Weathering of granites in a temperate climate (NW Portugal): granitic saprolites and arenization

Saprolites or in situ weathering products from granitoid rocks of Northwest Portugal were studied by means of X-ray diffraction (XRD) of their clay fraction and observations of polished thin sections combined with transmission electron microscopy (TEM), scanning electron microscopy with energy dispersive spectrometer (SEM-EDS) microanalyses. Their principal features are: (1) depths of more than 10 m; (2) mean material loss of 40% as calculated by an isovolumetric method; (3) low clay content (mean value of 7%) and a high degree of mineralogical evolution. The clay fraction is characterized by a predominance of kaolinite and gibbsite, with subordinate 2:1 minerals (illite, chlorite, vermiculitic mixed layers, vermiculite). The significance of the secondary minerals in granitic saprolites from Northwest Portugal is compared to published data from Atlantic Europe. This allows the definition of a climatic zonality for weathering products and identifies ‘arenization’ as an important weathering process.