New Constraints On Electron-Beam Induced Halogen Migration In Apatite
New Constraints On Electron-Beam Induced Halogen Migration In Apatite
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DOI：
10.2138/am-2015-4949
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发表时间：
2015
影响因子：
3.1
通讯作者：
M. Stock;M. Humphreys;M. Humphreys;V. Smith;Roger D. Johnson;D. Pyle;Eimf
中科院分区：
文献类型：
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作者：
M. Stock;M. Humphreys;M. Humphreys;V. Smith;Roger D. Johnson;D. Pyle;Eimf
摘要在磷灰石的电子探针分析（EPMA）中，氟和氯的X射线计数率变化很大。由于这种变化的速率、时间和幅度是磷灰石取向和组成以及EPMA操作条件的函数，因此这代表了磷灰石中挥发性元素分析的一个重要问题。尽管这种效应被认为是对电子束暴露的内在晶体学反应，但计数率变化的机制和原因仍不清楚。我们通过直接检查电子束曝光对磷灰石的影响来解决这个问题，通过执行二次离子质谱（西姆斯）以前受到电子束照射的点的深度分布。在氟磷灰石的照射过程中，定向为c轴平行于电子束，卤素逐渐集中在样品表面，即使在相对低功率（15 nA，10-15 kV）的光束。这种表面富集对应于EPMA FKa X射线计数率的增加。经过长时间的照射，表面区域开始失去卤素，并逐渐耗尽，相应的电子探针计数率下降。在正常的EPMA操作条件下，在氟磷灰石中没有卤素再分布，氟磷灰石的c轴垂直于电子束，或氯磷灰石。我们推断，阴离子富集的结果从卤素的迁移远离电荷积累的中心所造成的电子从电子探针束的注入，辅助由电子-物质相互作用引起的热梯度。表面富集的过程是最好的解释卤素迁移通过间隙晶体学网站在c轴通道。这表明，一旦热场和电场被移除，卤素可能会在很长的时间尺度上或随着样品加热而松弛回到其原始位置。
Abstract Fluorine and chlorine X‑ray count rates are known to vary significantly during electron probe microanalysis (EPMA) of apatite. Since the rate, timing, and magnitude of this variation are a function of apatite orientation and composition, as well as EPMA operating conditions, this represents a significant problem for volatile element analysis in apatite. Although the effect is thought to be an intrinsic crystallographic response to electron-beam exposure, the mechanisms and causes of the count rate variability remain unclear. We tackle this by examining directly the effects of electron-beam exposure on apatite, by performing secondary ion mass spectrometry (SIMS) depth profiles of points previously subject to electron-beam irradiation. During irradiation of fluorapatite, oriented with the c-axis parallel to the electron beam, halogens become progressively concentrated at the sample surface, even under a relatively low power (15 nA, 10-15 kV) beam. This surface enrichment corresponds to an observed increase in EPMA FKa X‑ray count rates. After prolonged irradiation, the surface region starts to lose halogens and becomes progressively depleted, corresponding with a drop in EPMA count rates. Under normal EPMA operating conditions there is no halogen redistribution in fluorapatite oriented with the c-axis perpendicular to the electron beam, or in chlorapatite. We infer that anionic enrichment results from the migration of halogens away from a center of charge build-up caused by the implantation of electrons from the EPMA beam, assisted by the thermal gradient induced by electron-matter interactions. The process of surface enrichment is best explained by halogen migration through interstitial crystallographic sites in the c-axis channel. This suggests that once the thermal and electric fields are removed, halogens may relax back to their original positions on very long timescales or with sample heating.