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

The crystal structure of hejtmanite, Ba2Mn4Ti2(Si2O7)2O2(OH)2F2, from Mbolve Hill, Mkushi River area, Central Province, Zambia (holotype material) has been refined on a twinned crystal to R1 = 1.88% on the basis of 4539 [|F| > 4σ|F|]. Hejtmanite is triclinic, C⎯1, a = 10.716(2), b = 13.795(3), c = 11.778(2) Å, α = 90.07(3), β = 112.24(3), γ = 90.03(3)°, V = 1612(2) Å3. 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 a.p.f.u. The empirical formula, calculated on the basis of 20 (O + F) anions, is of the form AP 2MO 4MH 2(Si2O7)2(XO)4(XP)2, Z = 4: (Ba1.82K0.07Sr0.02)Σ1.91 (Mn2.33Fe2+ 1.65Zr0.04Mg0.03)Σ3.95(Ti1.88Nb0.10Zr0.02)Σ2(Si2.02O7)2O2 [(OH)1.89F0.11]Σ2 F2. 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 a.p.f.u. per (Si2O7)2 [as defined by Sokolova (2006)]. In the O sheet, five [6]MO sites are occupied mainly by Mn, less Fe2+ and minor Zr and Mg, with = 2.198 Å (ϕ = O,OH), ideally giving Mn4 a.p.f.u. In the H sheet, two [6]MH sites are occupied mainly by Ti, with = 1.962 Å (ϕ = O,F), ideally giving Ti2 a.p.f.u.; four [4]Si sites are occupied by Si, with = 1.625 Å. The MH octahedra and Si2O7 groups constitute the H sheet. The two [12]Ba-dominant AP(1,2) sites, with = 2.984 Å (ϕ = O, F), ideally give Ba2 a.p.f.u. Two XO M(1,2) and two XO A(1,2) sites are occupied by O atoms and OH groups with minor F, respectively, ideally giving (XO)4 = (XO M)2 + (XO A)2 = O2(OH)2 p.f.u. Two XP M(1,2) sites are occupied by F, giving F2 a.p.f.u. TS blocks link via a layer of Ba atoms which constitute the I block. Simplified and endmember formulae of hejtmanite are Ba2(Mn,Fe2+)4Ti2 (Si2O7)2O2(OH,F)2 F2 and Ba2Mn4Ti2(Si2O7)2O2(OH)2F2, Z = 4. Hejtmanite is a Mn-analogue of bafertisite, Ba2Fe2+ 4Ti2(Si2O7)2O2(OH)2F2.