Thermal Analysis, Compressibility, and Decomposition of Synthetic Bastnäsite-(La) to Lanthanum Oxyfluoride

Understanding basic material properties of rare earth element (REE) bearing minerals such as their phase stability and equations of state can assist in understanding how economically viable deposits might form. Bastnäsite is the most commonly mined REE bearing mineral. We synthesized the lanthanum-fluoride end member, bastnäsite-(La) (LaCO3F), and investigated its thermal behavior and decomposition products from 298 K to 1173 K under ambient pressure conditions through thermogravimetric analysis, differential scanning calorimetry, evolved gas analysis, and high temperature powder X-ray diffraction. We also investigated the compressibility of bastnäsite-(La) via single crystal X-ray diffraction in diamond anvil cells at an ambient temperature up to 11.3 GPa and from 4.9 GPa to 7.7 GPa up to 673 K. At ambient pressure, bastnäsite-(La) was stable up to 598 K in air, where it decomposed into CO2 and tetragonal γ-LaOF. Above 948 K, cubic α-LaOF is stable. High temperature X-ray diffraction data were used to fit the Fei thermal equation of state and the thermal expansion coefficient α298 for all three materials. Bastnäsite-(La) was fit from 298 K to 723 K with V0 = 439.82 Å3, α298 = 4.32 × 10−5 K−1, a0 = −1.68 × 10−5 K−1, a1 = 8.34 × 10−8 K−1, and a2 = 3.126 K−1. Tetragonal γ-LaOF was fit from 723 K to 948 K with V0 = 96.51 Å3, α298 = 2.95×10−4 K−1, a0 = −2.41×10−5 K−1, a1 = 2.42×10−7 K−1, and a2 = 41.147 K−1. Cubic α-LaOF was fit from 973 K to 1123 K with V0 = 190.71 Å3, α298 = −1.12×10−5 K−1, a0 = 2.36×10−4 K−1, a1 = −1.73 × 10−7 K−1, and a2 = −17.362 K−1. An ambient temperature third order Birch–Murnaghan equation of state was fit with V0 = 439.82 Å3, K0 = 105 GPa, and K’ = 5.58.