Illite neoformation in plagioclase during weathering: Evidence from semi-arid Northeast Brazil

Illite minerals in soils and weathered rocks are usually interpreted as secondary clays derived from the transformation of primary rock-forming minerals with similar properties (e.g. muscovite), or as direct products of hydrothermal alteration after feldspar (i.e. sericite). With the help of detailed investigations involving X-ray diffraction, scanning electron microscopy and X-ray element mapping, this study testifies to the neoformation of illite inside plagioclase feldspar crystals after their partial dissolution in the weathering zone of Luvisols in semi-arid Northeast Brazil. Our results thus reveal that meteoric weathering can produce illite not only from mica, i.e., by a transformation process, but also within non-alkali feldspar by a neoformation process, at least under the semi-arid conditions that have long prevailed in the study area. Given that illite in weathering mantles has recurrently been attributed, often by default, to hydrothermal processes, results shown here provide new focus to an alternative pathway of illite formation primarily controlled by climate.