Kaolinite to smectite transformation: A crystal chemistry study by analytical electron microscopy

Kaolinite and smectite coexist in variable proportions along the Tamame de Sayago deposit in Zamora, Spain, suggesting a close genetic relationship. Kaolinite is formed through the weathering of the Variscan granite, while smectite is subsequently derived from kaolinite through a superimposed hydrothermal process that contributes to Si and Mg content. The transformation of smectites into kaolinite has been widely investigated, however, the reverse transformation of kaolinite into smectite is rarely documented. In this study, a group of representative samples from the deposit were studied via analytical electron microscopy and high-resolution transmission electron microscopy to determine the crystal chemistry of particles and explain the genetic relationship between the two clay minerals. The results showed that the crystal chemistry of particles varied, both among the particles of a sample and at different points within single particles. Chemical composition varied progressively from compositions of kaolinite to kaolinite/smectite and ultimately montmorillonite, with most particles displaying intermediate compositions. SiO 2 and MgO contents increased progressively from kaolinite to montmorillonite through intermediate compositions. The morphologies and compositions of particles suggest the predominance of a solid-state transformation of kaolinite to smectite via interstratified kaolinite-smectite in areas less affected by hydrothermal fluids. Point analyses revealed the existence of domains with different compositions within single pseudohexagonal particles, suggesting the existence of areas with different degrees of transformation in single particles. In areas closer to faults, through which Si and Mg could circulate, dissolution-crystallisation was likely the main process responsible for smectite neoformation. Occasionally, epitaxial smectite growth was observed along the edges of kaolinite crystals. The simultaneous operation but different intensities of three processes solid-state transformation, epitaxy, and dissolution-precipitation generated particles with a complex crystal chemistry related to the existence of interstratified phases. • Crystallochemistry of clay minerals was related to hydrothermal alteration of kaolinised granite. • Transformation from kaolinite to beidellite included complex interstratified. • Particles crystal-chemistry varied among sample particles and within particles. • Solid-state transformation, epigenesis, and dissolution-precipitation processes were present.