A mineralogic study of the Tajo Basin's Arcases Unit has been realized from borehole's data. The mineralogy of these sediments corresponds mainly to inherited phases resulting from alteration of granite. Biotite and muscovite micas are transformed to smectites (ferriferous beidellites and to a lesser extent montmorillonites), mixed-layer l-Sm and illites, at the source area. Feldspars alter mainly to dioctahedrical smectites an minar kaolinite. Electronic microscopy observations of micas transforming to illite-smectite and the presence of mixed-layer CI-Sm, suggest that part of the transformations continued within the basin after sedimentation.
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