The effect of fluorine on rim growth rates in the ternary CaO-MgO-SiO2 system

An increase in fluorine concentration significantly enhances rim growth rates between periclase and wollastonite, as previously reported by Franke and Joachim-Mrosko (2022). However, the mechanisms responsible for this acceleration, as well as the quantitative relationship between volatile content and rim growth dynamics, remain only partially understood. A more detailed understanding of these processes is critical for our understanding of the role of volatiles in metamorphic and metasomatic reactions.

To address this, we conducted high-p-T experiments at 1000 °C and 1.5 GPa for 20 minutes, with fluorine contents ranging from 1 to 2 wt%. For example, at 1 wt% fluorine, rim growth reached an overall thickness of approximately 50 µm. Building on a previous study (Franke and Joachim-Mrosko, 2022), our preliminary results suggest a linear increase in rim growth rate between 0 and 1 wt% fluorine. However, at fluorine concentrations exceeding 1 wt%, the slope of this linear trend seems to suddenly decrease. Moreover, fluorine is observed to accumulate at the rim–wollastonite interface, indicating a coupled transport of fluorine and MgO.

The reduced growth rate above 1 wt% fluorine may be attributed to a microstructural transition from a layered to a more lamellar rim microstructure, which likely modifies grain and phase boundary properties and thus affects rim growth kinetics.

Reference:
Franke, M., & Joachim-Mrosko, B. (2022). Am. Mineral.