Content of iron cations in three biotite specimens of true trioctahedral mica were determined by Mössbauer spectroscopy, electron microprobe and wet-chemistry methods. International certified reference materials were analyzed simultaneously with micas to evaluate the accuracy of the wet-chemistry method. High precision Mössbauer spectroscopic Fe³⁺/Fe²⁺ ratios coupled with the electron microprobe iron determinations were compared with the wet-chemical data. Comparisons of data show that in wet-chemistry method powdered micas dissolve more readily during acid attack than the granular micas and thus yield higher precision and accuracy.