From structure topology to chemical composition. XX. Titanium silicates: the crystal structure of hejtmanite, Ba2Mn4Ti2(Si2O7)2O2(OH)2F2, a Group-II TS-block mineral

The crystal structure of hejtmanite, Ba2Mn4Ti2(Si2O7)(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 [vertical bar F vertical bar > 4 sigma vertical bar F vertical bar] reflections. Hejtmanite is triclinic, C (1) over bar, a = 10.716(2), b = 13.795(3), c = 11.778 (2) angstrom, alpha = 90.07(3), beta = 112.24(3), gamma = 90.03(3)degrees, V = 1612(2) angstrom(3). Chemical analysis (electron microprobe) gives: Ta2O5 0.09, Nb2O5 1.27, ZrO2 0.65, TiO2 14.35, SiO2 23.13, BaO 26.68, SrO 0.19, FeO 11.28, MnO 15.12, Cs2O 0.05, K2O 0.33, F 3.82, H2Ocalc. 1.63, O = F -1.61, total 97.10 wt.%, where the H2O 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 A(2)(P)M(4)(O)M(2)(H)(Si2O7)(2)(X-O)(4)(X-P)(2), Z=4: (Ba1.82K0.07Sr0.02)(Sigma 1.91)(Mn2.33Fe1.652+Zr0.04Mg0.03)(Sigma 3.95)(Ti1.88Nb0.10Zr0.02)(Sigma 2)(Si2.02O7)(2)O-2[(OH)(1.89)F-0.11](Sigma 2)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 (Si2O7)(2) [as defined by Sokolova (2006)]. In the O sheet, five M-[6](O) sites are occupied mainly by Mn, less Fe2+ and minor Zr and Mg, with <M-O-phi> = 2.198 angstrom (phi = O,OH), ideally giving Mn-4 apfu. In the H sheet, two M-[6](H) sites are occupied mainly by Ti, with <M-H-phi> = 1.962 angstrom (phi = O, F), ideally giving Ti-2 apfu; four Si-[4] sites are occupied by Si, with <Si-O> = 1.625 angstrom. The MH octahedra and Si2O7 groups constitute the H sheet. The two [12] Ba-dominant A(P)(1,2) sites, with <A(P)-phi> = 2.984 angstrom (phi = O, F), ideally give Ba-2 apfu. Two X-M(O)(1,2) and two X-A(O) (1,2) sites are occupied by O atoms and OH groups with minor F, respectively, ideally giving (X-O)(4) = (X-M(O))(2) + (X-A(O))(2)=O-2(OH) 2 pfu. Two X-M(P)(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, Fe2+)(4)Ti-2 (Si2O7)(2)O-2(OH, F)(2)F-2 and Ba2Mn4Ti2(Si2O7)(2)O-2(OH)(2)F-2, Z = 4. Hejtmanite is a Mn-analogue of bafertisite, Ba2Fe42+Ti2(Si2O7)(2)O-2(OH)(2)F-2.