X-ray powder refinement of a natural garnet from Diamantina, Minas Gerais, Brazil

The chemical composition, [Mg_0.015 (2),Al_1.20 (2),Si_2.99 (1),Ca_3.00 (2),Ti_0.027 (3),Mn_0.017 (3), Fe_0.75 (3)]O₁₂ (magnesium alumin­ium silicon calcium titanium manganese iron dodeca­oxide), of a natural garnet collected at Diamantina, Minas Gerais, Brazil was determined by electron microprobe analysis. The structure refinement from X-ray powder data is insensitive to the small amounts of Ti, Mn and Mg. The determined cation distribution using the Rietveld method is {Ca₃²⁺}[Al_1.338 (8)³⁺,Fe_0.662 (8)³⁺](Si₃⁴⁺)O₁₂^2-, which is in agreement with a solid solution of the grossular-andradite series. The arrangement of the cations around the O atom is best described as a highly distorted tetrahedral.