H_2O and CO_2 in minerals of the hauyne-sodalite group: an FTIR spectroscopy study

摘要

This paper reports an infrared spectroscopic study of a set of sodalite-group minerals. The specimens have been identified using a combination of X-ray diffraction and microchemical analysis. As expected, the Si/Al ratio is approx 1; the extra framework cation content is characterized by a well-defined Na 8888888 (Ca+K) substitution. The lattice parameters of the studied samples range from sodalite (sample LM11) with a = 8.889(2) A, to hauyne (sample HR3S) with a = 9.1265(2) A. The specimens, having the SO_4~(2-) group as a dominant anion, show a clear correlation between the a cell edge and the K content. Single-crystal FTIR spectroscopy shows that haiiyne and nosean typically contain enclathrated CO_2 molecules, in addition to H_2O and minor carbonate, while sodalite is virtually CO_2-free. Detailed microspectro-metric mappings show a non-homogeneous distribution of volatile constituents across the crystals, which may be related to the presence of fractures in the crystals. Because of such zoning, a relatively wide variation is observed when calibrating extinction coefficients on the basis of a bulk analytical method such as CHN elemental analysis.