Mechanism of Kaolinite-to-Illite Transformation in the Central–Southern Xihu Sag

Kaolinite-to-illite transformation is a common clay-mineral reaction during sedimentary basin diagenesis and can be used to constrain fluid–rock interaction and the evolution of diagenetic environments. Here we integrate XRD, SEM/TEM, EDS/EPMA, numerical simulations, and physical simulation experiments to constrain the controlling factors, regional temperature thresholds, and mechanisms of kaolinite illitization. Results show that during early diagenesis under relatively open-system conditions, meteoric-water flushing and organic-acid activity enhance feldspar dissolution, providing Si and Al sources for kaolinite formation. With increasing burial depth and temperature, the diagenetic system evolves from open to semi-closed/closed conditions; restricted fluid circulation promotes the accumulation of K⁺ released by continued feldspar dissolution in pore waters, thereby providing the key prerequisite for illite nucleation and stabilization. The onset temperature of illitization differs significantly among regions. Illite precipitation is jointly controlled by temperature, K⁺ concentration, and pH, and the critical K⁺ concentration required for illitization decreases with increasing temperature and pH. Combined mineralogical and geochemical constraints indicate that kaolinite illitization proceeds predominantly via a dissolution–reprecipitation mechanism, and its extent is governed by the coupled effects of temperature thresholds, effective K⁺ supply, and the openness of the diagenetic system. These findings provide a basis for characterizing clay-mineral diagenetic behavior and reconstructing paleo-fluid conditions.