CORRENSITE - A SINGLE-PHASE OR A MIXED-LAYER PHYLLOSILICATE IN THE SAPONITE-TO-CHLORITE CONVERSION SERIES - A CASE-STUDY OF SANCERRE-COUY DEEP DRILL HOLE (FRANCE)

Transmission electron microscopy (TEM), analytical electron microscopy (AEM), and decomposition-simulation of X-ray diffraction (XRD) patter ns were used to characterize trioctahedral clay from Sancerre-Couy tha t had previously been considered to be a mixed-layer material of chlor ite-smectite (C-S). Corrensite should not be regarded as a regular, 50 :50 mixture of chlorite and smectite mixed layers but as a true phase in the thermodynamic sense with a discrete stability field and paragen etic relationships with saponite and chlorite. C-S was not found to ex ist; the clay is composed of saponite or mixtures of corrensite, chlor ite, and minor amounts of chlorite-corrensite mixed layers (C-C). Corr ensite layers from different samples have a constant AEM composition ( except for Fe/Mg ratio). The b parameter of corrensite is not compatib le with a simple association of chlorite and smectite layers. Chlorite -like and smectite-like sublayers of corrensite do not behave independ ently during crystal growth. Selected-area electron diffraction (SAED) patterns and decomposition of XRD profiles provide evidence of C-C mi xed layers. The conversion of corrensite to chlorite occurred by two s imultaneous processes: (1) intergrowth of coherently stacked layers of chlorite leading to a mixture of discrete chlorite and corrensite cry stallites; and (2) chlorite-corrensite random mixed layering in the ca se of very intimate associations of chlorite and corrensite domains. I ntergrowths largely predominate over mixed layers because the solubili ty between corrensite and chlorite layers is low. These C-C mixed laye rs cannot be considered as interstratified minerals sensu stricto as m ay be the case for the smectite-to-illite conversion series.