Single-crystal absorption and reflection infrared spectroscopy of birefringent grossular-andradite garnets

Reduction in symmetry of optically anomalous garnets from Ia3̄d to I1̄ or Fddd by cation ordering implies that a large number of vibrations should be infrared active. Yet, only the pattern characteristic of isotropic isometric garnets was observed in single-crystal polarized and unpolarized IR reflectance spectra from 80 to 4000 cm^-1 of four birefringent garnets. These samples are nearly identical to those whose structures have been crystallographically refined. A single-crystal IR absorption spectrum that resolves the entire fundamental region is presented and it is found that this compares closely with data from reflection measurements. The contrasting results from spectroscopy and crystallography may indicate that strain causes anomalous optical anisotropy in garnet in that X-ray diffraction requires juxtaposition of ~1000 unit cells, whereas lattice vibrations can be produced by isolated molecules. -from Authors