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THE CRYSTAL CHEMISTRY OF SHCHERBAKOVITE FROM THE KHIBINA MASSIF, KOLA PENINSULA, RUSSIA

Yulia A. Uvarova^1,, Elena Sokolova¹, Frank C. Hawthorne¹, Ruslan P. Liferovich² and Roger H. Mitchell²

¹ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
² Department of Geology, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada

E-mail address: umuvarov{at}cc.umanitoba.ca

The crystal structure of shcherbakovite from Mount Rasvumchorr, Khibina massif, Kola Peninsula, Russia, ideally K₂ Na Ti₂⁴⁺ O (OH) [Si₄O₁₂], a 8.1538(4), b 10.5569(5), c 13.9882(6) Å, V 1204.1(2) ų, space group Imma, Z = 4, D_calc 3.194 g/cm³, has been refined to R₁ = 3.2% for 960 unique (F_o > 4F) reflections collected on a Bruker single-crystal P4 diffractometer equipped with a CCD detector and MoK X-radiation. Shcherbakovite occurs in late (hydrothermal) shallowly dipping veins of natrolite and is associated with natrolite, aegirine, K-feldspar, strontian apatite, titanite, spherulitic baryllite and rare pyrrhotite and chalcopyrite. Electron-microprobe analysis gave SiO₂ 40.57, TiO₂ 18.87, Fe₂O₃ 1.05, MnO 0.06, BaO 6.7, CaO 0.20, K₂O 13.22, Na₂O 5.59, Nb₂O₅ 10.49, SrO 0.08, ZrO₂ 0.84, Ta₂O₅ 0.06, (H₂O)_calc 0.73, sum 97.83 wt.%. The amount of H₂O was determined from crystal-structure analysis. There is one tetrahedrally coordinated T site, <T–O> 1.607 Å, occupied by Si. There are two octahedrally coordinated sites, M(1), occupied by (Ti_0.68 Nb_{0.32 1.00}), with <M(1)–O> = 2.016 Å, and M(2), occupied by (Ti_0.72 Nb_0.15 Fe_0.08³⁺ Zr_{0.04 1.01}), with <M(2)–O> = 1.999 Å. The M(1) and M(2) sites are separated by 0.477 Å, and hence cannot be occupied simultaneously at a local level. There are three interstitial A sites: the A(1) site is [9]-coordinated and is occupied by (K_0.66 Ba_0.23 Na_0.07 Ca_0.02), with <A(1)–O> = 2.952 Å; the A(2) site is [8]-coordinated and is occupied by K, with <A(2)–O> = 2.861 Å, and the A(3) site is [6]-coordinated and is occupied by Na, with <A(3)–O> = 2.493 Å. Shcherbakovite is a K-analogue of batisite, ideally Ba Na₂ Ti₂⁴⁺ O₂ [Si₄O₁₂]. Shcherbakovite is related to batisite by substitution of K for Ba and K for Na. "Noonkanbahite", ideally Ba K Na Ti₂⁴⁺ O₂ [Si₄O₁₂], has never been approved by the IMA as a new mineral species. Based on the crystal chemistry of this structure type, the general formula of these minerals may be written as A B C M_{2 2} [Si₄O₁₂], with the following end-member compositions: batisite A = Ba, B = Na, C = Na; shcherbakovite, A = K, B = K, C = Na; "noonkanbahite", A = Ba, B = K, C = Na; unnamed, A = K, B = Na, C = Na.

Keywords: shcherbakovite, batisite, end-member, crystal-structure refinement.

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