Stacking Order in a K/Mg Interstratified Vermiculite from Malawi

C. de la Calle1, J. L. Martin de Vidales2 and C. H. Pons3
1 Instituto de Ciencia de Materiales, Sede D, C.S.I.C., c) Serrano 113, 28006 Madrid, Spain
2 Departamento de Quimica Agricola, Facultad de Ciencias, Universidad Autonoma de Madrid, 28049 Madrid, Spain
3 CRMD CNRS-Université d'Orléans (UM 0131), U.F.R. Sciences Rue de Chartres BP 6759, 45067 Orléans Cedex 2, France

Abstract: The stacking order of a bi-ionic K/Mg vermiculite from Malawi (Nyasaland), has been determined from Weissenberg data. The sample corresponds to a K/Mg interstratified vermiculite containing 50% K layers (dK = 1.01 nm) and 50% Mg layers with two layers of water (dMg = 1.44 nm). The observed intensities along (0, 0), (h, 0), (0, k) and (1, k) reciprocal rods were compared to the calculated intensities given by model defect structures. It was found that: 1) The (h, 0), (0, k) and (1, k) rods reveal the interstratification phenomenon which was previously observed on the (0, 0) rod; 2) The exchange of Mg by K does not occur randomly in a single interlayer, but occurs in interlayer domains sufficiently extensive to allow the reorganisation of the layer stacking and development of the ordered K-vermiculite structure from the semi-ordered structure characteristic of magnesium vermiculites; 3) For this sample, the interlayer water of the Mg phase occurs in two coexisting configurations; one configuration has water molecules forming octahedral coordination around the Mg2+, the other has water which is not linked to the Mg2+ cation forming two planes.

Key Words: Crystal structure • Interstratification • Ion exchange • Ordering • Stacking • Vermiculite

Clays and Clay Minerals; October 1993 v. 41; no. 5; p. 580-589; DOI: 10.1346/CCMN.1993.0410508
© 1993, The Clay Minerals Society
Clay Minerals Society (www.clays.org)