Kazanskyite, Ba & TiNbNa 3Ti(Si 2O 7) 2O 2(OH) 2(H 2O) 4, a Group-III Ti-disilicate mineral from the Khibiny alkaline massif, Kola Peninsula, Russia : description and crystal structure

Kazanskyite, Ba&TiNbNa 3Ti(Si 2O 7) 2O 2(OH) 2(H 2O) 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 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 α 1.695, β 1.703, γ 1.733 (λ590 nm), 2V meas = 64.8(7)°, 2V calc = 55.4°, with no discernible dispersion. It is not pleochroic. Kazanskyite is triclinic, space group P1, 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) Å 3. The strongest lines in the X-ray powder-diffraction pattern [d(Å)(I)(hkl)] are: 2.813(100)(124,122), 2.149(82)(222,220,207,220,222), 3.938(70)(113,112), 4.288(44)(111,110,110,111), 2.128(44)(223,221,134,221,134,221,223), 3.127(39)(116,115), 3.690(36)(114), 2.895(33)(123,121) and 2.955(32)(120,120,122). Chemical analysis by electron microprobe gave Nb 2O 59.70, TiO 219.41, SiO 228.21, Al 2O 30.13, FeO 0.28, MnO 4.65, BaO 12.50, SrO 3.41, CaO 0.89, K 2O 1.12, Na 2O 9.15, H 2O 9.87, F 1.29, O = F-0.54, sum 100.07 wt.%; H 2O was determined from structure refinement. The empirical formula is (Na 2.55Mn 0.31Ca 0.11Fe 2+0.03) Σ3(Ba 0.70Sr 0.28K 0.21Ca 0.03) Σ1.22(Ti 2.09Nb 0.63Mn 0.26Al 0.02) Σ3Si 4.05O 21.42H 9.45F 0.59, calculated on 22 (O + F) a.p.f.u., Z = 2. The structural formula of the form A P2M H2M O4(Si 2O 7) 2X O4X PMX PA(H 2O) n is (Ba 0.56Sr 0.22K 0.15Ca 0.03& 0.04) Σ1(& 0.74Ba 0.14Sr 0.06K 0.06) Σ1(Ti 0.98Al 0.0 2) Σ1(Nb 0.63Ti 0.37) Σ1(Na 2.55Mn 0.31Ca 0.11Fe 2+0.03) Σ3(Ti 0.74Mn 0.26) Σ1(Si 2O 7) 2O 2(OH 1.41F 0.59) Σ2(H 2O)(& 0.74H 2O 0.26) Σ1(H 2O) 2.74. Simplified and ideal formulae are as follows: Ba(&,Ba)Ti(Nb,Ti)(Na,Mn) 3(Ti,Mn)(Si 2O 7) 2O 2(OH,F) 2(H 2O) 4 and Ba & TiNbNa 3Ti(Si 2O 7) 2O 2(OH) 2(H 2O) 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 TS block consists of HOH sheets (H is heteropolyhedral and O is octahedral). The TS 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 (Si 2O 7) 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 H 2O 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 Ivanovich Kazansky, a prominent Russian ore geologist and an expert in Precambrian metallogeny.