H (sub 2) O and CO (sub 2) in minerals of the haueyne-sodalite group; an FTIR spectroscopy study

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 approximately 1; the extra framework cation content is characterized by a well-defined Na<-->(Ca+K) substitution. The lattice parameters of the studied samples range from sodalite (sample LM11) with a=8.889(2) Aa, to hauyne (sample HR3S) with a=9.1265(2) Aa. The specimens, having the SO (super 2-) (sub 4) as a dominant anion, show a clear correlation between the a cell edge and the K content. Single-crystal FTIR spectroscopy shows that hauyne and nosean typically contain enclathrated CO (sub 2) molecules, in addition to H (sub 2) O and minor carbonate, while sodalite is virtually CO (sub 2) -free. Detailed microspectrometric 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.