Sanchez-Pastor, Nuria; Aldushin, Kirill; Jordan, Guntram; Schmahl, Wolfgang W.
K+-Na+ exchange in phlogopite on the scale of a single layer.
In: Geochimica et Cosmochimica Acta, Vol. 74, Nr. 7: S. 1954-1962
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Hydrothermal atomic force microscopy (HAFM) was used to investigateK+-Na+ ion exchange in phlogopite in-situ. The exchange of K+ for Na+caused the interlayer distance to swell by approximately 5 A. A distinctreaction 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 toinhomogeneities, the data indicate a diffusion mechanism within theinterlayers. Diffusion coefficients ranged between 2 x 10(-8) and 35 x10(-8) cm(2)/s, depending on the depth of the interlayer, the solutioncomposition, and temperature. An activation energy of 15 kJ/mol wascalculated from the temperature dependence of the diffusioncoefficients. In addition to the regular 5 angstrom swelling,bulge-shaped irregular swelling of up to 200 nm could be observed. Thisirregular swelling might be an initial stage of delamination.Reducing the Na+-concentration in the solution at a constantK+-concentration was found to reduce the exchange rate. he exchangeceases completely when the equilibrium ratio r(K+/Na+) of the solutionis reached. The measured r(K+/Na+) of 0.013 indicates a lowerK+-selectivity for interlayers that are closer to the surface. Thislower selectivity is most likely related to a lower strain energyassociated with the expansion of interlayers close to the surface.Reversing the exchange reaction caused the interlayers to shrink totheir original height. The kinematics of the front of the reversereaction were significantly enhanced. In parts, swollen Na+-areas wereengulfed and trapped by the shrunken K+-areas. No morphologicalindications of remnant alterations other than these trapped islands andthe irregular swelling were observed.