Theoretical and ²⁷Al CPMAS NMR investigation of aluminum coordination changes during aluminosilicate dissolution

Ab initio molecular orbital calculations were performed, and ²⁷Al CP MAS-NMR spectra were evaluated in order to investigate the possible tetrahedral to octahedral coordination change of Al at the feldspar-water interface under acidic conditions. Aluminum coordination is octahedral in solution, and tetrahedral in feldspar crystals. Whether this change in coordination can occur on feldspar surfaces as part of the dissolution mechanism has been debated. Molecular orbital calculations were performed on aluminosilicate clusters with a few surrounding water molecules to partially account for solvation effects at the feldspar-water interface. The calculations on both fully-relaxed and partially-constrained clusters suggest that the energy difference between ^[4]Al and ^[6]Al where both are linked to three Si-tetrahedra (i.e., Q³Al) in the feldspar structure, is small enough to allow for the conversion of Q^{3 [4]}Al to Q^{3 [6]}Al in a hydrated layer of feldspar, prior to the release of Al ions to the aqueous solution. The introduction of a few water molecules to the clusters introduced the possibility of multiple optimized geometries for each Al coordination, with energy differences on the order of several hydrogen bonds. The calculation of activation energies and transition states between Q^{3 [4]}Al, Q^{3 [5]}Al, and Q^{3 [6]}Al was complicated by the introduction of water molecules and the use of fully-relaxed aluminosilicate clusters. Calculated isotropic shifts for Q^{1 [6]}Al, Q^{2 [6]}Al, and Q^{3 [6]}Al suggest that the ^[6]Al observed on aluminosilicate glass surfaces using ²⁷Al CP MAS-NMR is Q^{1 [6]}Al and therefore formed as part of the dissolution process. The formation of ^[6]Al in situ on a feldspar surface (as opposed to re-precipitation from solution) has significant implications for the dissolution mechanism and surface chemistry of feldspars.