The effect of phosphorus on manganocolumbite and mangaotantalite solubility in peralkaline to peraluminous granitic melts

Abstract Solubility experiments of Mn-columbite (MnNb2O6) and Mn-tantalite (MnTa2O6) were conducted under water-saturated conditions in synthetic haplogranitic melts containing different amounts of phosphorus at 800 °C and 100 MPa. All experiments were carried out in cold-seal rapid quenching pressure vessels (RQV) with water as a pressure medium. Experimental results show that: (1) the solubilities of MnNb2O6 and MnTa2O6 in peralkaline melts are higher than those in peraluminous melts; (2) phosphorus has strong influence on the solubilities of MnTa2O6 and MnNb2O6 in peralkaline melts, KSpNb ${\rm{K}}_{{\rm{Sp}}}^{{\rm{Nb}}}$ and KSpTa ${\rm{K}}_{{\rm{Sp}}}^{{\rm{Ta}}}$ decrease from 104.89 × 10–4 mol2/kg2 and 107.62 × 10–4 mol2/kg2 for melts without P2O5 to 16.11 × 10–4 mol2/kg2 and 7.96 × 10–4 mol2/kg2 for melts containing ~4.00 wt% P2O5, respectively; (3) phosphorus has less influence on the solubilities of MnTa2O6 and MnNb2O6 in peraluminous melt, KSpNb ${\rm{K}}_{{\rm{Sp}}}^{{\rm{Nb}}}$ decrease from 4.50 × 10–4 mol2/kg2 for melts without P2O5 to 0.73 × 10–4 mol2/kg2, and KSpTa from 3.57 × 10–4 mol2/kg2 to 0.14 × 10–4 mol2/kg2 for melts contaning ~5.00 wt% P2O5. Taking the structural role of phosphorus into account, P decreases the solubility of Mn-columbite and Mn-tantalite via competing for network modifiers.

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