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K+-Na+ exchange in phlogopite on the scale of a single layer

Geochimica Et Cosmochimica Acta 74(7): 1954-1962

Autoren/Herausgeber: Sanchez-Pastor N
Aldushin K
Jordan G
Schmahl WW
Erschienen: 2010

Hydrothermal atomic force microscopy (HAFM) was used to investigate K+-Na+ ion exchange in phlogopite in-situ. The exchange of K+ for Na+ caused the interlayer distance to swell by approximately 5 A. A distinct reaction front could be resolved between the K+-areas and the swollen (hydrated) Na+-areas, indicating a single reaction step mechanism. Although the fronts revealed kinematic variability due to inhomogeneities, the data indicate a diffusion mechanism within the interlayers. Diffusion coefficients ranged between 2 x 10(-8) and 35 x 10(-8) cm(2)/s, depending on the depth of the interlayer, the solution composition, and temperature. An activation energy of 15 kJ/mol was calculated from the temperature dependence of the diffusion coefficients. In addition to the regular 5 angstrom swelling, bulge-shaped irregular swelling of up to 200 nm could be observed. This irregular swelling might be an initial stage of delamination.|Reducing the Na+-concentration in the solution at a constant K+-concentration was found to reduce the exchange rate. he exchange ceases completely when the equilibrium ratio r(K+/Na+) of the solution is reached. The measured r(K+/Na+) of 0.013 indicates a lower K+-selectivity for interlayers that are closer to the surface. This lower selectivity is most likely related to a lower strain energy associated with the expansion of interlayers close to the surface.|Reversing the exchange reaction caused the interlayers to shrink to their original height. The kinematics of the front of the reverse reaction were significantly enhanced. In parts, swollen Na+-areas were engulfed and trapped by the shrunken K+-areas. No morphological indications of remnant alterations other than these trapped islands and the irregular swelling were observed. (C) 2009 Elsevier Ltd. All rights reserved.

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