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<jats:title>Abstract</jats:title><jats:p>Dissolution of feldspar and mica and precipitation of kaolinite require a through flow of meteoric water to remove cations such as Na<jats:sup>+</jats:sup> and K<jats:sup>+</jats:sup> and silica. Compaction driven pore-water flow is in most cases too slow to be significant in terms of transport of solids. The very low solubility of A1 suggests that precipitation of new authigenic clay minerals requires unstable Al-bearing precursor minerals. Chlorite may form diagenetically from smectite and from kaolinite when a source of Fe and Mg is present. In the North Sea Basin, the main phase of illite precipitation reducing the quality of Jurassic reservoirs occurs at depths close to 4 km (130-140°C) but the amount of illite depends on the presence of both kaolinite and K-feldspar. Clay mineral reactions in shales and sandstones are very important factors determining mechanical and chemical compaction and are thus critical for realistic basin modelling.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Clay mineral evolution in diagenetic and very low-grade metamorphic sedimentary rocks from the Maláguide Complex was studied using the following parameters: illite and chlorite crystallinity, <jats:italic>b</jats:italic> cell dimension of illite, chemistry of chlorite, and the occurrence and distribution of the following phyllosilicates: I-S mixed-layer minerals, kaolin polytypes, chlorite-bearing mixedlayer minerals (tosudite and mica-chlorite interstratifications), vermiculite-like minerals, paragonite and biotite. The distribution of IC values permits the subdivision of the study area into three zones: diagenetic, anchizone and epizone. On the contrary, the distribution of CC values cannot be used in the study of the metamorphic zonation. The mineralogy and <jats:italic>b</jats:italic> dimension of illite indicate that the Maláguide samples display a low to low-medium pressure metamorphism, also characterized by steep temperature gradients.</jats:p><jats:p>The correlation between IC data and distribution of other significant minerals has permitted the establishment of a zonation within the chlorite zone, which is confirmed by the appearance of some index minerals, such as paragonite, biotite, chloritoid and andalusite.</jats:p>