Kubicki, J. D. and Blake, G. A. and Apitz, S. E. (1996) Ab initio calculations on aluminosilicate Q^3 species: Implications for atomic structures of mineral surfaces and dissolution mechanisms of feldspars. American Mineralogist, 81 (7-8). pp. 789-799. ISSN 0003-004X http://resolver.caltech.edu/CaltechAUTHORS:20120912-115931973
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Molecular orbital calculations on various aluminosilicate Q^3 T-OH and bridging O species were performed to model atomic structural changes on mineral surfaces that occur as a function of pH. Calculated vibrational frequencies are reported for the terminal T-O, T-OH, and O-H bonds of the central T cation as a test of our models, and the predicted frequencies compare well with experimental vibrational spectra of aluminosilicates. Optimized bond lengths and T-O-T angles to the central Q3 Si^4+ and Al^+3 cations in these molecules change significantly as the T-OH bond is protonated and deprotonated. Protonation of terminal bonds tends to shorten and strengthen the remaining three T-Obr bonds that would attach the central cation to the bulk mineral. This result is in contrast to the T-Obrweakening that has been suggested previously as a mechanism for protonassisted dissolution (e.g., Furrer and Stumm 1986; Wieland et al. 1988). Proton affinities (PA) of the Q^3 T-OH, T-OH_2, and T-OH-T species are predicted to help delineate the process of proton-assisted dissolution in quartz and feldspars. Theoretical results predict that the PAs of Al-OH_2 and Al-OH-Si species are comparable; thus, as Al-OH_2 surface species become stable, protons are also energetically favored to attach to bridging 0 atoms. In addition, we found that substitution of Al^+3 for Si^4+ in the second-nearest-neighbor site of a Q^3 Si-OH can increase the calculated PA, although the effect is diminished by the presence of a charge-balancing Na+ cation. This result has implications for models that attempt to describe the behavior of aluminosilicate surfaces in terms of the component oxides. Addition of Na+ to charge balance molecules strongly affects calculated structures, proton affinities, and vibrational spectra. The role of charge-balancing cations was often omitted in previous theoretical studies of aluminosilicates, but the magnitude of the charge-balancing effect could alter the results and conclusions of earlier calculations in this area.
|Additional Information:||© 1996 Mineralogical Society of America. Manuscript received: February 13, 1995; manuscript accepted: March 27, 1996. J.D.K. acknowledges the National Research Council Research Associateship program. S.E.A. and J.D.K. acknowledge support from ONT and ONR. G.A.B. acknowledges NSF grant EAR-9316432. Computational resources were supplied by the Jet Propulsion Laboratory. The comments of D.A. Sverjensky on an early draft of the manuscript and the review of D.M. Sherman greatly aided the development of this paper.|
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|Deposited By:||Jason Perez|
|Deposited On:||12 Sep 2012 21:36|
|Last Modified:||12 Sep 2012 21:36|
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