Crystal structure of low-symmetry rondorfite |
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Authors: | R K Rastsvetaeva A E Zadov N V Chukanov |
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Institution: | (1) Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiĭ pr. 59, Moscow, 119333, Russia;(2) NPO Neokhim, Dmitrovskoe sh. 71, Moscow, 127238, Russia;(3) Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432, Russia |
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Abstract: | The crystal structure of an aluminum-rich variety of the mineral rondorfite with the composition Ca16Mg2(Si7Al)(O31OH)]Cl4 from the skarns of the Verkhne-Chegemskoe plateau (the Kabardino-Balkarian Republic, the Northern Caucasus Region, Russia) was solved in the triclinic space group with the unit-cell parameters a = 15.100(2) Å, b = 15.110(2) Å, c = 15.092(2) Å, α = 90.06(1)°, β = 90.01(1)°, γ = 89.93(1)°, Z = 4, sp. gr. P1. The structural model consisting of 248 independent atoms was determined by the phase-correction method and refined to R = 3.8% with anisotropic displacement parameters based on all 7156 independent reflections with 7156 F > 3σ(F). The crystal structure is based on pentamers consisting of four Si tetrahedra linked by the central Mg tetrahedron. The structure can formally be refined in the cubic space group (a = 15.105 Å, sp. gr. Fd \(\overline 3 \), seven independent positions) with anisotropic displacement parameters to R = 2.74% based on 579 reflections with F > 3σ(F) without accounting for more than 1000 observed reflections, which are inconsistent with the cubic symmetry of the crystal structure. |
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