Refinement of the Crystal Structure of Vlasovite from Burpala Massif (Russia)
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Refinement of the Crystal Structure of Vlasovite from Burpala Massif (Russia) E. V. Kanevaa,*, N. V. Vladykina, E. Mestob, M. Lacalamitab, F. Scordarib, and E. Schingarob a Vinogradov
Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk, 664033 Russia b University of Bari Aldo Moro, Bari 70125, Italy *e-mail: [email protected] Received April 3, 2017
Abstract—The structural model and chemical composition of vlasovite from the Burpala massif (Russia) have been determined using X-ray diffraction methods and electron probe microanalysis. The crystal structure has been refined in the sp. gr. Cc to R = 1.4% (Rw = 1.8%) with the following unit-cell parameters: a = 11.0396(3) Å, b = 10.1042(2) Å, c = 8.5696(2) Å, β = 100.307(1)°, V = 940.48(4) Å3, and Z = 4. DOI: 10.1134/S1063774518070106
INTRODUCTION Vlasovite (Na2Zr(Si4O11)) belongs to the structural chemical group of alkaline zirconosilicates, characterized by the presence of a three-dimensional heteropolyhedral (mixed) framework in the structure. The mineral was named in honor of the Russian mineralogist and geochemist K.A. Vlasov (1905–1964), a founder of the Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements in Moscow, who studied the Khibiny massif on the Kola Peninsula (Russia). This mineral generally forms irregularly shaped grains and aggregates, but sometimes is found in the form of transparent colorless or brownish crystals. In 1961, Tikhonenkova and Kazakov [1] described for the first time vlasovite as a new mineral from albitized eudialyte-microcline fenites and pegmatoid nepheline syenites of the exocontact zone of the Lovozero massif. The vlasovite crystal structure was determined for the first time on a crystal from the Lovozero massif [2, 3]. The vlasovite from the Lovozero massif was found to be monoclinic: it crystallizes into the sp. gr. C2/c (vlasovite-1M). In 1967, Fleet and Cann [4] found vlasovite crystals as an accessory mineral on the Ascensian Island in the Atlantic Ocean. This phase turned out to be triclinic with the sp. gr. P 1 vlasovite1A). It was also found to transform to the monoclinic phase after heating to 29°C. However, the rather large values of the refinement R factor (more than 10%) reported in [4] suggest that the structure was not accurately and/or completely modeled. Large thermal displacements of sodium atoms occupying the Na(2) site were noted in [3]; the transition from the monoclinic
to triclinic phase was explained by possible ordering of sodium in framework voids. The vlasovite from the Kipawa alkaline complex in Quebec (Canada) [5] was described as the primary phase in syenites. In 2003, Gobechiya et al. [6] refined the structure of a vlasovite sample from the Kipawa alkaline complex. Its structure was solved in the sp. gr. C2/c; however, there was a discrepancy between the sodium contents obtained by refining the site occupancy and the concentration determined by electron-probe analysis. According to the structural refinement, vla
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