Evidence of irreversible CO2 intercalation in montmorillonite

► Combined same-sample data ascertain anomalous CO2 intercalation in montmorillonite. ► Supercritical CO2 trapping via extreme confinement is partly irreversible. ► CO2-H2O mixing in the interlayer can result in its irreversible expansion. ► Long-term CO2 trapping in the interlayer can result in carbonation.