X-ray powder diffraction
analyses indicate that none of the samples is a pure clay being
characterized by the presence of a significant non-clay fraction dominated
by calcite and quartz, and to a minor extent by dolomite, feldspars
and gypsum. The clay fraction is composed of a very diverse set of clay
minerals represented by illite, smectites, interstratified illite/smectite,
kaolinite and chlorites. T