This paper is based on microprobe analysis (Ta, Nb, Fe, Mn, Ti, W, Sn) and Fe (super 3+) EPR spectra of cassiterite samples from 14 tin deposits from the Iberian Hercynian Massif. The interpretation of microprobe data, atomic ratios (Fe+Mn)/(Nb+Ta), and Fe (super 3+) EPR spectra reveal that the coupled substitution: 3Sn (super 4+) = (Fe,Mn) (super 2+) +2(Nb,Ta) (super 5+) is dominant in cassiterites from pegmatites, and high-temperature quartz veins and in the dark zones of cassiterites. The mechanisms: 2Sn (super 4+) = Fe (super 3+) + (Nb,Ta) (super 5+) and Sn (super 4+) +O (super 2-) = Fe (super 3+) +OH (super -) are dominant in those from medium-low temperature quartz veins and in the light zones of cassiterites.

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