富氯造岩矿物中溴的分子结构

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

卤族元素诸如氯和溴作为地球化学示踪剂,常用于指示岩浆、变质岩和热液的来源和演化过程。而认清溴在造岩矿物中的形态和结构有助于丰富和完善其在地质环境演变中的示踪作用。但是,溴在造岩矿物中的含量极低导致大多数结构分析方法都无法使用,因此造岩矿物中微量溴的结构研究极具挑战性。本文采用81Br魔角旋转核磁共振(MAS NMR)光谱和同步辐射吸收光谱(XAS)技术,首次对富氯造岩矿物中的微量溴进行了结构分析。结果表明溴离子在方硼石中的微区结构不同于该矿物中三配位Cl原子的结构环境,而与Mg3B7O13Br中八面体配位的溴离子相似,表明即使在微量条件下也存在域偏析。而对其他富氯造岩矿物的Br K边X光吸收近边结构(XANES)光谱白线峰的位置和扩展X射线吸收精细结构(EXAFS)分析表明微量溴离子替代了这些矿物中氯的位置,导致局部结构扭曲膨胀。溴离子在造岩矿物中的这一微观结构研究结果可为探索氯和溴在地质演变过程的指示作用提供新的科学依据。

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.