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□TiNbNa3Ti(Si2O7)2O2(OH)2(H2O)4, is a Group-III 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 μm thick and up to 330 μm across. It is colourless to pale tan, with a colourless 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/cm3. Kazanskyite is biaxial positive with α 1.695, β 1.703, γ 1.733 (λ 590 nm), 2Vmeas. = 64.8(7)°, 2Vcalc. = 55.4°, with no discernible dispersion. It is nonpleochroic. Kazanskyite is triclinic, space group P⎯1, a 5.4260(9), b 7.135(1), c 25.514(4) Å, α 90.172(4), β 90.916(4), γ 89.964(3)°, V 977.61(3) Å3. The strongest lines in the X-ray powder-diffraction pattern [d(Å)(I)(hkl)] are: 2.813(100)(12-4,1-2-2), 2.149(82)(22-2,2-20,207,220,2-22), 3.938(70)(1-13,112), 4.288(44)(11-1,1-10,110,1-11), 2.128(44)(22-3,2-2-1,1-34,221,2-23), 3.127(39)(1-16,115), 3.690(36)(1-14), 2.895(33)(1-23,121) and 2.955(32)(1-20,120,1-22). 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 (Na2.55Mn0.31Ca0.11Fe2+0.03)Σ3 (Ba0.70Sr0.28K0.21Ca0.03)Σ1.22(Ti2.09Nb0.63Mn0.26Al0.02)Σ3 Si4.05O21.42H9.45F0.59, calculated on 22 (O + F) a.p.f.u., Z = 2. The structural formula of the form AP2MH2MO4(Si2O7)2XO4XP MXPA(H2O)n is (Ba0.56Sr0.22K0.15Ca0.03□0.04)Σ1 (□0.74Ba0.14Sr0.06K0.06)Σ1 (Ti0.98Al0.02)Σ1(Nb0.63Ti0.37)Σ1 (Na2.55Mn0.31Ca0.11Fe2+0.03)Σ3(Ti0.74Mn0.26)Σ1(Si2O7)2O2(OH1.41F0.59)Σ2(H2O)(□0.74H2O0.26)Σ1(H2O)2.74. Simplified and ideal formulae are as follows: Ba(□,Ba)Ti(Nb,Ti)(Na,Mn)3(Ti,Mn)(Si2O7)2O2 (OH,F)2(H2O)4 and Ba□TiNbNa3Ti(Si2O7)2O2 (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 R1 index of 8.09 %. The crystal structure of kazanskyite is a combination of a TS (titanium silicate) block and an I (intermediate) block. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block exhibits linkage and stereochemistry typical for Group III (Ti = 3 a.p.f.u.) of 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, AP(1,2), occupied mainly by Ba (less Sr and K) at 96 and 26%. There are two I blocks: the I1 block is a layer of Ba atoms; the I2 block consists of H2O groups and AP(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 Ivanovich Kazansky (Вадим Иванович Казанский), a prominent Russian ore geologist and an expert in Precambrian metallogeny.