Abstract
As is usual for peculiar chemical compositions, melilite-type compounds exhibit a two-dimensional incommensurately modulated structure which can be described with two wave vectors: q 1 =α(a* + b*) and q 2 =α(−a* + b*), where a* and b* are the tetragonal reciprocal axes of the basic cell. The low-temperature dependence of the modulation wave vector of a natural melilite crystal with chemical composition (Ca1.89Sr0.01 Na0.08K0.02)(Mg0.92Al0.08)(Si1.97Al0.03)O7 has been studied by X-ray single-crystal diffraction methods in the temperature range 298–100 K. The value of the α coefficient shows a continuous linear increase, ranging from 0.281(1) at 298 K to 0.299(1) at 100 K. No plateau-like temperature dependence was observed throughout the temperature studied, thus indicating that no independent phase with a specific q stabilizes in this natural crystal. A comparison with the low-temperature behaviour of synthetic Ca2MgSi2O7 is given.
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Acknowledgments.
The authors wish to thank M.Ohmasa (Dept. of Life Sciences, Himeji Institute of Technology, Kamigori, Ako-gun, Hyogo, Japan) for providing unpublished data. The paper benefited from the careful reviews by Andrew Putnis and two anonymous referees. The study was financially supported by CNR (Istituto di Geoscienze e Georisorse – sez. Firenze), and by MURST, CoFin 2001, project Structural complexity and properties of minerals: microstructures, modularities, modulations.
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Bindi, L., Bonazzi, P. Low-temperature study of natural melilite (Ca1.89Sr0.01Na0.08K0.02) (Mg0.92Al0.08)(Si1.97Al0.03)O7: towards a commensurate value of the q vector. Phys Chem Minerals 30, 523–526 (2003). https://doi.org/10.1007/s00269-003-0346-y
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DOI: https://doi.org/10.1007/s00269-003-0346-y