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Speciation of Arsenic and Antimony in Basaltic Magmas
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2020-05-01 , DOI: 10.1016/j.gca.2020.02.022
Bryan J. Maciag , James M. Brenan

Abstract This study applies X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structures (EXAFS) spectroscopy at the K-edge to determine the speciation of arsenic and antimony in a suite of basaltic glasses synthesized over a range of oxygen fugacity (fO2) at 1200 °C and 0.1 MPa. Experiments were executed in evacuated fused silica ampoules using a variety of solid metal metal-oxide buffers to achieve fO2’s ranging from FMQ −3.3 to FMQ +5.7 (where FMQ is the fayalite magnetite quartz buffer). The oxidation state was calculated using linear combination fitting (LCF) to spectral reference material with known oxidation states using the XANES spectra. Speciation results were corrected for the quench effect of iron. Trivalent arsenic and antimony were determined to be the dominant oxidation state in the samples, with pentavalent arsenic contributing to less than 10% of the budget of arsenic unless the fO2 is greater than FMQ +5.3 ± 0.9, while pentavalent antimony was not observed. Additionally, no reduced oxidation states of arsenic or antimony were found in the glasses even at the lowest fO2 investigated. Structural parameters such as the coordination number and bond length were determined by fitting theoretical electron scattering paths to the EXAFS spectra. Arsenic is coordinated by three oxygens at 1.78 ± 0.01 A forming AsIIIO3E (where E is the lone pair of electrons) trigonal pyramids. Antimony is coordinated by three oxygens at 1.98 ± 0.01 A, interpreted to be in a trigonal pyramid structure similar to As3+. As both metalloids are primarily present in the trivalent state over the range of terrestrial fO2 (FMQ −3 to FMQ +5) it is expected that each will behave incompatibly during basaltic melting or crystallization, owing to (1) the tendency for these elements to form oxyanions, and (2) their poor match in ionic radius and charge for the major cation sites in mafic minerals.

中文翻译:

玄武岩浆中砷和锑的形态

摘要 本研究在 K 边缘应用 X 射线吸收近边结构 (XANES) 和扩展 X 射线吸收精细结构 (EXAFS) 光谱来确定在一系列合成的玄武岩玻璃中砷和锑的形态。 1200 °C 和 0.1 MPa 时的氧逸度 (fO2)。实验在真空熔融石英安瓿中使用各种固体金属金属氧化物缓冲液进行,以达到 FMQ -3.3 至 FMQ +5.7(其中 FMQ 是铁橄榄石磁铁矿石英缓冲液)的 fO2。使用 XANES 光谱对具有已知氧化态的光谱参考材料使用线性组合拟合 (LCF) 计算氧化态。铁的淬火效应校正了形态结果。三价砷和锑被确定为样品中的主要氧化态,五价砷占砷预算的 10% 以下,除非 fO2 大于 FMQ +5.3 ± 0.9,而未观察到五价锑。此外,即使在所研究的最低 fO2 下,玻璃中也未发现砷或锑的氧化态降低。通过将理论电子散射路径与 EXAFS 光谱拟合来确定结构参数,例如配位数和键长。砷由 1.78 ± 0.01 A 的三个氧配位形成 AsIIIO3E(其中 E 是孤对电子)三角金字塔。锑由 1.98 ± 0.01 A 的三个氧配位,解释为类似于 As3+ 的三角金字塔结构。
更新日期:2020-05-01
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