Thermodynamic Study of Na-K-Ca Exchange Reactions in Vermiculite

Abstract

Abstract--The exchange of Na, K, and Ca ions in vermiculite has been studied between 50 and 1500C at a total normality of 0.1. The free energy changes were negative in the Na ~ K, Ca ~ K, and Na ~ Ca exchange reactions, and the cation preference in the vermiculite was Na < Ca < K in the temperature range examined. The enthalpy changes, calculated by application of the van't Hoffequation to the relation between equilibrium constant and temperature, were positive. The cation preference in the vermiculite seems to have been largely controlled by the increase in entropy which was due to the positive values of the entropy change term in the solid for the Na-K and Ca-K exchanges and the positive values of the entropy change in the solution for the Na-Ca exchange. From a thermodynamic analysis of the cationmixing properties in vermiculite, the observed free energy change in the Na-K exchange was determined solely by the differences of the intrinsic electrostatic binding energy of the cations on the clay surface. That in the Na-Ca exchange was interpreted by taking into account an extra interaction energy of Ca-Ca pairs in addition to the intrinsic energy change. Furthermore, an additional interaction energy between K ions and the clay surface appears to have been added to the free energy change in the Ca-K exchange. In the Na ~ K and Ca ~ K exchange reactions a structural modification occurred in the K-equivalent fraction (RI0 range 0.05--0.6, and a regularly interstratified phase of 15-A and 10-A members was formed at 0.05 < Y(K < 0.4.

Ion Exchanged Vermiculites with Lower Expansion Onset Temperatures

“…The XRF results emphasize the fact that Palabora vermiculite is not pure because the content of potassium, expressed as K 2 O, exceeds 0.35% [27,33].…”

“…Previous kinetic studies have already shown a very slow rate of exchange in similar vermiculites due to the high layer charge density [33,44,45]. All experiments were run at the pH of the salt solution and it was not controlled accurately since the exchange rate is not affected by the pH in the range of 4 to 9 [28].…”

“…Vermiculite selectivity, in the above series, is determined mostly by the entropy increase rather than enthalpy change since both enthalpy and entropy are positive during the ion exchange [28,33]. For instance, vermiculite's preference for divalent cations is mainly due to increase in the (configurational) entropy during replacement of monovalent cations by corresponding half-divalent cations [28].…”

“…Determined standard free energies (∆G°) [28,33] suggest the following affinity of alkali and alkaline metal ions to vermiculite layers:…”

Illite-Smectite Mixed-Layer Minerals in the Hydrothermal Alteration of Volcanic Rocks: I. One-Dimensional XRD Structure Analysis and Characterization of Component Layers

“…The average layer charge of the Palabora vermiculite is 0.83/O 10 (OH) 2 based on data of Inoue (1984). The Goto rectorite is the same material as that studied by Lagaly (1979) Kakkonda.…”