POTENTIAL HYDROGEN BONDING SITES IN KYANITE, ANDALUSITE, AND SILLIMANITE

It has been shown that water can be incorporated within the structures of nominally anhydrous silicates, including the aluminosilicate polymorphs, kyanite, andalusite, and sillimanite [1]. Although numerous studies have investigated the stabilities of the aluminosilicate polymorphs, little is know about the incorporation of hydrogen within these minerals. This study examines potential hydrogen bonding sites within the aluminosilicate polymorphs based on an analysis of the Laplacian of the electron density distribution, following the method of Ross et al. [2]. The CRYSTAL98 program [3] was used to generate analytical electron density distributions for kyanite, andalusite and sillimanite, and TOPOND [4] was used to locate the (3,-3) critical points. The (3,-3) critical points correspond to regions of nonbonding electron pairs and help pinpoint potential sites of electrophilic attack by hydrogen, for example [5]. Although (3,-3) critical points are found on all oxygen atoms in the aluminosilicate polymorphs, our results indicate that O1 in andalusite and O2 in sillimanite are prime sites for protonation. These oxygens are solely coordinated to three aluminum atoms and have bond valences of 1.88 v.u. and 1.87 v.u., respectively. The location of the (3,-3) critical points indicate that the potential sites are large enough to accommodate hydrogen and also indicate that the O-H bonds in andalusite and sillimanite are highly directional. In andalusite, the O-H vectors lie within (001) and are approximately parallel to [-1 1 0] and [110]. In sillimanite, the O-H vectors also lie within (001) but are oriented closer to [010]. The location of potential sites for hydrogen in kyanite is less clear. However, the positions of the (3,-3) critical points on O2 and O6, which are bonded to four aluminum atoms, have highly optimized geometries for protonation.

[1] Bell D.R. and Rossman G.R. (1992) Science, 255, 1391-1397. [2] Ross N.L., Gibbs, G.V., and Rosso, K.M. (2003) Am. Mineral., in press. [3] Saunders V.R., Dovesi R., Roetti C., Causa M., Harrison N. M., Orlando R., and Zicovich-Wilson C. M. (1998) CRYSTAL98 User's Manual. Univ. of Torino. [4] Gatti C. (1997) TOPOND96 User's Manual, CNR-CSRSRC. [5] Bader R.F.W., MacDougall P.J. and Lau C.D.H. (1984) J. Am. Chem. Soc., 106, 1594-1605.