『Abstract
　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.

Keywords: Weathering; Illite; Potassium dynamics; Semi-arid climate; Northeast Brazil』

1. Introduction
2. Materials and methods
　2.1. The study area and its environment
　2.2. Laboratory methods
3. Results
　3.1. Profile chemistry and mineralogy
　3.2. Micromorphological and mineralogical characterization of plagioclase weathering
　　3.2.1. Gneiss at 400 cm depth
　　3.2.2. Saprolite at 135 cm depth
　3.3. Chemical composition of illites formed by plagioclase weathering
4. Discussion: Potassium dynamics in saprolite and implications for weathering pathways in semi-arid climates
　4.1. Origin of potassium in secondary illite
　4.2. Potassium dynamics during weathering
　4.3. Illite neoformation, weathering and climate
5. Conclusion
Acknowledgements
References