The speciation of Fluorine in silicate melts

Understanding the behaviour of the halogens (F, Cl, Br, I) is required to explain the dynamic interplay between the silicate interior and volatile-rich hydrospheres and atmospheres of planets, moons, and exoworlds (Clay et al., 2017; 2019). We identify fluorine as a starting point because it is the most abundant and compatible of the halogens in the Earth system, and can directly impact the nature of Earth’s interior. For example, fluorine affects the thermal stability of mineral phases and the viscosity of silicate melts (Webster et al., 2018). Fluorine is a highly electronegative, monoisotopic, negatively charged atom with a low boiling point and ionic radius similar to both oxygen and hydroxyl ions (OH-) (Crepisson et al., 2014; Dalou et al., 2012). Therefore, understanding the pathways followed by fluorine might illuminate the pathways followed by water within silicate systems (McCubbin et al., 2015). Thus, the mechanics of fluorine dissolution in silicate melts (speciation) require attention.
We have conducted experiments at high temperatures (1250°C) across a range of pressures (0-3 GPa) in a simple Ca-Mg-Al-Si-O system (CMAS) which reproduces the viscosities of intermediate to mafic silicate melts. The speciation of fluorine is studied using solid-state nuclear magnetic resonance (NMR) spectroscopy. Our results show most of the fluorine is bound with aluminium in the melt structure, but minor abundances of Mg-F and Ca-F are also observed in the 19F NMR spectrum. There is no observable effect of pressure and/or temperature. These observations challenge the assumption that fluorine and hydroxyl ions are kin, because OH- form stable compounds with Mg2+ (i.e. as Mg(OH)2, Mookherjee et al. 2008). These data predict decoupling of F- and OH- irrespective of their similar ionic radius and charge. Therefore, assuming the relative behaviours of F- and OH- follow an equilibrium exchange relationship might be (counterintuitively) flawed.