High-pressure phase transition in clinochlore: IIa polytype stabilization

Structural variations of natural clinochlore with pressure have been studied by in situ single-crystal X-ray diffraction (XRD) in a diamond-anvil cell in the pressure range 0–20 GPa at room temperature. High-resolution XRD data allowed for the identification of a polytypic phase transition at about 9 GPa. Around 4.32(5) GPa, the unit-cell parameters exhibited a significant deviation from linear behavior, particularly the c and β values, abruptly interrupted when the phase transition occurs. XRD 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 in the 1960s as the least stable chlorite polytype but has never been observed in natural chlorites. The phase transition also has a significative effect on the bulk modulus, with a reduction from K0 = 81.2(13)t to K0 = 56.0(6) GPa 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 the 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.