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

• Published in: The Canadian Mineralogist
• Main Authors: F. Cámara, E. Sokolova, Y. Abdu, F. C. Hawthorne, A. P. Khomyakov
• Other Authors: F.C. Hawthorne, A.P. Khomyakov
• Format: Article in Journal/Newspaper
• Language: English
• Published: Mineralogical Association of Canada 2013
• Subjects: Crystal structure; Group IV; Khibiny alkaline massif; Kola peninsula; Kolskyite; New mineral; Russia; Ti-disilicate; TS block; Settore GEO/06 - Mineralogia; Khibiny; Kola Peninsula; Kukisvumchorr; kola peninsula
• Online Access: http://hdl.handle.net/2434/477092; https://doi.org/10.3749/canmin.51.6.921

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 ...