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Tantalum mineralization invariably is associated with granites and pegmatites that are rich in lithium. This association potentially is explained, at least in part, by chemical interactions between Li and Ta in melts. The effect of lithium on the solubility of tantalite (MnTa₂O₆) in water-saturated granitic melts was therefore experimentally determined at 750° to 1035°C and 2 kbar. High concentrations of Nb, Zr, Hf and W are also common in rare-metal granites and pegmatites, hence the effect of Li on the solubilities of columbite (MnNb₂O₆), zircon (ZrSiO₄), hafnon (HfSiO₄) and wolframite (MnWO₄) in granitic melts were also investigated. The haplogranitic melt compositions used in these experiments have constant mole percent Si and Al, a constant Na/K ratio, and the Al/(Li+Na+K) ratio was 1.0 for all experiments, i.e., the lithium content was the only compositional variable. Columbite and tantalite solubilities in water-saturated granitic liquids increase by a factor of ~2 to 3 with the addition of 2 wt% Li₂O (Fig. 3.6-10a). By contrast, the lithium content of the melt does not affect the solubility of wolframite and the solubilities of zircon and hafnon decrease with increasing Li content of the melt (Fig. 3.6-10b). This most likely reflects the lower field strength of Zr⁴⁺ and Hf⁴⁺,
 

Fig.3.6-10: a) Log solubility products (mol2/kg2) of wolframite [MnO]·[WO4], tantalite [MnO]·[Ta2O5] and columbite [MnO]·[Nb2O5] in granitic melts as a function of lithium concentration. b) Solubilities of zircon and hafnon in granitic melts as a function of lithium concentration, expressed as (mol/kg) Zr and Hf, respectively.

compared to Nb⁵⁺ and Ta⁺⁵, and consequently Zr and Hf are less able to compete with Li for non-bridging oxygens. The lack of dependence of wolframite solubility on Li content is consistent with W having a 6+ valence and behaving as a network former.

It is unlikely that natural melts are saturated with columbite-tantalite at 750°C, but this temperature is higher than the crystallization temperature of highly evolved granites. The solubility data can be extrapolated to 600°C, a more reasonable temperature, then be compared to abundances of Li-F-Mn-Fe-Ta-Nb in natural rocks and glasses that are representative of the liquids from which rare-metal granites and pegmatites crystallize. It is estimated that at 600°C these melt compositions could be saturated in columbite, but not tantalite. By contrast, tantalite saturation is predicted if the melts contained less Li and F. A potential explanation of the genesis of tantalum mineralization is that Ta is retained in the melt because of high Li-F concentrations. Tantalite crystallization is delayed until a Li±F±P mineral crystallizes, which lowers tantalite solubility, and results in a general association of Ta with Li.