In-situ HP-HT synchrotron XRD of chlorite: phase transition and geological significance of a disordered layered mineral studied with Diamond Anvil Cell and Large Volume Press

Chlorite is one of the most significant water-bearing minerals in subduction zones, and its behavior under high-pressure (HP) and high-temperature (HT) conditions plays a crucial role in modeling dehydration processes [1-2]. A previous study on single crystals of natural clinochlore from Val d’Ala (Italy) determined its bulk modulus at both room temperature and 600 K [3]. Building upon these results, we now present the first single-crystal P–V–T equation of state for clinochlore up to 4.5 GPa, derived by combining previous data with a synchrotron single-crystal X-ray diffraction (SC-XRD) study conducted at room pressure across a temperature range from 100 K to 930 K on the same sample (XRD1 beamline, Elettra). As reported in Chrappan Soldavini et al. [3], clinochlore exhibits two HP-induced phase transitions. The second one is marked by a distinct volume reduction (~0.7%) at ~8.5 GPa and a notable change in the bulk modulus. This transformation has been attributed to a polytypic phase transition involving the stabilization of a chlorite polytype theoretically predicted by Brown & Bailey [4], but not previously observed. Although this transition occurs beyond the stability field of clinochlore in geological systems, its negative slope in P–T space suggests that it could become relevant under HT conditions characteristic of subduction environments, where oceanic crust transfer water into the deep mantle. We report two in-situ experiments conducted at the ID06-LVP beamline (ESRF), using a large-volume press coupled with synchrotron powder X-ray diffraction, both starting from the same clinochlore sample and simulating subduction-like P–T paths. Both experiments revealed the same phase transition observed in previous SC-XRD studies and confirm a negative slope. Moreover, an in-situ diffraction studies on Fe-rich chlorite single crystal (ID15b beamline, ESRF) confirmed the same transition around 8.5 GPa, indicating that chlorite composition has a limited effect on its HP behavior. Additional in-situ powder X-ray diffraction experiments (MCX beamline, Elettra) were conducted on natural chlorite samples with varying iron and/or chromium contents to explore the crystal-chemical influence on their thermal expansion behavior. Our findings provide new insights into the crystal-chemical and thermoelastic properties of chlorite, highlighting the role of polytypic transitions in subduction processes. We propose that this structural transformation, which leads to a denser structure, is associated with an expanded stability field of this hydrous silicate. This increased stability might delay its breakdown under current Earth’s geothermal conditions, allowing it to transform directly into a new dense hydrous silicate at mantle pressures and temperatures, as recently identified and reported in Gemmi et al. [5]. The potential broadening of the chlorite stability field could have significant implications for water storage and transport into the mantle, contributing to the development of a deep-water cycle connecting the Earth's crust to the mantle transition zone. The presence of a hydrous layer in the mantle is supported by the occurrence of hydrous silicates in diamond inclusions, as reported by Pearson et al. [6].