From structure topology to chemical composition. IV. Titanium silicates: crystal chemistry of barytolamprophyllite

The crystal structure of barytolamprophyllite, ideally (BaK) Na(3) Ti(3) (Si(2)O(7))(2)O(2) (OH)(2), a 19.8971(7), b 7.1165(3), c 5.4108(2) angstrom, beta 96.676(2)degrees, V 760.96(3) angstrom(3), space group C2/m, Z = 2, D(calc). 3.521 g.cm(-3), from Mt. Yuksporr, Khibina alkaline massif, Kola Peninsula, Russia, has been relined to R(1) = 2.82% oil the basis of 1175 unique reflections (F(o) > 4 sigma F) collected on a Bruker AXS SMART APEX diffractometer with a CCD detector and MoK alpha radiation. An electron-microprobe analysis gave (wt.%): SiO(2) 29.31, Al(2)O(3) 0.16, Nb(2)O(5) 0.11, TiO(2) 28.39, Fe(2)O(3) 0.49, FeO 2.66, MnO 2.42, MgO 0.26, CaO 0.88, SrO 4.11, BaO 16.37, K(2)O 3.52, Na(2)O 7.90, F 1.69, H(2)O 0.77 (calculated from structure refinement), O=F-0.71, fora total of 98.33. The empirical formula is (Ba(0.88)K(0.61)Sr(0.33)Ca(0.05)square(0.13))(Sigma 2.00) (Na(2.09)Fe(0.30)(2+)Mn(0.28)(2+) Ca(0.08) Mg(0.05)square(0.20))(Sigma 3.00) (Ti(2.91)Fe(0.05)(3+) O(2) [F(0.73) (OH)(0.70) O(0.57)]Sigma(2.00), Z = 2, calculated on the basis of 4 Si apfu. The crystal structure of barytolamprophyllite call be described as a combination of two blocks, a TS (titanium silicate) block and all 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 angstrom, two Ti-dominant sites, the [5]-coordinated M(H) site and [6]-coordinated M degrees(1) site, and two [6]-coordinated Na-dominant sites, M degrees(2) and M degrees(3). The M(H) site (= Ti(1.97)Al(0.03) pfu) is coordinated by five O atoms, with < M(H)-O > = 1.914 angstrom; the M degrees(1) site (= Ti(0.94)Fe(0.05)(3+) Nb(0.01) pfu) is coordinated by four O atoms and two OH groups, with < M degrees(1)-O > = 1.993 angstrom; the M degrees(2) and M degrees(3) sites (= Na(1.00) and Na(1.09)Fe(0.30)(2+)Mn(0.28)(2+) Ca(0.08) Mg(0.05) square(0.20) pfu, respectively) are coordinated by six O atoms with < M degrees(2)-O > = 2.441 angstrom, and four O atoms and two OH groups, with < M degrees(3)-O > = 2.296 angstrom, respectively. The MH square pyramid and (Si(2)O(7)) groups constitute the H sheet. The M degrees(1-3) octahedra form the close-packed 0 sheet. Linkage of H and 0 sheets occurs via common vertices of MH square pyramids and (Si(2)O(7)) groups with M'(1-3) octahedra. The second or intermediate I block includes the 110]-coordinated A(P) site (= Ba(0.88) K(0.61) Sr(0.33) Ca(0.05)square(0.13) pfu) with < A(P)-O > = 2.843 angstrom. The TS block and the I block (which ideally is a layer of Bit and K atoms) alternate along a. The ideal formula of barytolamprophyllites is (BaK) Na(3) Ti(3) (Si(2)O(7))(2) O(2) (OH)(2), Z = 2. The topology of its structure is identical to that of lamprophyllite, (SrNa) Na<INF>3</INF> Ti<INF>3</INF> (Si<INF>2</INF>O<INF></INF>)<INF>2</INF> O<INF>2</INF> (OH)<INF>2</INF>, and nabalamprophyllite, Ba Na Na<INF>3</INF> Ti<INF>3</INF> (Si<INF>2</INF>O<INF>7</INF>)<INF>2</INF> O<INF>2</INF> (OH)<INF>2</INF>. 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+) Na<INF>3</INF> Ti<INF>3</INF> (Si<INF>2</INF>O<INF>7</INF>)<INF>2</INF> O<INF>2</INF> (OH)<INF>2</INF>, where A2+ = Sr2+, Ba2+, Ca2+; A+ = Na+, K+.