A raspberry-red grossular from the Sierra de Cruces Range in the state of Coahuila in Mexico has been investigated. Electron-microprobe analysis and ^(57)Fe Mossbauer and UVYVIS spectroscopy were used in order to investigate the nature of the unusual red colour. The composition of the garnet is approximately (Ca_(2.94)Mg_(0.09)Mn^(2+)_(0.10))_(3.13)(Al_(1.90)Fe^(3+) _(0.05)Mn^(3+)_(0.01))_(1.96)Si_(2.97)O_(12). The UV/VIS spectrum is characterised by an absorption envelope centred around 19,000 cm1 in the green region, which can be fitted with two bands located at 19,503 and 17,302 cm^(-1), originating from two spin-allowed dd-electron transitions. They result from the presence of Mn^(3+) on the octahedral site of point symmetry 3.
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Geiger et al. (1999) studied this question.