Vibrational spectroscopy of pyrope-majorite garnets: Structural implications (original) (raw)

Infrared reflectance, thin-film absorbance, and polarized Raman spectra were acquired from ∼100 to ∼4000 cm -1 from polycrystalline garnets along the pyrope (Py = Mg3Al2Si3O12)-majorite (Mj = Mg3(MgSi)Si3O12) join. We measured Py100, Mj39, Mj45, Mj80, and natural single-crystal, nearly end- member pyrope. Consistency is obtained between measured absorption spectra and absorption coef- ficients calculated through Kramers-Kronig analysis of reflectivity ( R) data, if R-values are scaled to account for losses through scattering. The widths of the lattice modes double from each end-member toward the middle of the binary, whereas frequencies are nearly independent of composition. Strong polarization of Raman peaks of all our samples except pyrope shows that the space group is reduced from cubic, even for the Al-rich majorites. The largest number of IR or Raman peaks is observed for Mj80. The number of peaks for Al-rich majorites lies between those seen for cubic pyrope and I41/a majorite...