Kazanskyite, Ba□TiNbNa3Ti(Si2O7)2O2(OH)2(H2O)4, a Group-III Ti-disilicate mineral from the Khibiny alkaline massif, Kola Peninsula, Russia: description and crystal structure

Kazanskyite, Ba square TiNbNa3Ti(Si2O7)(2)O-2(OH)(2)(H2O)(4), is a Group-111 TS-block mineral from the Kirovskii mine, Mount Kukisvumchorr, Khibiny alkaline massif, Kola Peninsula, Russia. The mineral occurs as flexible and commonly bent flakes 2-15 pm thick and up to 330 pm across. It is colourless to pale tan, with a white streak and a vitreous lustre. The mineral formed in a pegmatite as a result of hydrothermal activity. Associated minerals are natrolite, barytolamprophyllite, nechelyustovite, hydroxylapatite, belovite-(La), belovite-(Ce), gaidonnayite, nenadkevichite, epididymite, apophyllite-(KF) and sphalerite. Kazanskyite has perfect cleavage on {001}, splintery fracture and a Mohs hardness of 3. Its calculated density is 2.930 g cm(-3). Kazanskyite is biaxial positive with alpha 1.695, beta 1.703, gamma 1.733 (gamma 590 nm), 2V(meas) = 64.8(7)degrees, 2V(calc) = 55.40 degrees, with no discernible dispersion. It is not pleochroic. Kazanskyite is triclinic, space group P (1) over bar, a 5.4260(9), b 7.135(1), c 25.514(4) angstrom, alpha 90.172(4), beta 90.916(4), gamma 89.964(3)degrees, V 977.61(3) angstrom(3). The strongest lines in the X-ray powder-diffraction pattern [d(angstrom)(I)(hkl)] are: 2.813(100)(12 (4) over bar ,1 (2) over bar(2) over bar), 2.149(82)(22 (2) over bar ,2 (2) over bar0,207,220,2 (2) over bar2), 3.938(70)(1 (1) over bar3,112), 4.288(11 (1) over bar, 1 (1) over bar0,110,1 (1) over bar1), 2.128(44)(22 (3) over bar ,2 (2) over bar(1) over bar, 1 (3) over bar4,221,1 (3) over bar4,221,2 (2) over bar3), 3.127(39)(1 (1) over bar6,115), 3.690(36)(1 (1) over bar4), 2.895(33)(1 (2) over bar3,121) and 2.955(32)(1 (2) over bar0,120,1 (2) over bar2). Chemical analysis by electron microprobe gave Nb2O5 9.70, TiO2 19.41, SiO2 28.21, Al2O3 0.13, FeO 0.28, MnO 4.65, BaO 12.50, SrO 3.41, CaO 0.89, K2O 1.12, Na2O 9.15, H2O 9.87, F 1.29, O = F -0.54, sum 100.07 wt.%; H2O was determined from structure refinement. The empirical formula is (Na2.55Mn0.31Ca0.11Fe0.032+)(Sigma 3)(Ba0.70Sr0.28K0.21Ca0.03)Sigma(1.22)(Ti2.09Nb0.63Mn0.26Al0.02)(Sigma 3) Si4.05O21.42H9.45F0.59, calculated on 22 (0 + F) a.p.f.u., Z = 2. The structural formula of the form A(2)(P)M(2)(H)M(4)(O)(Si2O7)(2)(X4XMXAP)-X-O-X-P(H2O)(n) is (Ba0.56Sr0.22K0.15Ca0.03 square(0.04))(Sigma 1)(Ti0.74Ba0.14Sr0.06K0.06)(Sigma 1) (Ti0.98Al0.02)(Sigma 1)(Nb0.63Ti0.37)(Sigma 1)(Na2.55Mn0.31Ca0.11Fe3)(Sigma 1)(Ti0.74Mn0.26)(Sigma 1)(Si2O7)(2)O-2 (OH1.41F0.59)(Sigma 2)(H2O)(square 0.74H2O0.26)(Sigma 1)(H2O)(2.74). Simplified and ideal formulae are as follows: Ba(square,Ba)Ti(Nb,Ti)(Na,Mn)(3)(Ti,Mn)(Si2O7)(2)O-2(OH,F)(2)(H2O)(4) and Ba square TiNbNa3Ti (Si2O7)(2)O-2(OH)(2)(H2O)(4). The Raman spectrum of the mineral contains the following bands: 3462 cm(-1) (broad) and 3545 and 3628 cm(-1) (sharp). The crystal structure was solved by direct methods and refined to an R-1 index of 8.09%. The crystal structure of kazanskyite is a combination of a TS (titanium silicate) block and an I (intermediate) block. The IS block consists of HOH sheets (H is heteropolyhedral and O is octahedral). The IS block exhibits linkage and stereochemistry typical for Group-III (Ti = 3 a.p.f.u.) Ti-disilicate minerals. The TS block has two different H sheets where (Si2O7) groups link to [5]-coordinated Ti and [6]-coordinated Nb polyhedra, respectively. There are two peripheral sites, A(P)(1,2), occupied mainly by Ba (less Sr and K) at 96% and 26%. There are two I blocks: the I-1 block is a layer of Ba atoms; the I-2 block consists of H2O groups and A(P)(2) atoms. The TS and I blocks are topologically identical to those in the nechelyustovite structure. The mineral is named in honour of Professor Vadim lvanovich Kazansky (sic), a prominent Russian ore geologist and an expert in Precambrian metallogeny.