FROM STRUCTURE TOPOLOGY TO CHEMICAL COMPOSITION. IV. TITANIUM SILICATES: THE ORTHORHOMBIC POLYTYPE OF NABALAMPROPHYLLITE FROM THE LOVOZERO MASSIF, KOLA PENINSULA, RUSSIA

Abstract

The crystal structure of nabalamprophyllite-2 O , an orthorhombic polytype of nabalamprophyllite, ideally (BaNa) Na 3 Ti 3 (Si 2 O 7 ) 2 O 2 (OH) 2 , a 19.564(2), b 7.1173(5), c 5.4144(4) A, V 753.90(4) A 3 , space group Pnmn , Z = 2, D calc. 3.410 g.cm −3 , from the Yubileinaya vein, Karnasurt Mountain, Lovozero alkaline massif, Kola Peninsula, Russia, has been refined to a R 1 of 5.1% on the basis of 669 unique reflections ( F o > 4σ F ), collected on a Bruker P 4 diffractometer with a CCD 4K Smart detector and Mo K α radiation. An electron-microprobe analysis gave (wt.%): SiO 2 29.79, Al 2 O 3 0.16, Nb 2 O 5 0.71, TiO 2 27.85, Fe 2 O 3 1.05, MnO 3.52, MgO 0.20, CaO 0.15, SrO 6.09, BaO 16.55, K 2 O 1.14, Na 2 O 10.89, F 1.53, H 2 O 0.99 (calculated from structure refinement), O=F −0.64, total 99.98. The empirical formula is (Ba 0.87 Sr 0.47 Na 0.28 K 0.20 Ca 0.02 □ 0.16 ) ∑2 (Na 2.56 Mn 0.40 2+ Mg 0.04 ) ∑3 (Ti 2.81 Fe 0.11 3+ Nb 0.04 Al 0.03 ) ∑3 (Si 2 O 7 ) 2 [(OH) 0.89 F 0.65 O 0.46 ] ∑2 , Z = 2, calculated on the basis of 4 Si apfu . The crystal structure of nabalamprophyllite-2 O can be described as a combination of a TS block and an I block. The TS (titanium silicate) block consists of HOH sheets (H: heteropolyhedral, O: octahedral), and occurs in 24 Ti disilicate minerals. In the TS block, there are five 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 O (1) site, and two [6]-coordinated Na-dominant sites, M O (2) and M O (3). The M H site (Ti 1.97 Al 0.03 apfu ) is coordinated by five O atoms, with H –O> = 1.91 A ; the M O (1) site (Ti 0.85 Fe 0.11 3+ Nb 0.04 apfu ) is coordinated by four O atoms and two OH groups, with O (1)–O,OH> = 2.00 A ; the M O (2) and M O (3) sites (Na 1.00 and Na 1.56 Mn 0.40 2+ Ca 0.08 Mg 0.04 apfu , respectively) are coordinated by six O atoms with O (2)–O> = 2.44 A and four O atoms and two OH groups, with O (3)–O,OH> = 2.30 A, respectively. The M H square pyramids and (Si 2 O 7 ) groups constitute the H sheet. The M O (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 O (1–3) octahedra. The I block includes the [10]-coordinated A P sites (Ba 0.87 Sr 0.47 Na 0.28 K 0.20 Ca 0.02 □ 0.16 apfu ) with A P –O> = 2.82 A. The TS block and the I block [which ideally is a layer of (BaNa) atoms] alternate along a . The ideal formula of nabalamprophyllite-2 O is (BaNa) Na 3 Ti 3 (Si 2 O 7 ) 2 O 2 (OH) 2 , in accord with the general formula of nabalamprophyllite-2 M , Ba Na Na 3 Ti 3 (Si 2 O 7 ) 2 O 2 (OH) 2 , space group P 2/ m , previously known as nabalamprophyllite. The structure topology of nabalamprophyllite-2 O differs from that of nabalamprophyllite-2 M in (1) disorder/order of the A P cations in the I block, and (2) different stacking order of TS blocks. In nabalamprophyllite-2 O , Ba and Na atoms are disordered at one A P site; in nabalamprophyllite-2 M , Ba and Na atoms are dominant each at one of two A P sites. The difference in stacking order of TS blocks in nabalamprophyllite polytypes is identical to that in lamprophyllite-2 O and lamprophyllite-2 M . In Ti disilicates with the TS block, hainite, (Ca 3 REE) Na (NaCa)Ti (Si 2 O 7 ) 2 OF 3 , and rinkite, (Ca 3 REE) Na (NaCa)Ti (Si 2 O 7 ) 2 OF 3 , can be considered as polytypes.