New Data on the Isomorphism in Eudialyte-Group Minerals. VIII: Crystal Structure of the Fe 2+ -Analog of Feklichevite, a
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CTURE OF INORGANIC COMPOUNDS
New Data on the Isomorphism in Eudialyte-Group Minerals. VIII: Crystal Structure of the Fe2+-Analog of Feklichevite, a Potentially New Mineral from the Khibiny Massif R. K. Rastsvetaevaa,*, N. V. Chukanovb, L. V. Sipavinac, and M. V. Voroninc a Shubnikov
Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia b Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia c Korzhinskii Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia *e-mail: [email protected] Received April 3, 2020; revised April 8, 2020; accepted April 8, 2020
Abstract—A potentially new calcium-rich eudialyte-group mineral from the Khibiny massif has been investigated by X-ray diffraction (XRD) analysis. The trigonal unit-cell parameters are found to be as follows: a = 14.221(1) Å, c = 30.04(1) Å, and V = 5261.3 Å3; sp. gr. R3m. The crystal structure is refined to the final reliability factor R = 2.8% in the anisotropic approximation of atomic displacements, using 1714 reflections with F > 3σ(F). The idealized formula of the mineral (Z = 3) is (Na12Ca3)Ca6Fe32 + Zr3NbSi25O75(H2O,OH,F,Cl)3. The dominance of divalent iron form is confirmed by Mössbauer spectroscopy data. The closest analog of the mineral under study is feklichevite from the Kovdor alkaline massif; the main difference is the dominance of divalent iron over trivalent iron in the former. DOI: 10.1134/S1063774520050181
INTRODUCTION Eudialyte-group minerals are abundant in alkaline massifs (in particular, on the Kola peninsula); their characteristic feature is extremely complex composition. They are considered as potential raw materials for zirconium and some others rare (including rare-earth) elements, in view of which studies on eudialyte-concentrate leaching are under way [1, 2]. The eudialyte group consists of ring zirconium and titanium silicates with the trigonal symmetry (R3m, R3 m [увеличить цифру 3]); it includes more than 30 mineral specimens [3]. The general formula of these minerals is [N1N2N3N4N5]3M16M23M3M4Z3(Si9O27)2(Si3O9)2Ø4–6X1X2, where letters denote key structural sites, containing various statistically distributed components; 18 components (specifically, Si, Na, K, Ca, Sr, Fe2+, Fe3+, Mn, Ti, Zr, Nb, W, Сe, H, C, O, F, and Cl) may dominate in particular sites [4]. The microporous structures of the eudialyte-group minerals are based on heteropolyhedral frameworks, including three- and nine-membered silicon–oxygen tetrahedra ([Si3O9] and [Si9O27], respectively) and six-membered rings of M1О6 octahedra with shared edges. All rings are connected by discrete ZО6 octahedra, which are
most often occupied by zirconium, and additionally by the M2On polyhedron (n = 4–7; M = Fe2+, Fe3+, Mn2+, Na, Zr). Other sites are located in this unique framework. The M1 site is mainly occupied by cations of Ca, Mn2+, rare-earth elements, Na, Sr, and Fe2+
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