Estimating values of Fe3+/ΣFe in pyroxenes with the electron microprobe.

Values of Fe3+/ ΣFe (ΣFe = Fe3+ + Fe2+) were determined for six (6) pyroxenes, (five (5) diopsides and one (1) enstatite), using the single dissolution technique of Fritz and Popp, 1985 (FP85). These same pyroxenes were analyzed with the electron microprobe (EMP), and values of Fe3+/ΣFe were determined using charge balance from average EMP compositions. Typically, these averages were based on thirty (30) analyses on multiple grains of the same sample, although some pyroxenes were characterized with as few as 9 and as many as 120 analyses. Differences between average values of Fe3+/ΣFe determined using the two methods were ≤ 0.02 for pyroxenes with Fe-contents >9 wt% FeO T (all Fe as FeO T). Pyroxenes with lower Fe contents of approximately 4 to 6 wt% FeO T exhibit differences that fall between 0.02 and 0.06. One pyroxene, with the lowest Fe-content (1.79 wt% FeO T ; 0.055 atoms per formula unit or apfu), exhibits the largest difference Fe3+/ ΣFe difference between EMP and FP85 of 0.14. Values of the standard deviation (1σ), based on average values of Fe3+/ ΣFe (EMP), range from 0.03 to 0.22, which are also inversely correlated to the Fe content. For pyroxenes with FeO T > 4.0 wt%, and with compositions similar to those examined in this study, it is possible to use charge balance to determine values of Fe3+/ΣFe(EMP) to better than ±0.07. • Pyroxene stoichiometries, calculated from electron microprobe (EMP) analyses, yield values of Fe3+/SFe (charge balance). • Fe3+/SFe (EMP) for 6 pyroxenes (5 diopsides and 1 enstatite) range from 0.84 ±0.22 to 0.17 ±0.04 (1s). • The precision (1s) is sensitive to Fe content, with 1s ≤0.05 for pyroxenes with > 6 wt% FeO T. • Values of Fe3+/SFe, determined independently, agree with Fe3+/SFe(EMP) to within 0.07 for pyroxenes with > 4 wt% FeO T. • Change balance should, therefore, yield useful values of Fe3+/SFe for a variety of natural pyroxenes. [ABSTRACT FROM AUTHOR]