Refined structure of afwillite from the northern Baikal region
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CTURE OF INORGANIC COMPOUNDS
Refined Structure of Afwillite from the Northern Baikal Region R. K. Rastsvetaevaa, N. V. Chukanovb, and A. E. Zadovc a Shubnikov
Institute of Crystallography, Russian Academy of Sciences, Leninskiœ pr. 59, Moscow, 119333 Russia e-mail: [email protected] b Institute of Problems of Chemical Physics in Chernogolovka, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia c NPO Neokhim, Dmitrovskoe sh. 71, Moscow, 127238, Russia Received September 10, 2007
Abstract—The crystal structure of the mineral afwillite [Ca12(H2O)8][SiO4]4[SiO2(OH)2]4 from the northern Baikal region was refined in a triclinic unit cell with the following parameters: a = 16.330(2) Å, b = 5.6389(6) Å, c = 11.685(1) Å, α = 90.08(1)°, β = 126.446(2)°, γ = 89.95(1)°, and sp. gr. P1. The triclinic unit cell is related to the monoclinic unit cell determined earlier (sp. gr. Cc, a = 16.278(1) Å, b = 5.6321(4) Å, c = 13.236(1) Å, and β = 134.898(3)°) by the transformation matrix [–100/010/101]. The structural model was determined using the phase-correction procedure and refined anisotropically to R = 2.8% based on 3270 independent reflections with F > 3σ(F). The crystal-chemical formula of afwillite was revised. The characteristic features of the IR spectrum of afwillite were explained based on the structural data. PACS numbers: 91.60.Ed DOI: 10.1134/S1063774509030080
The low-temperature calcium hydrosilicate afwillite was discovered in 1923 in Kimberley (South Africa) and was described as a new mineral with the formula 3ë‡é · 2SiO2 · 3ç2é [1]. More recently, this mineral has been studied by X-ray diffraction [2]. This study showed that the mineral contains OH– and H2O groups and has the formula ë‡3(SiO3OH)2 · 2H2O. The structure refinement [3] based on the same experimental data set revealed the presence of H atoms, but their positions were not determined because of the low quality of the X-ray data. In a later study of the mineral based on high-quality X-ray diffraction data, all six hydrogen atoms were located in difference-electrondensity maps and the hydrogen-bond system was revealed [4]. The structure of the mineral was studied in a monoclinic system with the unit-cell parameters a = 16.278(1) Å, b = 5.6321(4) Å, c = 13.236(1) Å, β = 134.898(3)°, and sp. gr. Cc. We decided to reinvestigate the structure of afwillite, because the IR spectrum of this mineral cannot be explained based on the hydrogen-bond system revealed in [4]. In the IR spectrum of afwillite (Fig. 1), the main maximum of the split band of Si–O stretching vibrations is observed at 911 cm–1, which corresponds to typical orthosilicates (or neosilicates, i.e., silicates consisting of isolated SiO4 tetrahedra). The hydrogen-containing groups show a strong acidic character as is evidenced by a series of bands in the 1200–3200-cm–1 region. The positions of some of these bands (at 3150, 2750, 1745, and 1095 cm–1) coincide with the theoreti-
cally calculated characteristic modes of the ç3é+ group [5]. For many years these bands were assig
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