13.795(3), c = 11.778 (2) , = 90.07(3), = 112.24(3), = 90.03(3), V = 1612(2) 3 . Chemical analysis (electron microprobe) gives: Ta 2 O 5 0.09, Nb 2 O 5 1.27, ZrO 2 0.65, TiO 2 14.35, SiO 2 23.13, BaO 26.68, SrO 0.19, FeO 11.28, MnO 15.12, Cs 2 O 0.05, K 2 O 0.33, F 3.82, H 2 O calc . 1.63, O = F 1.61, total 97.10 wt.%, where the H 2 O content was calculated from the crystal-structure refinement, with (OH F) = 4 apfu. The empirical formula, calculated on the basis of 20 (O F) anions, is of the form (Si 2 O 7 ) 2 (X O ) 4 (X P ) 2 , Z=4: (Ba 1.82 K 0.07 Sr 0.02 ) Σ1.91 (Mn 2.33 Zr 0.04 Mg 0.03 ) Σ3.95 (Ti 1.88 Nb 0.10 Zr 0.02 ) Σ2 (Si 2.02 O 7 ) 2 O 2 [(OH) 1.89 F 0.11 ] Σ2 F 2 . The crystal structure is 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 TS-block minerals: Ti ( Nb) = 2 apfu per (Si 2 O 7 ) 2 [as defined by Sokolova (2006)]. In the O sheet, five [6] M O sites are occupied mainly by Mn, less Fe 2 and minor Zr and Mg, with <MOφ> = 2.198 (φ = O,OH), ideally giving Mn 4 apfu. In the H sheet, two [6] M H sites are occupied mainly by Ti, with <M H φ> = 1.962 (φ = O,F), ideally giving Ti 2 apfu; four [4] Si sites are occupied by Si, with < SiO> = 1.625 . 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 <A P φ> = 2.984 (φ = O, F), ideally give Ba 2 apfu. Two (1,2) and two (1,2) sites are occupied by O atoms and OH groups with minor F, respectively, ideally giving (X O ) 4 = ( ) 2 ( )2=O 2 (OH) 2 pfu. Two (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 end-member formulae of hejtmanite are Ba 2 (Mn,Fe 2 ) 4 Ti 2 (Si 2 O 7 ) 2 O 2 (OH,F) 2 F 2 and Ba 2 Mn 4 Ti 2 (Si 2 O 7 ) 2 O 2 (OH) 2 F 2 , Z = 4. Hejtmanite is a Mn-analogue of bafertisite, Ba 2 4 Ti 2 (Si 2 O 7 ) 2 O 2 (OH) 2 F 2 .