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This paper is based on microprobe analysis (Ta, Nb, Fe, Mn, Ti, W, Sn) and Fe3+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 Fe3+EPR spectra reveal that the coupled substitution: 3Sn4+= (Fe,Mn)2++ 2(Nb,Ta)5+is dominant in cassiterites from pegmatites, and high-temperature quartz veins and in the dark zones of cassiterites.The mechanisms:2Sn4+= Fe3++ (Nb,Ta)5+andSn4++ O2−= Fe3++ OH−are dominant in those from medium-low temperature quartz veins and in the light zones of cassiterites.

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