Stacking Order in a 14.30-Å Mg-Vermiculite

Cristina de la Calle1, Hélène Suquet2 and Charles-Henri Pons3
1 Instituto de Ciencia de Materiales, CSIC Calle Serrano 113. 28006 Madrid, Spain
2 Laboratoire de Réactivité de Surface et Structure CNRS U.A. 1106, Université P. et M. Curie 4 Place Jussieu, 75252 Paris Cedex 05, France
3 Laboratoire de Cristallographie, CNRS U.A. 810, Université d'Orléans B.P. 6759, rue de Chartres, 45067 Orléans Cedex 2, France

Abstract: The stacking order of a 14.30-Å Mg-vermiculite from Santa-Olalla, Spain, has been determined from Weissenberg photographs. The results prove that the Mg-vermiculite structure is not a 2-layer polytype structure, but a semi-ordered one. Because the structure is semi-ordered, its resolution needed a dual approach: (1) a direct approach using an electron density projection along the y axis in conjunction with a one-dimensional electron-density projection onto the z axis; and (2) an indirect approach in which the observed intensities along the (0,k) and (1,k) reciprocal rods were compared to the calculated intensities given by model defect structures. The semi-ordered structure of the Mg-vermiculite results from ±b/3 shifts in the stacking of the silicate layers. The shifts are randomly either along +b or -b.

Key Words: Crystal structure • Ordering • Polytype • Stacking • Vermiculite

Clays and Clay Minerals; December 1988 v. 36; no. 6; p. 481-490; DOI: 10.1346/CCMN.1988.0360601
© 1988, The Clay Minerals Society
Clay Minerals Society (www.clays.org)