Uncommon fibrous-clays in duricrust and near surface sediments: features and genesis (Esquivias, Madrid basin)

Mineralogy, chemical composition and textures of silcretes, calcretes/palustrine limestones and argillaceous beds are studied to consider the features and the genesis of fibrous-clay minerals. The mineralogy, petrology and geochemical studies were carried out by X-ray diffraction, optical microscopy, and, scanning (SEM) and transmission (TEM) electron microscopy with dispersive analysis (EDS). Sepiolite and palygorskite were found in the argillaceous beds and silcretes while only palygorskite was found in the calcretes/palustrine limestones. The chemical compositions of sepiolite and palygorskite are anomalous. The sepiolite shows Mg/Al relations lower than the standard, and the palygorskite higher than standard. Smectites occurs in the argillaceous beds, and the dissolution of these smectites can generate appropriated cations for the formation of sepiolite and palygorskite. The absence of smectites in the silcretes and calcretes/palustrine limestones could suggest that the fibrous clay minerals were formed by direct precipitation from interstitial water of these rocks. Part of the palygorskite included in the calcretes/palustrine limestones formed latter than calcite, and its formation could have been favored by the dissolution of the calcite. The silcretes are mainly constituted of opal-CT, palygorskite and sepiolite. They are formed by silicification of the calcrete/palustrine limestones and the palygorskite is the relic of these host rocks. Opal CT and sepiolite are neoformed minerals which were produced during the silicification, or after, by aging from a magnesium rich-silica gel.