From natural observations to an experimental investigation of a potential miscibility gap between Fluorapatite and Fluorbritholite-(Ce)

Evidence for a miscibility gap between fluorapatite (FAp) and fluorbritholite-(Ce) (FBri) has been observed in various natural occurrences, inferred by significant compositional gaps between Ca + P (FAp) and REE + Si (FBri) chemical components.

This presentation delves into the observation of coupled exsolution microtextures of FAp in FBri and vice versa at the Rodeo de los Molles REE Prospect, in Argentina. The existence of a miscibility gap as a function of temperature between FAp and FBri was explored via a series of simple experiments performed at 500°, 600°, 700°, and 1100 °C and 200 MPa. A series of combinations involving synthetic CePO4 or FAp and CeF3 plus H2O and SiO2 and a series of reactants, i.e. Na2Si2O5, NaOH, NaF, CaF, KOH, and Ca(OH)2 was utilized. Experiments involving CePO4 and Na2Si2O5 and NaOH were the most reactive, while experiments involving KOH and Ca(OH)2 produced neither FBri nor FAp. This suggests that Na might play a major role in the formation of FBri.

      On a T vs FAp-FBri composition plot, the miscibility gap of natural samples is situated between FAp92FBri8 and FAp30FBri70 at T below 350 °C. At T above 350 °C the gap narrows down to FAp80FBri20 and FAp68FBri32. In the experiments, FBri-(Ce) shows a continuous compositional range from FAp78FBri22 to FAp28FBri72 for T = 500 °C, FAp76FBri24 to FAp52FBri48 for T = 600 °C and FAp80FBri20 to FAp60FBri40 for T = 700 °C. This indicates that there is no miscibility gap ≥ 500 °C between fluorapatite and fluorbritholite-(Ce).