Niobium solubility and pyrochlore, lueshite and Ca2Nb2O7 stability in CaCO3 – Na2CO3 – NaNbO3 +/− CaF2 at 0.1 GPa, with application to the formation of calciocarbonatite-hosted pyrochlore ore deposits

Experimental data are presented for an evaluation of the solubility of niobium in the ternary system CaCO3 – Na2CO3 – NaNbO3 (or calcite – sodium carbonate – lueshite) and in the quaternary system CaCO3 – Na2CO3 – NaNbO3 – CaF2 (fluorite) or over the temperature range 700 to 1050 °C and 0.1 GPa pressure. Phase relationships are given for the ternary system and as a ternary section in the quaternary system with 5 wt% CaF2 to define the primary crystallizing phase fields for calcite, lueshite, pyrochlore and the Ca-Nb perovskite phase Ca2Nb2O7. In the ternary system the data show the presence of a large field of calcite plus liquid, lueshite plus liquid, and Ca2Nb2O7 plus liquid. Niobium (Nb) solubility along the calcite - Ca2Nb2O7 cotectic decreases from ~7.5 to 0.56 wt% Nb2O5 with decreasing Ca/(Ca+Na). With the addition of 5 wt% CaF2 to the ternary system, the data show the presence of a large field of calcite plus liquid, and pyrochlore plus liquid, with the stability field of lueshite greatly reduced in size. Niobium (Nb) solubility along the calcite - pyrochlore cotectic decreases uniformly with decreasing Ca/(Ca+Na) and temperature from ~32 wt% Nb2O5 at Cc60Lu40 to 0.8 wt% Nb2O5 at Cc65Nc35 at which point it remains near constant at ~0.56 wt% Nb2O5 with further decreasing Ca/(Ca+Na) and temperature. Two parental calciocarbonatite melt compositions are modeled with our new Nb solubility versus temperature curve to suggest that pyrochlore will crystallize when the carbonatite melt reaches 1000-1400 ppm Nb at ~850 °C.