Talc, as an important class of clay minerals constituting subducting oceanic crust, has long been known to undergo interlayer expansion by ~6% to contain net ~13 wt.% water into the ‘so-called’ 10 Å phase. Although subduction fluid is mildly alkaline and includes various salts and other dissolved species, its effect on the stability of subducting minerals has not yet been considered. Here, we report that subducting talc, when exposed to alkaline salty water conditions, breaks down to form a super-hydrated 15 Å phase at ~3.0 GPa and ~350 °C, corresponding to a depth of ~90–95 km along a cold subduction geotherm. The 15 Å phase remains stable down to ~125 km depth, where it transforms into the previously known 10 Å phase. Our combined experimental and computational results show that the super-hydrated 15 Å phase contains net ~31 wt.% water through interlayer expansion by ~60%. Our work thus demonstrates mineral transformation under more realistic subduction environments, which calls for reevaluation of subduction-related geochemistry and seismicity as well as water transportation into the deep Earth.