The Experimental Transformation of Mica toward Smectite*; Relative Importance of Total Charge and Tetrahedral Substitution

M. Robert
Laboratoire des Sols, Station Centrale d' Agronomie, CNRA, 78, Versailles, France
* This general term refers to all expansible minerals of the montmorillonite group.

Abstract: This article emphasizes the influence of crystal-chemical composition on the ease of transformation of micas to vermiculite or more smectite-like minerals. The swelling test, with glycerol, of Mg-saturated mineral is used to characterize the degree of transformation.

The main structural factors of this evolution are tetrahedral substitution of Si by Al, and total charge. There is a relation between these two factors, i.e. the lower the tetrahedral substitution, the greater can be the charge without affecting the smectite swelling behavior.

In this respect there is a contrast between tri and dioctahedral micas. In the first, tetrahedral Al is so high (> 1·20 for Si4O10) that transformation into smectite must imply modification of the tetrahedral layer. For dioctahedral micaceous phyllites (illites, glauconites) where tetrahedral charge is lower, transformation can be easier. Only a lowering of total charge is needed and reduction-oxidation seems to play a very important role in this process.

Clays and Clay Minerals; June 1973 v. 21; no. 3; p. 167-174; DOI: 10.1346/CCMN.1973.0210305
© 1973, The Clay Minerals Society
Clay Minerals Society (www.clays.org)