From structure topology to chemical composition. III. Titanium silicates : the Crystal chemistry of barytolamprophyllite

The crystal structure of barytolamprophyllite, ideally (BaK) Na3 Ti3 (Si2O7)2O2 (OH)2, a 19.8971(7), b 7.1165(3), c 5.4108(2) Å, β 96.676(2)°, V 760.96(3) Å3, space group C2/m, Z = 2, Dcalc. 3.521 g.cm-3, from Mt. Yuksporr, Khibina alkaline massif, Kola Peninsula, Russia, has been refined to R1 = 2.82% on the basis of 1175 unique reflections (Fo > 4σF) collected on a Bruker AXS SMART APEX diffractometer with a CCD detector and MoKα radiation. An electron-microprobe analysis gave (wt.%): SiO2 29.31, Al2O3 0.16. Nb2O5 0.11, TiO2 28.39, Fe2O3 0.49, FeO 2.66, MnO 2.42, MgO 0.26, CaO 0.88, SrO 4.11, BaO 16.37, K2O 3.52, Na2O 7.90, F 1.69, H2O 0.77 (calculated from structure refinement), O=F -0.71,for a total of 98.33. The empirical formula is (Ba0.88 K0.61 Sr0.33 Ca0.05 □0.13) Σ2.00 (Na2.09 Fe2+0.30 Mn2+ 0.28 Ca0.08 Mg0.05 □ 0.20) Σ3.00 (Ti2.91 Fe3+0.05 Al0.03 Nb0.01) Σ3.00 (Si2O7)2 O2 [F0.73 (OH)0.70 O0.57] Σ2.00 Z = 2, calculated on the basis of 4 Si apfu. The crystal structure of barytolamprophyllite can be described as a combination of two blocks, a TS (titanium silicate) block and an I (intermediate) block. The TS block consists of HOH sheets (H heteropolyhedral, O octahedral), which occur in 24 Ti disilicate minerals. In the TS block, there are six cation sites, a [4]-coordinated Si site with <Si-O> = 1.631 Å, two Ti-dominant sites, the [5]-coordinated MH site and [6]-coordinated M°(1) site, and two [6]-coordinated Na-dominant sites, M°(2) and M°(3). The MH site (= Ti1.97 Al0.03 pfu) is coordinated by five O atoms, with <MH-O> = 1.914 Å; the M°(1) site (= Ti0.94 Fe3+ 0.05 Nb0.01 pfu) is coordinated by four O atoms and two OH groups, with <M°(1)-O> = 1.993 Å; the M°(2) and M°(3) sites (= Na1.00 and Na1.09 Fe2+0.30 Mn2+0.28 Ca0.08 Mg0.05 □0.20 pfu, respectively) are coordinated by six O atoms with <M°(2)-O> = 2.441 Å, and four O atoms and two OH groups, with <M°(3)-O> = 2.296 Å, respectively. The MH square pyramid and (Si2O7) groups constitute the H sheet. The M°(1-3) octahedra form the close-packed O sheet. Linkage of H and O sheets occurs via common vertices of MH square pyramids and (Si2O7) groups with M°(1-3) octahedra. The second or intermediate I block includes the [10]-coordinated AP site (= Ba0.88 K0.61 Sr0.33 Ca0.05□0.13 pfu) with <AP-O> = 2.843 Å. The TS block and the I block (which ideally is a layer of Ba and K atoms) alternate along a. The ideal formula of barytolamprophyllite is (BaK) Na3 Ti3 (Si2O7)2 O2 (OH)2 Z = 2. The topology of its structure is identical to that of lamprophyllite, (SrNa) Na3 Ti3 (Si2O7)2 O2 (OH)2, and nabalamprophyllite, Ba Na Na3 Ti3 (Si2O7)2 O2 (OH)2. We explain why, for lamprophyllite, barytolamprophyllite and nabalamprophyllite, the chemical composition of the I block is of the form (A2+ A+) with a total charge of 3+, and the ideal formula of the minerals is (A2+A+) Na3 Ti3 (Si2O7)2 O2 (OH)2, where A2+ = Sr2±, Ba2+, Ca2+; A+ = Na+, K+.