Natural clinochlore structural variations with pressure have been studied by in-situ single crystal X-ray diffraction in diamond-anvil cell in the pressure interval 0-20 GPa at room temperature. High resolution data allowed for the identification of a polytypic phase transition at about 9 GPa. Around 4.32(5) GPa, the sample showed a significant deviation from linear behavior of the unit cell parameters, particularly in the c and β values, abruptly interrupted when the phase transition occurs. The X-ray diffraction patterns showed a drastic reduction of diffuse scattering due to the stabilization of the high-pressure structure, suggesting that the atomic reorganization of the layers led to a disorder reduction. The phase transition showed complete reversibility during the experiment. Ab-initio structural refinements identified the transition as polytypic, from the initial IIb-4 triclinic polytype (space group C1) to the IIa-1 monoclinic structure (space group C2/m), with unit cell parameters a=5.2058(6) Å, b=9.0208(4) Å, c=13.560(7) Å, β=97.34(3)°. The latter was theoretically derived back in the ’60s as the least stable chlorite polytype and has never been observed before in natural chlorites. The phase transition also has a significative effect on the bulk modulus, with a reduction from K0=81.2(13) GPa to K0=56.0(6), for the high-pressure structure. An isothermal run at 600 K from ambient pressure to 14 GPa showed the same phase transition at 7.8(5) GPa. Its occurrence at lower pressures suggests a negative P/T slope for this transition. Therefore, at high-temperature and high-pressure conditions compatible with impact phenomena, the polytypic phase transition could prevent chlorite from early destabilization and dehydration.

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