Program > Browse abstracts by speaker > Sun Weipin

Niobium (Nb), essential for high-tech applications, is commonly found in carbonatites where pyrochlore as the predominant ore mineral is prevalent. Previous work indicates that pyrochlore is highly soluble in carbonatite melts [1]. This study experimentally examines how varying conditions affect pyrochlore solubility and reveals the processes that could induce its saturation in these systems.

Our starting materials comprised sintered oxides with the ideal pyrochlore composition mixed with a model sodic-calcitic carbonatite melt. Piston-cylinder experiments were conducted on homogeneous 1:1 mixture in Pt capsules at 1000–1300°C and 1 GPa. The products, saturated with pyrochlore crystals alongside quenched carbonate melt, allowed us to determine pyrochlore solubility. Melt composition effects were evaluated by systematically varying Ca# and CaF₂ (up to 3.84 wt%) and by adding SiO₂ (up to 13.04 wt%).

Results show that pyrochlore solubility increases with higher temperature (lnKsp: –20.82 to –16.20) and decreases with lower melt Si content (lnKsp: –5.79 to –5.17). Variations of F contents in melts appear to have little effect. Although Ca# seems to play a minor role, higher Mg concentrations promote the crystallization of other Nb oxide phases, such as fersmite (CaNb₂O₆) and oboniobite (Mg₄Nb₂O₉), instead of pyrochlore. This work systematically explores how key physicochemical parameters affect pyrochlore solubility in carbonate melts, and it suggests that phlogopite formation during metasomatism consuming Si and Mg might trigger pyrochlore crystallization, thereby offering significant implications for understanding Nb enrichment in carbonatites.

[1] Mitchell, R.H., Kjarsgaard, B.A., 2004. Solubility of niobium in the system CaCO3-CaF2 -NaNbO3 at 0.1 GPa pressure: Implications for the crystallization of pyrochlore from carbonatite magma. Contrib. Mineral. Petrol. 148, 281–287.