FOLLOW THE ELECTRONS: USING QUANTUM MECHANICS TO MAP OUT SILICATE MINERAL DISSOLUTION MECHANISMS

In the most fundamental sense, an elementary chemical reaction redistributes electronic charge from existing chemical bonds to form new chemical bonds. When silicate minerals dissolve, electron density must transfer from bonds involving connecting oxygens to new ones that form between the attacking reagents and silicate mineral components. Modern quantum mechanical models not only inform us about how the electrons flow during these reactions but they also now provide a clear mapping of the sites where the attacking reagents will affix to the mineral. This analysis allows us to recognize “proton-promoted” and “ligand-promoted” dissolution mechanisms as less general cases of electrophilic and nucleophilic attacks. Furthermore, quantum mechanical methods provide results that are consistent with donor-acceptor theory, which gives us an excellent qualitative basis for visualizing how bonds strenghten or weaken as the reagents approach sites of attack. Donor-acceptor theory shows that adding hydrogens to highly-underbonded, terminal oxygens strengthens the bonds between the nearby cations and the bulk structure, whereas adding hydrogens to bridging oxygens weakens the bonds between the nearby cation and the underlying structure. Thus, quantum mechanics provides a much-needed link between some well known chemical principles and silicate mineral dissolution reactions.