Complete substitution of Ca by Sr has been experimentally obtained at 750°C, 1 kbar in hydroxyl potassium-richterite. The refinement of the site occupancies by the whole-powder-pattern refinement method (Rietveld method) shows Sr to be completely ordered at the M(4) site. Cell parameters vary as a function of the Sr → Ca substitution in a strongly anisotropic way: a and b increase whereas c remains constant. There is also a major increase in the β angle. The variation of distance is linear. The structure refinements are good in the space group C2/m with the usual amphibole unit cell; in addition, there is no evidence for the existence of a supercell. Thus Ca and Sr show no evidence of any long-range ordering
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Ventura et al. (1993) studied this question.
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