Optimizing the electron microprobe analysis of hydrous alkali aluminosilicate glasses

Journal Article · Sun Sep 01 04:00:00 EDT 1996 · American Mineralogist

DOI:https://doi.org/10.2138/am-1996-9-1016· OSTI ID:543690

Morgan, G.B. VI; London, D ^[1]

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1. Univ. of Oklahoma, Norman, OK (United States)

The time-dependent loss of NaK{alpha} X-ray intensity during electron-beam irradiation of hydrous alkali aluminosilicate glasses is apparently more significant during the initial few seconds of beam exposure than it is for anhydrous glasses, and it is pronounced for incident beam currents >2-5 nA (using 15-20 {mu}m beam diameters). Exponential fits of NaK{alpha} intensity vs. time show a progressive decrease in the apparent zero-time intercepts for incident beams from 2 to 20 nA, and thus methods for correcting Na concentrations solely on the basis of curve fitting and extrapolation to zero-time values may underestimate Na contents by almost 10% (relative) for higher beam currents. Similar exponential fits to the intensity-time data for AlK{alpha} and SiK{alpha} show that {open_quotes}grow-in{close_quotes} is greater for Al than for Si. For incident currents {ge}5 nA, the magnitudes of all intensity changes also increase with total H{sub 2}O content of glass. On the basis of these observations, the optimal conditions for analysis of hydrous alkali aluminosilicate glasses include a 2 nA beam with 20 {mu}m diameter and counting times of 20-40 s for the analysis of alkali aluminosilicate components, with Na and Al analyzed first (simultaneously, if possible). These methods minimize Na loss and grow-in for Al and Si to the point that little or no correction is needed, provide good statistical accuracy, and work with a wide variety of standard materials (i.e., glass standards with compositions and H{sub 2}O contents comparable to the unknowns are not needed). 19 refs., 6 figs., 2 tabs.

• Sponsoring Organization: USDOE
• OSTI ID: 543690
• Journal Information: American Mineralogist, Journal Name: American Mineralogist Journal Issue: 9-10 Vol. 81; ISSN AMMIAY; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
58 GEOSCIENCES
ELECTRON MICROPROBE ANALYSIS
GEOCHEMISTRY
GLASS
PHASE STUDIES