High-pressure and high-temperature structure and equation of state of Na₃Ca₂La(CO₃)₅ burbankite

Abstract

In this study we report the synthesis of single crystals of burbankite, Na3Ca2La(CO3)5, at 5 GPa and 1073 K. The structural evolution, bulk modulus and thermal expansion of burbankite were studied and determined by two separate high-pressure (0–7.07(5) GPa) and high-temperature (298–746 K) in situ single-crystal X-ray diffraction experiments. The refined parameters of a second-order Birch–Murnaghan equation of state (EoS) are V0= 593.22(3) Å3 and KT0= 69.8(4) GPa. The thermal expansion coefficients of a Berman-type EoS are α0= 6.0(2) ×10-5 K−1, α1= 5.7(7) ×10-8 K−2 and V0= 591.95(8) Å3. The thermoelastic parameters determined in this study allow us to estimate the larger density of burbankite in the pressure-temperature range of 5.5–6 GPa and 1173–1273 K, with respect to the density of carbonatitic magmas at the same conditions. For this reason, we suggest that burbankite might fractionate from the magma and play a key role as an upper-mantle reservoir of light trivalent rare earth elements (REE3+).