Clay minerals occur as replacements of olivine, pyroxenes, plagioclase and interstitial materials, and as vesicle fillings, in altered basalts from the Middle Atlas (Morocco). Phyllosilicates are the main components of this alteration process. They have been characterized here by optical microscopy, X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy as saponite, talc, corrensite and chlorite. The homogeneity of the chemical compositions of these phyllosilicates, in different alteration environments, probably means that they are controlled by the composition of the hydrothermal fluid. Talc-saponite-corrensite (with dominant saponite) is the clay mineral association most frequently observed (corrensite being more abundant in the vesicular levels of the basalts). Such an association, with no evidence of albite and zeolite, suggests that these basalts have suffered minimal alteration at relatively low temperatures. Another association, chlorite-corrensite, was detected in a deeper (vesicular) sample, probably resulting from a slightly higher temperature. Lastly, only discrete phyllosilicates (i.e. no random mixed-layer minerals) were observed. This suggests that near-equilibrium conditions prevailed during this alteration stage and that a dissolution-crystallization process was the main mechanism of transformation.

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