New constraints on electron-beam induced halogen migration in apatite

• Published in: The American mineralogist Vol. 100; no. 1; pp. 281 - 293
• Main Authors: Stock, Michael J, Humphreys, Madeleine C. S, Smith, Victoria C, Johnson, Roger D, Pyle, David M
• Format: Journal Article
• Language: English
• Published: Mineralogical Society of America 01.01.2015
• Subjects: Apatite; beam damage; chlorapatite; chlorine; Durango Mexico; electron probe data; electron-probe microanalysis; electrons; fluorapatite; fluorine; halogen migration; halogens; instruments; ion probe data; irradiation; mass spectra; Mexico; Mineralogy; nonsilicates; phosphates; secondary ion mass spectrometry; spectra; techniques
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-2015-4949

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 FKα 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.