(NAS Colloquium) Geology, Mineralogy, and Human Welfare (1999) / Chapter Skim
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Surface Geochemistry of the Clay Minerals
Surface Geochemistry of the Clay Minerals
Pages 3358-3364
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From page 3358... ...
Unraveling the surface geochemistry of hydrated clay minerals is an abiding, if difficult, topic in earth sciences research. Recent experimental and computational studies that take advantage of new methodologies and basic insights derived from the study of concentrated ionic solutions have begun to clarify the structure of electrical double layers formed on hydrated clay mineral surfaces, particularly those in the interlayer region of swelling 2:1 layer type clay minerals.
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From page 3359... ...
In dioctahedral clay minerals, this hydroxyl group points toward the empty metal site in the octahedral sheet whereas, in trioctahedral clay minerals, it points perpendicularly to the siloxane surface (9~. The reactivity of the siloxane surface depends on the nature of the local charge distribution in the clay layer (12~.
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From page 3360... ...
Because the water protons could be identified separately from those in the clay mineral structure and were distinguishable from other interlayer atoms, it was possible to show rather clearly that the Ca2+ are octahedrally coordinated to their nearest-neighbor water molecules, as also occurs in concentrated, as opposed to dilute, aqueous solutions of CaCl2 (13, 184. Moreover, four of the solvating water molecules form hydrogen bonds with the siloxane surfaces, as expected for clay minerals with isomorphic substitutions only in the tetrahedral sheets (12~.
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From page 3361... ...
These developments encouraged the undertaking of systematic simulation studies of low-charge smectite hydrates, particularly the ubiquitous Wyoming montmorillonites, which have both octahedral and tetrahedral charge sites (6, 27-34~. Prototypical Wyoming montmorillonite corresponds to At = 7.75, n2 = 3.75, n3 = n4 = 0, nS = 0.5, and n6 = 0 in the chemical formula given above for 2:1 layer type clay minerals, with x = 0.75 as the layer charge.
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From page 3362... ...
Dashed lines indicate hydrogen bonds between water molecules. Portions of the opposing two siloxane surfaces also are shown, with the beige sphere at the bottom of the figure (center)
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From page 3363... ...
Pyrophyllite, the uncharged analog of montmorillonite, and talc, the uncharged analog of vermiculite, both have hydrophobic siloxane surfaces (36, 374. Studies of the effect of layer charge on the adsorption of both water and hydrocarbon molecules by smectites (36, 38)
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From page 3364... ...
The characteristics of adsorbed water on 2:1 clay minerals also reveal the competition between interlayer cations and clay mineral structure for intercalated water molecules, as well as that between hydrophilic and hydrophobic interactions. This competition produces a complex electrical double layer structure whose origins and behavior are beginning to be understood at a fundamental level as the basis for progress toward improved design in applications (7, 8)
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