PNNL

Abstract

Potential protonation sites within the three aluminosilicate polymorphs, kyanite, sillimanite, and andalusite, have been examined through analysis of (3,-3) bond critical point information in comparison with polarized FTIR spectroscopy of single crystals of kyanite and sillimanite from previous studies and examination with the polarized infrared spectrum of andalusite from this study. Seven peaks were observed, four strong peaks at 3440 cm-1, 3460 cm-1, 3530 cm-1, and 3600 cm-1 and three weak peaks at 3480 cm-1, 3520 cm-1, and 3650 cm-1, when the electric vector is parallel with a, six peaks, three strong at 3440 cm-1, 3460 cm-1, and 3530 cm-1 and three weak peaks at 3480 cm-1, 3520 cm-1, and 3560 cm-1, when the electric vector is parallel with b, and no peaks when the electric vector is parallel with c. The results indicate that hydrogen is located in the (001) plane of andalusite and sillimanite and the (11-1) plane in kyanite as determined from the polarized FTIR spectrum of the three minerals. The concentration of water in the samples of andalusite examined varied between 10 and 15 ppm H20 by weight. Examination of the (3,-3) critical points in comparison with the polarized FTIR indicates that hydrogen will prefer bonding to the O1 and O2 oxygen atoms in andalusite and the O2 and O4 oxygens in sillimanite, which correspond to the oxygen with the highest Laplacian value and the underbonded oxygen in the two structures. In kyanite, comparison of the FTIR spectrum and the bond critical points indicate that hydrogen will bond to the two four-coordinated oxygens, O4 and O6.