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 Ia 3 d , with a crystal-chemical formula Na 1.93 (Cs 10.48 Rb 0.31 K 0.04 ) Σ=10.83 (Al 14.45 Si 33.97 ) Σ=48.42 O 96 ·3.92H 2 O, and an H 2 O 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 ν 3 and ν 1 stretching modes of the H 2 O molecule, respectively. A very intense and sharp peak at 1620 cm –1 is assigned to the ν 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 2 O, 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 2 O 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 εiCO 2 = 11,000±3000 l mol –1 cm –2 ; the linear molar absorption coefficient for the same integration range is εlCO 2 = 1600±500 l mol –1 cm –1 .