Bp. Mcaloon et Am. Hofmeister, SINGLE-CRYSTAL IR SPECTROSCOPY OF GROSSULAR-ANDRADITE GARNETS, The American mineralogist, 80(11-12), 1995, pp. 1145-1156
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.