Fe3+-Al3+ substitution in the octahedral site of 14 natural garnet samples was investigated through measurement of complete single-crystal infrared (IR) reflectance spectra across the grossular-andradite [Ca3(AlxFe1−x)2Si3O12] binary. Frequencies of all 17 IR-active fundamental modes depend nearly linearly on composition. Two peaks assigned to translations of the octahedral cations against the O framework exhibit two-mode behavior. Observation of similar frequencies for bands involving Al and Fe3+ and of much larger intensities for bands associated with Fe3+ suggests that the Fe-O bond has greater strength, which compensates for the greater mass of Fe. The nearly linear behavior of the modes indicates that mixing should approach ideality for this series, as was previously observed in thermodynamic studies. The existence of ideal mixing suggests that high degrees of ordering are not present, and thus that ordering might not cause the anomalous birefringence associated with this series.

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