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

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) = alpha(a* + b*) and q(2) = alpha(-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)O-7 has been studied by X-ray single-crystal diffraction methods in the temperature range 298-100 K. The value of the a 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.