RARE EARTH OXYPHOSPHATES: PRESSURE-TEMPERATURE CONSTRAINT FOR IMPACT MELTS?

Rare earth (RE) orthophosphates – monazite and xenotime - are among the most refractory minerals known. Their synthetic analogues are extensively studied for applications ranging from laser hosts to matrix for actinides immobilization. The family of rare earth oxyphosphates (REOPs) compounds were first discovered almost half a century ago [1] after melting RE orthophosphates in a solar furnace. Their composition can be expressed by the general formula (RE₂O₃)_xRE(PO₄) with RE/P ratios 7:3, 3:1, 4:1 and 5:1 (no Ce oxyphosphates were identified). Surprisingly, REOPs were studied very little. The structures were determined only for Nd₃PO₇ and closely related Eu₃PO₇ prepared as single crystals.

The experimental and computational study of structures, thermodynamic and physical properties of REOPs is underway as part of Multi University Research Initiative "Planetary- and Geologically-Inspired Discovery of Refractory Materials." REOPs were synthesized by co-precipitation and annealing at 1400 °C and by laser melting of REPO₄. The first transmission electron microscopy (TEM) results indicate that Y oxyphosphates retain crystallinity after prolonged annealing and ball milling and are stable under the beam.

The temperature of impact melts above 2370 °C has been proposed based on observation of microstructures indicating melted ZrO₂ after zircon decomposition [2]. Our work prompts for TEM investigation of xenotime from impact sites. If oxyphosphates inclusions are identified, they can be used as pressure-temperature constraints when thermodynamic data for REOPs become available.

[1] J.J. Serra, J. Coutures, A. Rouanet, Thermal treatment of lanthanide orthophosphates (LnPO4) and the formation of new (oxyphosphate) compounds. High Temp. - High Pressures, 8(3) 337 (1976). [2] N.E. Timms, et al., Cubic zirconia in >2370 °C impact melt records Earth's hottest crust. Earth Planet. Sci. Lett., 477, 52 (2017).