Dehydroxylation of Fe3+, Mg-rich dioctahedral micas: (I) structural transformation

Celadonite and glauconite samples heated at different temperatures were stu died by X-ray and electron diffraction. For dioctahedral micas the in-plane component of the translation between layers (ccos beta/a), which is strong ly dependent on the position of the vacant octahedral site, significantly d ecreases at temperatures greater than the temperature of maximum dehydroxyl ation. The simulation of XRD patterns from different structural models reve als the actual crystal structure of dehydroxylated samples as well as the d ynamics of the structural transformations. In the nonheated state the sampl es consist of tv (trans-vacant) 2:1 layers. During dehydroxylation, cations migrate from cis- into trans-octahedra and have 5-fold coordination. In th e averaged unit-cell the 'residual' anions formed after the dehydroxylation reaction occupy the former OH sites with probability equal to 0.5. The mig ration of octahedral cations is accompanied by the transformation of the C- centred layer unit-cells into primitive ones. In contrast to Fe, Al and Mg cations have a greater ability to migrate.