Interstratification in Malawi Vermiculite: Effect of Bi-Ionic K-Mg Solutions

Jose-Luis Martin de Vidales1, Eladio Vila2, Antonio Ruiz-Amil2, Cristina de la Calle2, 3 and Charles-Henri Pons4
1 Departamento de Química Agrícola, Geología y Geoquímica, Facultad de Ciencias Universidad Autónoma de Madrid, 28049 Madrid, Spain
2 Instituto de Ciencia de Materiales, C.S.I.C., Serrano 113, 28006 Madrid, Spain
3 Laboratoire de Réactivité de Surface et Structure, URA 1106, CNRS, Université P. et M. Curie 4, Place Jussieu, Tour 54–55, 2ème Étage, 75252 Paris Cédex 05, France
4 Laboratoire de Cristallographie, Université d′Orléans, CNRS U.A. 810, B.P. 6759 Rue de Chartres, 45067 Orléans, Cédex 2, France

Abstract: The conversion of Malawi vermiculite into K-vermiculite by treatment with bi-ionic K-Mg solutions of 1 N total ion concentration (KCl and MgCl2 mixed solutions of ionic strength equal to 0.5) was studied by following the 00l X-ray powder diffraction (XRD) reflections. Flakes of Mg-saturated samples were treated at 160°C during 24 hr with bi-ionic solutions, with the K concentration varying from zero to pure 1 N KCl solution. The K-Mg interlayer exchange began at a critical value xK = .0196 (K/Mg = 1/100) of the molar fraction of K in the solution. Above the critical concentration and extending to pure 1 N KCl, the XRD diagrams were characteristic of a 10-Å/14-Å interstratification that had a marked tendency towards regularity. Experiments with KCl and MgCl2 mixed solutions of ionic strength equal to 0.75 and 1.0 showed that the exchange began at the same critical value xK as the experiments with ionic strength equal to 0.5, if the K added was equivalent. X-ray fluorescence analysis further showed that the amount of K adsorbed was proportional to the molar fraction xK and to the proportion of K-saturated layers (10 Å) in the interstratification. To explain the mechanism of this quasi-regular interstratification, a crystallochemical rather than a thermodynamic mechanism is proposed.

Key Words: Interstratification • Ion exchange • Magnesium • Potassium • Vermiculite • X-ray powder diffraction

Clays and Clay Minerals; October 1990 v. 38; no. 5; p. 513-521; DOI: 10.1346/CCMN.1990.0380508
© 1990, The Clay Minerals Society
Clay Minerals Society (www.clays.org)