Pyrochlore crystallization in carbonatites; the role of fluorine

Carbonatite complexes are the principal source of mineable niobium, mainly as pyrochlore, yet no explanations exist for the considerable variations in niobium content between various carbonatites, and little is known about the solubility of Nb in carbonatite magmas or the factors that control pyrochlore crystallization. The solubility of Nb 2 O 5 and of pyrochlore in synthetic carbonatite magma (CaCO 3 -Na 2 CO 3 liquids) at 1 kb has now been determined and the effect of dissolved fluorine evaluated. Solubility of Nb 2 O 5 in water-bearing calcitic liquids is at least 5 wt % and may be as high as about 7.5 wt %, but diminishes to less than 0.75 wt % in CaCO 3 -Na 2 CO 3 liquids that more closely resemble natural carbonatite magmas