First in-situ single crystal determination of IIa polytype in clinochlore and its relevance in the high-pressure behavior of chlorite

Chlorite is one of the most important water-bearing phases and its behavior at high pressure conditions has a significant role in the modeling of dehydration processes in subduction zones [1,2] and volatile transfer. Due to its propensity for stacking disorder, finding natural undistorted single crystals of chlorite is difficult, leading to a lack of available accurate data on its high-pressure crystallographic and physical properties. The adequate sample selection (natural clinochlore, Val d’Ala, Italy) allowed us to assess the stability conditions of a natural clinochlore single-crystal sample (nominally Mg5Al2Si3O10(OH)8), performing a high-pressure in-situ diffraction experiment with high-flux synchrotron radiation up to 20 GPa. Between 8 and 9 GPa, a polytypic phase transition from the IIb- to the IIa-structure occurs, resulting in the stabilization at high pressure of an uncommon chlorite polytype. It corresponds to the highest energy polytipe [3], to date never found in natural chlorite samples. The technically challenging experiment unequivocally allowed the ab-initio experimental structural determination of this high-pressure polymorph, and it confirms predictions of polytypic stability dating back to the 1960s and subsequently widely discussed based on X-ray powder diffraction data [4]. The transition is reversible upon decompression. Our results offer valuable insights into the high-pressure structural properties of chlorite, and we believe it has far-reaching implications in mineralogy and planetary sciences. We confirm that these phyllosilicates can undergo multiple reversible polytypic phase transitions to compensate and counterbalance important stress states without decomposition or amorphisation. Since phyllosilicates are considered primary candidates among the minerals present in meteorites for transferring volatiles during the accretion of planetary covers, the persistence of their metastable states in a suitable region of P-T-Time space could be a mechanism for the accumulation of volatile components in planetary bodies. [1] Poli, S. et al. (2002): Petrology of subducted slabs. Annu. Rev. Earth Pl. Sci., 30, 207-235.[2] Fumagalli, P. et al. (2014): Contrib. Mineral. Petrol., 167, 979.[3] Brown, B.E. et al. (1962): Am. Mineral., 47, 819-850.[4] Welch, M.D. et al. (2004): Am. Mineral., 89, 1337-1340