The single-crystal, polarized-light, FTIR spectrum of stoppaniite, the Fe analogue of beryl

We report here a single-crystal polarized-light study of stoppaniite, ideally (Fe,Al,Mg)4(Be6Si12O36)(H2O)2(Na,[]), from Capranica (Viterbo). Polarized-light FTIR spectra were collected on an oriented (hk0) section, doubly polished to 15 μm. The spectrum shows two main bands at 3,660 and 3,595 cm-1; the former is strongly polarized for E normal 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 ν3 stretching mode and the 1,620 cm-1 (associated with an overtone 2*ν2 at 3,230 cm-1) band to the ν2 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 ν2 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.