SINGLE-CRYSTAL IR SPECTROSCOPY OF GROSSULAR-ANDRADITE GARNETS

Fe3+-Al3+ Substitution in, the octahedral site of 14 natural garnet sa mples was investigated through measurement of complete single-crystal infrared (IR) reflectance spectra across the grossular-andradite [Ca-3 (AlxFe1-x)(2)Si3O12] binary. Frequencies of all 17 IR-active fundament al 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 invol ving Al and Fe3+ and of much larger intensities for bands associated w ith Fe3+ suggests that the Fe-O bond has greater strength, which compe nsates for the greater mass of Fe. The nearly linear behavior of the m odes indicates that mixing should approach ideality for this series, a s was previously observed in thermodynamic studies. The existence of i deal mixing suggests that high degrees of ordering are not present, an d thus that ordering might not cause the anomalous birefringence assoc iated with this series.