SINGLE-CRYSTAL ABSORPTION AND REFLECTION INFRARED-SPECTROSCOPY OF BIREFRINGENT GROSSULAR-ANDRADITE GARNETS

Reduction in symmetry of optically anomalous garnets from Ia3dBAR to I1BAR or Fddd by cation ordering, as indicated by crystallography, implies that a large number of vibrations (129 or 98) should be infrared (IR) 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 (Gr90An6Al4, Gr99An1, An94Gr5Sp1, and Gr60.8An36.5Sp1.3Al1Py0.3). These samples are nearly identical to those whose structures have been crystallographically refined. We also present the first single-crystal IR absorption spectrum that resolves the entire fundamental region and find this compares closely with data from reflection measurements. An extra band seen near 690 cm-1 is attributed to an overtone because it is present in isotropic garnets. The absence of orientational dependence and existence of 17 (or fewer) IR fundamentals (rather than the 98-129 expected) is consistent with an Ia3dBAR space group: ordering of divalent or trivalent cations is not apparent. The contrasting results from spectroscopy and crystallography may indicate that strain causes anomalous optical anisotropy in garnet (as occurs in diamond and quartz) in that X-ray diffraction requires juxtaposition of approximately 1000 unit cells, whereas lattice vibrations can be produced by isolated molecules.