Local structural environments of bromine in chlorine-rich minerals: Insights from Br K-edge XAS and 81Br MAS NMR spectroscopy

Halogens such as Cl and Br as geochemical tracers for constraining the sources and evolution of magmatic,metamorphic and hydrothermal systems require knowledge about their structural environments in both rock-forming minerals and coexisting melts/fluids.However,structural studies of Br in rock-forming minerals are challenging due to its exceedingly low concentrations that are not amendable to most structural techniques.In this study,we have conducted the first structural investigation of Br as a trace constituent in selected Cl-rich rock-forming minerals by use of combined 81Br magic angle spin nuclear magnetic resonance (MAS NMR) spectroscopy and Br K-edge synchrotron X-ray absorption spectroscopy (XAS).The whiteline positions of the Br K-edge X-ray absorption near-edge structure (XANES) spectra vary systematically with the coordination numbers of the Cl atoms,except for that in boracite.Similarly,fittings of extended X-ray absorption fine structure (EXAFS) data support the substitution of Br for Cl,with significant local expansions.However,the local structural environment of Br in boracite from EXAFS fittings differs from this of the three-coordinated Cl atom in this mineral but is similar to that of the octahedrally coordinated Br in cubic Mg3B7O13Br,suggesting a domain segregation even at the trace level.These local structural data provide new insights into the Br/Cl behavior in geological systems.