Order-disorder in clay mineral structures

A Plançon

doi:10.1180/000985501547286

Abstract

AbstractSome recent works dealing with the concept of order-disorder in clay minerals are considered, including those aspects of order-disorder which appeared in the Brindley & Brown (1980) monograph, i.e. disorder in the distribution of cations, disorder in layer stacking, orderdisorder in mixed-layer systems and finite crystal size as a lattice disorder. Heterogeneity of samples and polymorphous transformations are also considered as other types of disorder. Most of these works emphasize that accurate structural characterization can only be obtained if several techniques are combined (e.g. XRD and IR, EXAFS and Mo¨ssbauer spectroscopies, etc.). Another conclusion is that accurate structural determination provides the key to the genesis of clays.

Highlights

Some recent works dealing with the concept of order-disorder in clay minerals are considered, including those aspects of order-disorder which appeared in the Brindley & Brown (1980) monograph, i.e. disorder in the distribution of cations, disorder in layer stacking, orderdisorder in mixed-layer systems and finite crystal size as a lattice disorder
Recent work shows that the structural characterization of clay minerals becomes, with time, more and more precise and that the nature of the layers, their chemical composition and layer charge are insufficient to describe clay mineral particles
Is greater precision necessary for practical use? the use of clay minerals has not waited for their accurate structural characterization

Summary

Polymorphous transformations

Semiquantitative determination of trans-vacant and cis-vacant layers in illite-smectites and illitesmectite-vermiculites. Using models generated from powder XRD patterns, selected area electron diffraction (SAED), and thermogravimetric analysis, Muller et al (1999a, 2000a,b) showed that the mechanism is more complex for tv Fe-, Mg-rich minerals (like celadonite and glauconites). In this case the dehydroxylation causes a migration of Fe and Mg cations which leads to a fair distance between cations and a good screening of their repulsion. It is the shape of the molecule which seems to play the prominent role in the interlayer shift

Finite size of particles as a lattice disorder

Heterogeneity as a disorder

Findings

CONCLUSIONS