DETERMINATION OF Fe3+/FeTotal IN AMPHIBOLES USING THE ELECTRON MICROPROBE

Quantification of Fe³⁺/ Fe_Total (Fe_Total = Fe³⁺+Fe²⁺) in iron-bearing minerals is important in order to accurately apply certain mineral equilibria that are used to determine values of variables such as temperature, pressure, and oxygen fugacity. Given the excellent spatial resolution and relative accessibility of the electron microprobe (EMP), various workers have calibrated this instrument to determine Fe³⁺/Fe_Total in certain minerals. These efforts have typically involved quantifying characteristics of the Fe Lα and/or Fe Lβ peaks. For example, Fialin et al. (2001, Am. Min., 86, 456-465) demonstrated that the location of the Fe Lα peak changes as a function of Fe-content and values of Fe³⁺/Fe_Total, and we have applied this approach to determine values of Fe³⁺/Fe_Total in amphiboles.

The EMP was used to characterize the Fe Lα peaks for three suites of amphiboles, a kaersutite, Ti-bearing pargasite, and pargasitic hornblende. These amphiboles have been experimentally treated such that grains in each of these mineral suites are chemically identical, except for differences in the values of Fe³⁺/Fe_Total and H-content. A linear relation between the Fe Lα peak location - reported relative to the position of the hematite Fe Lα peak - and value of Fe³⁺/Fe_Total was observed for each of these three amphibole suites. The slopes and intercepts of these three lines vary as a function of Fe content. Thus, these amphiboles provided the basis to determine a relation between Fe-content and Fe Lα peak location, both measured with the EMP, and the value of Fe³⁺/Fe_Total as originally determined with ⁵⁷Fe Mössbauer spectroscopy. This relation reproduces the measured values of Fe³⁺/Fe_Total to within ± 0.07 and, therefore, should permit determination of values of Fe³⁺/Fe_Total in amphiboles with Fe-contents from 7 to 13 wt. % FeO with similar precision. The calibration presented here should be applicable to amphiboles with compositions similar to kaersutites, Ti-bearing pargasites and pargasitic hornblendes. Although further verification is necessary, this same calibration may be useful for determining values of Fe³⁺/Fe_Total in amphiboles with distinctly different compositions, and may even be more universally applicable.