FROM STRUCTURE TOPOLOGY TO CHEMICAL COMPOSITION. III. TITANIUM SILICATES: THE CRYSTAL CHEMISTRY OF BARYTOLAMPROPHYLLITE

Abstract

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) A, β 96.676(2)°, V 760.96(3) A 3 , space group C 2/ m , Z = 2, D calc. 3.521 g.cm −3 , from Mt. Yuksporr, Khibina alkaline massif, Kola Peninsula, Russia, has been refined to R 1 = 2.82% on the basis of 1175 unique reflections ( F o > 4σ F ) collected on a Bruker AXS SMART APEX diffractometer with a CCD detector and Mo K α 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, for a total of 98.33. The empirical formula is (Ba 0.88 K 0.61 Sr 0.33 Ca 0.05 □ 0.13 ) ∑2.00 (Na 2.09 Fe 2+ 0.30 Mn 2+ 0.28 Ca 0.08 Mg 0.05 □ 0.20 ) ∑3.00 (Ti 2.91 Fe 3+ 0.05 Al 0.03 Nb 0.01 ) ∑3.00 (Si 2 O 7 ) 2 O 2 [F 0.73 (OH) 0.70 O 0.57 ] ∑2.00 , 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 = 1.631 A, two Ti-dominant sites, the [5]-coordinated M H site and [6]-coordinated M °(1) site, and two [6]-coordinated Na-dominant sites, M °(2) and M °(3). The M H site (= Ti 1.97 Al 0.03 pfu ) is coordinated by five O atoms, with H –O> = 1.914 A; the M °(1) site (= Ti 0.94 Fe 3+ 0.05 Nb 0.01 pfu ) is coordinated by four O atoms and two OH groups, with = 1.993 A; the M °(2) and M °(3) sites (= Na 1.00 and Na 1.09 Fe 2+ 0.30 Mn 2+ 0.28 Ca 0.08 Mg 0.05 □ 0.20 pfu , respectively) are coordinated by six O atoms with = 2.441 A, and four O atoms and two OH groups, with = 2.296 A, respectively. The M H square pyramid and (Si 2 O 7 ) 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 M H square pyramids and (Si 2 O 7 ) groups with M°(1–3) octahedra. The second or intermediate I block includes the [10]-coordinated A P site (= Ba 0.88 K 0.61 Sr 0.33 Ca 0.05 □ 0.13 pfu ) with A P –O> = 2.843 A. 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) 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 3 Ti 3 (Si 2 O 7 ) 2 O 2 (OH) 2 , and nabalamprophyllite, Ba Na Na 3 Ti 3 (Si 2 O 7 ) 2 O 2 (OH) 2 . We explain why, for lamprophyllite, barytolamprophyllite and nabalamprophyllite, the chemical composition of the I block is of the form (A 2+ A + ) with a total charge of 3+, and the ideal formula of the minerals is (A 2+ A + ) Na 3 Ti 3 (Si 2 O 7 ) 2 O 2 (OH) 2 , where A 2+ = Sr 2+ , Ba 2+ , Ca 2+ ; A + = Na + , K + .