Hydrothermal Reaction of Crystalline Albite, Sodium Aluminosilicate Glass, and a Rhyolitic-Composition Glass With Aqueous Solutions: A Solid State Study
Yang, Wang-Hong Alex
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https://hdl.handle.net/2142/71147
Description
Title
Hydrothermal Reaction of Crystalline Albite, Sodium Aluminosilicate Glass, and a Rhyolitic-Composition Glass With Aqueous Solutions: A Solid State Study
Author(s)
Yang, Wang-Hong Alex
Issue Date
1988
Doctoral Committee Chair(s)
Kirkpatrick, R. James
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Geochemistry
Abstract
Structure and chemical composition of the products of the reaction of crystalline low albite and a glass of nearly albite composition with aqueous solutions of pH's from 1 to 11 at 250$\sp\circ$C and the mechanisms of these reactions using high-resolution solid-state multinuclear NMR spectroscopy in combination with SEM, XRD, IR spectroscopy, and electron microprobe analysis are investigated. For crystalline albite, there are no detectable bulk or surface structural changes due to aqueous attack, indicating that both cation exchange and disruption of the aluminosilicate framework occur only near the mineral/solution interface and the hydrated surface layer, if it exists, is not more than about 30 angstroms thick. The rate of this reaction decreases with increasing solution pH, supporting the idea that the dissolution of feldspar is initiated by cation-exchange. For the glass of nearly albite composition the hydration proceeds by cation exchange of protons for sodium, penetration of water molecules into the bulk glass, and a small amount of depolymerization of the aluminosilicate framework in the interior of the glass. The invasion of water molecules and the cation-exchange cause structural changes in the glass via solid-state adjustment. A shell of water molecules is probably formed around the large cations with a maximum average coordination number of six. The secondary phases formed from both albite and the glass are often amorphous and can be well characterized by NMR. The compositional and structural variations of the amorphous phases are important factors in these reactions and cannot be ignored in theoretical models of aluminosilicate dissolution. As expected, the aluminum coordination in the secondary phases changes from six-fold to four-fold as the solution pH increases.
A synthetic iron-free rhyolitic (Na$\sb2$O-K$\sb2$O-MgO-CaO-Al$\sb2$O$\sb3$-SiO$\sb2$) glass is highlighted. As for the glass of nearly albite composition, during hydrothermal reaction water molecules penetrate the rhyolitic glass and change its structure. The structural changes and the amount of dissolution are smaller than for the glass of nearly albite composition, due to the presence of divalent cations in the rhyolitic glass, which have smaller diffusion coefficients and appear to decrease the rate of mass transfer of the monovalent cations. (Abstract shortened with permission of author.)
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