Cordierite II: The role of CO ₂ and H ₂ O

Polarized single-crystal Raman spectra at room temperature and 5 K and polarized infrared spectra at room temperature were obtained from four natural cordierites of different compositions in the wave number region of the CO 2 symmetric stretching vibration and the H 2 O stretching vibrations. The CO 2 molecules are preferentially aligned parallel to the X axis, consistent with results from X-ray diffraction and optical studies. The CO 2 contents of six natural cordierites, previously studied by powder IR methods (Vry et al. 1990), were determined via Raman spectroscopy. A linear relationship was found between CO 2 content and the Raman intensity ratio of the normalized CO 2 stretching mode against a Si-O stretching mode. This permits a determination of the CO 2 contents in cordierite using micro-Raman measurements. The internal stretching modes between 3500 and 3800 cm -1 were assigned to various types of H 2 O molecules occurring in the channel cavity. Three different orientations of H 2 O molecules that have no interactions with alkali cations located at 0,0,0 in the sixmembered tetrahedral rings are classified in a static model as Class I H 2 O molecules. The H···H vector for two of them is parallel to [001], and their molecular planes lie in the XZ and YZ crystal planes. The third type has its H···H vector directed along the X axis and its molecular plane lies in the XZ plane. Two other types of H 2 O have interactions with the alkali cations located at 0,0,0. They are classified as Class II H 2 O. They distinguish themselves by the number of H 2 O molecules bonded to the alkali atoms. The formation of weak hydrogen bonds at low temperatures may explain the appearance of some Raman stretching modes below 200 K. The H 2 O molecules of Class I-Type I/II are probably dynamically disordered about [001] hopping between orientations in the XY and XZ planes down to 5 K. Class II H 2 O may also be disordered, but more measurements are required to describe its dynamic behavior.