Weathered metapelites (from Verrucano metasediments, northern Apennines, Italy) containing coexisting, altered carpholite, muscovite, and chlorite have been studied by scanning and analytical-transmission electron microscopy in order to compare the products and mechanisms of weathering. TEM images and SAED patterns demonstrate that primary magnesiocarpholite and phyllosilicates are well-crystallized with few defects. Fractures parallel to (100) in magnesiocarpholite are the sites of the weathering products halloysite and goethite. Halloysite occurs as hollow tubes with variable orientations which are apparently unrelated to the parent magnesiocarpholite structure. In sharp contrast, muscovite crystals consist primarily of alternating packets of layers of kaolinite and muscovite. The layers of kaolinite are parallel to those of original muscovite, with along-layer transitions, as consistent with direct replacement of muscovite layers by kaolinite. Chlorite is replaced by corrensite and goethite with along-layer transitions from chlorite to corrensite (001).
Microtextures imply that weathering of magnesiocarpholite occurs by dissolution at fracture-mineral interfaces. The reaction involves almost complete breakdown of the structure components of magnesiocarpholite, and crystallization of products with unrelated orientation and with the disordered structure of the halloysite. Muscovite and chlorite are replaced by more highly ordered kaolinite and corrensite, respectively. The changes in structure and composition required by direct replacement of muscovite by kaolinite imply a dissolution-neocrystallization process, occurring at the muscovite-kaolinite interface.
The contrasting mineralogical, orientation, and textural relations between halloysite and kaolinite thus occur because halloysite crystallized as a pore-filling mineral in fractures within the structurally dissimilar magnesiocarpholite, but kaolinite formed as a direct layer replacement due to the similarity in structures.