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3.8 g. Speciation of transition metals in silicate melts (H. Keppler and N. Bagdassarov)

The speciation of trace elements in silicate melts influences the partitioning between silicate melts and minerals or fluids. While there are abundant data on the speciation in quenched glasses, there are hardly any studies of trace element speciation in silicate melts. We have therefore measured the crystal field spectra of transition metals (Ni2+, Co2+) in a haplogranitic glass and melt to 1500 °C.

Small changes in transition metal speciation were observed below the bulk glass transformation temperature Tg, but major changes occur above Tg. With increasing temperature, the distortion of the tetrahedral and octahedral sites decreases and the tetrahedral/octahedral ratio increases. The decrease of polyhedral distortion around Co2+ and Ni2+ can be inferred from a decrease of integral extinction coefficients and the disappearance of band splittings. These effects can clearly be seen in Figure 3.8-7. The peak close to 20000 cm-1 is due to octahedral Co2+, while tetrahedral Co2+ absorbs around 15000 cm-1.

Our data show that the speciation of transition metals in silicate melts close to the glass transformation temperature is similar to the speciation in the glass at room temperature; however, the speciation several hundred °C above the glass transformation temperature has little similarity with the room temperature glass. Therefore, structural models derived from spectroscopic studies of glasses have only limited significance for natural magmas at high temperature.

Fig. 3.8-7: Crystal field spectrum of Co2+ in haplogranitic glass (20 °C) and melt (1400 °C). Sample thickness 1 mm, 1 wt% Co. Spectra were measured using a Bruker IFS 120 FT spectrometer coupled with an all-reflecting microscope and a heating stage.

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