Influence of fluorine on the solubility of manganotantalite (MnTa2O6) and manganocolumbite (MnNb2O6) in granitic melts — An experimental study

Solubility experiments of manganotantalite (MnTa2O6) and manganocolumbite (MnNb2O6) were conducted under nearly fluid-saturated conditions in synthetic haplogranitic (HPG) and natural granitic (NG) melts containing different amounts of fluorine at 800 to 1000 °C and 200 MPa. The experiments were carried out in cold seal pressure vessels (at 800 °C) with water or Ar as a pressure medium and in internally heated pressure vessels (1000 °C) with Ar as a pressure medium. The F contents vary from 0 to 11.43 wt.% in experiments using HPG melts and from 0.61 to 3.78 wt.% in experiments using NG melts. Additionally, the NG melt contains minor amounts of P2O5 (0.44 to 0.64 wt.%).In HPG melts (alumina saturation index including Mn (ASIMn): 0.88 to 0.99) the solubility product values at 800 °C yield log (KspTa) and log (KspNb) ranging from − 3.36 ± 0.10 to − 2.92 ± 0.10 mol²/kg² and from − 3.01 ± 0.11 to − 2.87 ± 0.06 mol²/kg², respectively. In NG melts the log (KspTa) and log (KspNb) vary from − 3.00 ± 0.04 to − 2.35 ± 0.13 mol²/kg² and from − 2.83 ± 0.06 to − 2.67 ± 0.18 mol²/kg² at 800 °C, respectively. The solubility products of both MnTa2O6 (KspTa) and MnNb2O6 (KspNb) are not significantly affected by the F content for a given melt composition (data are virtually identical within error). The variation of the solubility products observed in this study is rather attributed to the changes in melt structure (e.g. polymerization) and, in particular, to the alumina saturation index. By contrast, there is no evidence for the formation of complexes associating F and Nb and/or Ta. Our results also indicate that the structural changes caused by the incorporation of F in natural water-rich silicate melts are not expected to significantly influence the maximum concentrations of Nb and Ta in granitic or pegmatitic melts coexisting with manganotantalite and manganocolumbite.

Research Highlights
► F has no effect on tantalite and columbite solubility in H2O-rich granitic melts.
► In natural systems KspTa and KspNb may be higher than in synthetic Fe-free melts.
► Increasing concentration of Fe2+ in granitic melts leads to an increase of KspTa.