The single-crystal, polarized-light, FTIR spectrum of stoppaniite, the Fe3+-bearing beryl

We report here a single-crystal polarized-light study of stoppaniite, ideally (Fe, Al, Mg)(4)(Be6Si12O36) (H2O)(2)(Na, square), from Capranica (Viterbo). Polarized-light FTIR spectra were collected on an oriented (hk0) section, doubly polished to 15 mu m. The spectrum shows two main bands at 3,660 and 3,595 cm(-1); the former is strongly polarized for E perpendicular to c, while the latter is polarized for E //c. A sharp and very intense band at 1,620 cm(-1), plus minor features at 4,000 and 3,228 cm(-1) are also polarized for E //c. On the basis of literature data and considering the pleochroic behavior of the absorptions, the 3,660 cm(-1) band is assigned to the nu(3) stretching mode and the 1,620 cm(-1) ( associated with an overtone 2*nu(2) at 3,230 cm(-1)) band to the nu(2) bending mode of "type II" water molecules within the structural channels of the studied beryl. The sharp band at 3,595 cm(-1) is not associated with a corresponding nu(2) bending mode; thus it is assigned to the stretching vibration of O-H groups in the sample. The minor 4,000 cm(-1) feature can be assigned to the combination of the O-H bond parallel to c with a low-frequency metal-oxygen mode such as the Na-O stretching mode. The present results suggest that the interpretation of the FTIR spectrum of Na-rich beryl needs to be carefully reconsidered.