Carbon dioxide in pollucite, a feldspathoid with the ideal composition (Cs,Na)16Al16Si32O96•nH2O

We report a single-crystal Fourier-transform infrared (FTIR) study of a sample of pollucite from Maine, USA. Prior to our work, the sample had been characterized by single-crystal X-ray diffraction, neutron diffraction and electron-probe microanalysis. It is cubic Ia3d, with a crystal-chemical formula Na 1.93(Cs 10.48Rb 0.31K 0.04) Σ=10.83(Al 14.45Si 33.97) Σ=48.42O 96•3.92H 2O, and an H 2O content, determined by thermogravimetric analysis, of 1.6 wt.%. The single-crystal FTIR spectrum has a doublet of intense bands at 3670 and 3589 cm -1, which are assigned to the n 3 and n 1 stretching modes of the H 2O molecule, respectively. A very intense and sharp peak at 1620 cm -1 is assigned to the n 2 bending vibration. In the near-infrared region there is a relatively intense peak at 5270 cm -1, which is assigned to a combination (ν 2 + ν 3) mode of H 2O, and a weak but well defined doublet at 7118 and 6831 cm -1, which is assigned to the first overtones of the fundamental stretching modes. A relatively weak but extremely sharp peak at 2348 cm -1 shows that the pollucite contains CO 2 molecules in structural cavities. Mapping the sample using FTIR indicates that both H 2O and CO 2 are homogeneously distributed. Secondary ion mass spectrometry yielded an average CO 2 content of 0.09±0.02 wt.%. On the basis of this value, we determined the integrated molar absorption coefficient for the spectroscopic analysis of CO 2 in pollucite to be e iCO2 = 11,000±3000 l mol -1 cm -2; the linear molar absorption coefficient for the same integration range is e lCO2 = 1600±500 l mol -1 cm -1.