Goldilocks effect of fluorine and chlorine in albitisation

Abstract

Hydrothermal fluids enriched in halogens (e.g., F and Cl) are essential for ore formation as well as associated alteration processes. Chloride-bearing solutions facilitate mineral re-equilibration and metal mass transfer by forming soluble complexes with Cl−. Fluorine is enriched in a variety of hydrothermal ore deposits and their alteration halos, but the significance of this enrichment remains controversial. Fluorine may simply reflect the source of the fluids, but may also play an active role in controlling the metal budget, with four main hypotheses having been proposed: (i) fluoride increases the mobility of some metals by forming stable coordination complexes; (ii) fluoride acts as a precipitating agent, due to the low solubility of many fluoride minerals; and fluorine improves fluid flow either (iii) by promoting silicate dissolution and creating fluid pathways under acidic conditions (HF(aq)), or (iv) by affecting the kinetics of interface coupled dissolution-reprecipitation reactions by modifying the dissolution and nucleation processes. Here, we tested the least studied mechanism (iv) by performing experiments to examine the effects of fluoride and chloride ions on the fluid-mediated albitisation of perthite (intergrowth of albite and microcline) at 600 °C and 2 kbar. We found that the albitisation rate depends on the Cl/(Cl + F) ratio, but not in a linear manner: reaction rates reach a maximum in solutions featuring ‘Goldilocks’ Cl/(Cl + F) ratios between 0.5– 0.99 (i.e., Cl/F is between 1 and 100). Based on recent advances in halogen chemistry, we propose that the observed Goldilocks effect was caused by non-covalent interactions between the dihalogen ClF(aq) and Si(OH)4(aq). To test this hypothesis, we calculated the bonding energy between the Si(OH)4(g) and ClF(g) molecules via Car-Parrinello first principle calculations. The positive values obtained indicate that ClF(g) can form a stable halogen bond with Si(OH)4(g). If confirmed experimentally in hydrothermal fluids, such complexes could make process (iv) a powerful and widely applicable mechanism for increasing silicate reaction kinetics, thereby promoting efficient coupling between fluid flow and fluid-rock interaction, a key factor in the development of mineral systems.