Kolskyite, (Ca□)Na2Ti4(Si2O7)2O4(H2O)7, a group-IV Ti-disilicate mineral from the Khibiny alkaline massif, Kola Peninsula, Russia : description and crystal structure

Kolskyite, (Ca□)Na2Ti4(Si2O7)2O4(H2O)7, is a Group-IV TS-block mineral from the Kirovskii mine, Mount Kukisvumchorr, Khibiny alkaline massif, Kola Peninsula, Russia. The mineral occurs as single, platy crystals 2-40 μm thick and up to 500 μm across. It is pinkish yellow, with a white streak and a vitreous luster. The mineral formed in a pegmatite as a result of hydrothermal activity. Associated minerals are natrolite, nechelyustovite, kazanskyite, barytolamprophyllite, hydroxylapatite, belovite-(La), belovite-(Ce), gaidonnayite, nenadkevichite, epididymite, apophyllite-(KF), and sphalerite. Kolskyite has perfect cleavage on 001, splintery fracture, and a Mohs hardness of 3. Its calculated density is 2.509 g/cm3. Kolskyite is biaxial negative with α 1.669, β 1.701, γ 1.720 (λ 590 nm), 2Vmeas. = 73.6(5)° , 2Vcalc. = 74.0° , with no discernible dispersion. It is nonpleochroic. Kolskyite is triclinic, space group P1¯, a 5.387(1), b 7.091(1), c 15.473(3) Å, α 96.580(4), β 93.948(4), γ 89.818(3)° , V 585.8(3) Å3. The strongest lines in the X-ray powder-diffraction pattern [d(Å)(I)(hkl)] are: 15.161(100)(001), 2.810(19)(121, 12¯2), 3.069(12) (005), 2.938(10)((1¯2¯1,120, 12¯1), 2.680(9)((1¯2¯3 ,200,114, 2¯01), 1.771(9) (04¯1,040), 2.618(8)(12¯3,122), 2.062(7)(221,22¯2,2¯2¯3,2¯22), and 1.600(7)(3¯2¯1,3¯20,320). Chemical analysis by electron microprobe gave Nb2O5 6.96, ZrO2 0.12, TiO2 26.38, SiO2 27.08, FeO 0.83, MnO 2.95, MgO 0.76, BaO 3.20, SrO 5.21, CaO 4.41, K2O 0.79, Na2O 6.75, H2O 13.81, F 0.70, O = F -0.29, sum 99.66 wt.%; H2O was determined from structure solution and refinement. The empirical formula was calculated on 25 (O + F) apfu: (Na1.93 Mn0.04Ca0.03)Σ2(Ca0.67Sr0.45Ba 0.19K0.15)Σ1.46(Ti2.93Nb0.46Mn0.33Mg0.17Fe2+0.10Zr0.01)Σ4Si4.00O24.67H13.60F0.33, Z = 1. Simplified and ideal formulae are as follows: (Ca,□)2Na2Ti4(Si2O7)2O4(H2O)7 and (Ca□)Na2Ti4(Si2O7)2O4(H2O)7. The FTIR spectrum of the mineral contains the following bands: ∼3300 cm-1 (very broad) and ∼1600 cm-1 (sharp). The crystal structure was solved by direct methods and refined to an R1 index of 8.8%. The crystal structure of kolskyite 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 IV [Ti (+ Mg + Mn) = 4 apfu] of Ti-disilicate minerals. In the H sheet in kolskyite, Si2O7 groups link to [6]-coordinated Ti octahedra. In the O sheet, Ti-dominant and Na octahedra each form brookite-like chains. There is one peripheral AP site occupied mainly by Ca (less Sr, Ba, and K) at 68%. The I block consists of H2O groups and AP atoms. The I block is topologically identical to those in the kazanskyite and nechelyustovite structures. The mineral is named after the Kola Peninsula (Kolskyi Poluostrov in Russian). The chemical formula and structure of kolskyite were predicted by Sokolova & Cámara (2010); this is the first correct prediction of a TS-block mineral.