Accuracy of ferric/ferrous determinations in micas: A comparison of Moessbauer spectroscopy and the Pratt and Wilson wet-chemical methods

We compare ferric/ferrous determinations in mica granules and powders, as obtained by the Pratt and Wilson wet chemical (WC) methods and by Moessbauer spectroscopy (MS). The Pratt method is accurate whereas the Wilson method is not but both have the same precision (σ = 1.2 wt.% FeO). Assuming that the Pratt WC method gave accurate ferric/ferrous ratios leads to a calculated ferric/ferrous ratio of MS recoilless fractions at room temperature for a given biotite sample of f3+/f2+ = 1.009(5). Also, the Mica-Fe and Mica-Mg international standards are shown to be unsuitable, with significant size-fraction dependencies of their oxidation states. These results are discussed in the general context of evaluating accuracy and precision of WC methods by comparisons with MS and of the special problems related to accuracy and precision with MS itself