Variations in the chemical composition of lamprophyllite-group minerals and the crystal structure of fluorine-rich baryt
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Variations in the Chemical Composition of Lamprophyllite-Group Minerals and the Crystal Structure of Fluorine-Rich Barytolamprophyllite from New Peralkaline Dyke M. I. Akimenkoa, S. M. Aksenova, b, N. V. Sorokhtinaa, L. N. Kogarkoa, N. N. Kononkovaa, R. K. Rastsvetaevab, and K. A. Rozenbergb a Vernadsky
b
Institute of Geochemistry and Analytical Chemistry, Moscow, Russia Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia e-mail: [email protected] Received April 2, 2015
Abstract—The variations in the chemical composition of lamprophyllite-group minerals from a peralkaline dyke of the Mokhnatye Roga area (Kandalaksha region, Kola Peninsula), which are crystallized during the entire period of dyke formation and form several generations, have been investigated. The early generations differ in a steadily high fluorine content, while the later ones exhibit reduced amount of fluorine, impurity elements, and sodium, with a simultaneous increase in the potassium content. The crystal structure of fluorine-rich barytolamprophyllite (potentially a new representative of the lamprophyllite group, differing by the predominance of fluorine in the anion X site) has been analyzed by single crystal X-ray diffraction. This mineral is found to have a monoclinic unit cell with the following parameters: a = 19.5219(8) Å, b = 7.0915(2) Å, c = 5.3925(2) Å, β = 96.628(3)°, and sp. gr. C2/m. The structure is refined to R = 5.73% in the anisotropic approximation of the atomic displacement parameters using 3668I > 2σ(I). The idealized formula (Z = 2) is (Ba,Sr)2[Na(Na,Fe)2(Ti,Mg)F2][Ti2(Si2O7)2O2]. DOI: 10.1134/S1063774515060024
INTRODUCTION Lamprophyllite-group minerals (LGMs) are widely abundant in peralkaline rocks and their pegmatites. They have been found as rock-forming and accessory minerals in the massifs of the Kola alkaline province: Lovozero, Khibiny, Niva, Kovdor, etc. [1– 4]. LGMs are also widespread in other alkaline massifs of Russia, for example, in Inagli (southern Yakutia) and Botogol (northeast Pribaykal’e) [3, 5]. In addition, they have been found in the Gardiner (Greenland) and Pilanesberg (SAR) complexes, Bearpaw rocks (United States), alkaline rocks of the Ol Doinyo Lengai volcano (Tanzania) [3], and several tens of other geological objects. We should especially note the Eifel paleovolcanic area (Germany): three minerals of the aforementioned group, which had not been previously observed in other regions, were found in the alkaline basalts of this area [6]. Currently, the lamprophyllite group is assigned to the heterophyllosilicate family [6–10], the crystal structure of which is based on a three-layer HOH module. The inner O layer of this module consists of edge-sharing MО6 octahedra, while the outer heteropolyhedral H layers are formed by Si2O7 diorthogroups, linked by L five-vertex polyhedra. The
Si2O7 : L ratio is 1 : 1. Large A cations are located between three-layer modules. The general formula of lamprophyllite-group minera
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