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

Abstract

AbstractWe 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 Na1.93(Cs10.48Rb0.31K0.04)Σ=10.83(Al14.45Si33.97)Σ=48.42O96·3.92H2O, and an H2O 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 ν3 and ν1 stretching modes of the H2O molecule, respectively. A very intense and sharp peak at 1620 cm–1 is assigned to the ν2 bending vibration. In the near-infrared region there is a relatively intense peak at 5270 cm–1, which is assigned to a combination (ν2 + ν3) mode of H2O, 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 CO2 molecules in structural cavities. Mapping the sample using FTIR indicates that both H2O and CO2 are homogeneously distributed. Secondary ion mass spectrometry yielded an average CO2 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 CO2 in pollucite to be εiCO2 = 11,000±3000 l mol–1 cm–2; the linear molar absorption coefficient for the same integration range is εlCO2 = 1600±500 l mol–1 cm–1.