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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2018: 2.631

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

Online
ISSN
1945-3027
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Volume 100, Issue 1

Issues

New Constraints On Electron-Beam Induced Halogen Migration In Apatite

Michael J. Stock / Madeleine C.S. Humphreys
  • Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, U.K.
  • Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE, U.K.
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/ Victoria C. Smith
  • Research Laboratory for Archaeology and the History of Art, University of Oxford, South Parks Road, Oxford, OX1 3QY, U.K.
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/ Roger D. Johnson / David M. Pyle / Eimf
  • Edinburgh Ion Microprobe Facility, University of Edinburgh, Grant Institute, Edinburgh, EH9 3JW, U.K.
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Published Online: 2015-01-10 | DOI: https://doi.org/10.2138/am-2015-4949

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.

Keywords : Apatite; electron-probe microanalysis; secondary ion mass spectrometry; halogen migration; beam damage

About the article

Received: 2014-02-24

Accepted: 2014-07-15

Published Online: 2015-01-10

Published in Print: 2015-01-01


Citation Information: American Mineralogist, Volume 100, Issue 1, Pages 281–293, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-4949.

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© 2015 by Walter de Gruyter Berlin/Boston. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0

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