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Fe (super 3+) /Sigma Fe vs FeLalpha peak energy for minerals and glasses; recent advances with the electron microprobe

Michel Fialin, Christiane Wagner, Nicole Metrich, Eric Humler, Laurence Galoisy and Antoine Bezos
Fe (super 3+) /Sigma Fe vs FeLalpha peak energy for minerals and glasses; recent advances with the electron microprobe
American Mineralogist (April 2001) 86 (4): 456-465

Abstract

This paper describes a preliminary study that attempts to determine the oxidation state of Fe (Fe (super 3+) /Sigma Fe) with the electron microprobe (EMP) by measuring the self-absorption induced shift of the FeLalpha peak emitted from minerals and glasses. In transition metals of the first row, the L-spectra exhibit common distortions, namely peak position shifts, peak shape alterations, and changes in the Lbeta /Lalpha ratios, caused by the large difference in the self-absorption coefficients (mu /rho ) on either sides of the L (sub 3) absorption edges that are in close proximity to the Lalpha peak maxima. Measurements performed on alpha -Fe (sub 2) O (sub 3) and Fe (sub x) O oxides have shown that self-absorption effects are stronger for the later oxide, leading to enhanced Fe (super 2+) Lalpha peak shift toward longer wavelengths as the beam energy increases. First measurements performed on silicates have confirmed that enhanced self-absorption of FeLalpha occurs on Fe (super 2+) sites. The measurements consisted of plotting the FeLalpha peak position at a fixed beam energy (15 KeV) against the total Fe concentration for two series of Fe (super 2+) - and Fe (super 3+) -bearing silicates. In a first step, these data have shown that both Fe (super 2+) Lalpha and Fe (super 3+) Lalpha peaks shift continuously toward longer wavelengths as the Fe concentration increases, with enhanced shifts for Fe (super 2+) Lalpha . For silicates containing only Fe (super 2+) or Fe (super 3+) , no effects of the site geometry were detected on the variations of the FeLalpha peak position. A second set of plots has shown the variations of the peak position relative to the previous Fe (super 2+) -Fe (super 3+) curves of step 1, as a function of the nominal Fe (super 3+) /Sigma Fe, for a series of reference minerals (hydrated and non-hydrated) and basaltic glasses. Data from chain and sheet silicates (e.g., pyroxenes, amphiboles, micas) exhibited strong deviations compared to other phases (e.g., garnets, Al-rich spinels, glasses), due to reduced self-absorption of FeLalpha . Intervalence-charge transfer (IVCT) mechanisms between Fe (super 2+) and Fe (super 3+) sites may be the origin of these deviations. These crystal-structure effects limit the accuracy of the method for mixed Fe (super 2+) -Fe (super 3+) valence silicates. Precisions achieved for further Fe (super 3+) /Sigma Fe measurements strongly depend on the total Fe concentration. For basaltic glasses containing an average of 8 wt% Fe and 10% Fe (super 3+) /Sigma Fe, the precision is about + or -2% (absolute). For low Fe concentrations (below 3.5 wt%), the uncertainty in the peak position measured by the EMP spectrometers leads to error bars that are similar to with the separation of the curves fitted to the Fe (super 2+) and Fe (super 3+) plots, which is propagated as prohibitive lack of precision for Fe (super 3+) /Sigma Fe (>70% relative). A major limitation of microbeam methods in general deals with beam damage. This aspect has been carefully studied for basaltic glasses, and optimal beam conditions have been established (in general, electron doses higher than those corresponding to 130 nA and 30 mu m beam diameter should be avoided to prevent large beam induced oxidation phenomena). Additional work, in progress, concerns: (1) other beam-sensitive phases such as hydrated glasses; and (2) minerals in which FeLalpha is affected by large matrix effect corrections (e.g., Cr-and Ti-rich oxides where FeLalpha is strongly absorbed), for which the self-absorption-induced shift of FeLalpha is different from that of common silicates and glasses.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 86
Serial Issue: 4
Title: Fe (super 3+) /Sigma Fe vs FeLalpha peak energy for minerals and glasses; recent advances with the electron microprobe
Affiliation: Universite de Paris VI, Centre de Microanalyse Camparis, Paris, France
Pages: 456-465
Published: 200104
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 60
Accession Number: 2001-049388
Categories: General geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Secondary Affiliation: Laboratoire Pierre Suee, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200115
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