『Abstract
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., Q3 Al) in the feldspar structure, is small enough
to allow for the conversion of Q3[4]Al to
Q3[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
Q3[4]Al, Q3[5]Al,
and Q3[6]Al was complicated by the introduction
of water molecules and the use of fully-relaxed aluminosilicate
clusters. Calculated isotropic shifts for Q1[6]Al,
Q2[6]Al, and Q3[6]Al
suggest that the [6]Al observed on aluminosilicate
glass surfaces using 27Al CP MAS-NMR is Q1[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.』
1. Introduction
1.1 Background
1.2. Proposed dissolution mechanism
2. Methods
2.1. Model clusters
2.2. Computational methods
2.3. Experimental methods
3. Results
3.1. Minimum potential energy configurations
3.2. Reaction path calculations
3.3. NMR properties
4. Discussion
4.1. Comparisons to experimental data
4.2. Potential model artifacts
4.3. Implications
5. Conclusions
Acknowledgments
References