Illite is present among the clay minerals in almost all the investigated
samples (except Group 3), ranging from ~27% to ~65%, with an
average of ~38%. It is the only clay mineral in Group 2. In some samples,
on heating to 550 °C, the (001) peak of illite shows a slight collapse, suggesting
that expandable layers could also be present. In most of the samples,
the sharp peaks of illite indicate that the illites are generally wellcrystallized.
Kaolinite is only present in one sample (AR9, ~51%),
whereas chlorites are present in Groups 1 and 4. Quartz is probably
also present in very small amounts in the separated clay fraction.