The in‐situ method of Raman spectroscopy was used to study the layered mineral phengite, K(Al,Mg)2(Si,Al)4O10(OH)2, compressed in water under simultaneously high temperatures and pressures (respectively, up to 373 °С and 12.5 GPa). The implemented conditions were typical of modeling the ‘cold’ subduction zones in lithospheric slabs. The high pressures and temperatures were produced in an electrically heated diamond‐anvil cell. Measured Raman spectra have demonstrated a high Р–Т stability of the mineral. No non‐quenchable phengite states (no reversible or irreversible polymorphic transitions, overhydration or notable amorphization) were observed in the investigated samples. Copyright © 2017 John Wiley & Sons, Ltd. In‐situ method of Raman spectroscopy was used to study the layered phengite compressed in water under simultaneously high P–T (up to 373 °С and 12.5 GPa). The implemented conditions were typical of modeling the “cold” subduction zones in lithospheric slabs. Raman spectra have demonstrated a high Р–Т stability of the mineral. No non‐quenchable phengite states were observed.