Rare earth element mobility in and around carbonatites controlled by sodium, potassium, and silica

TLDR

Carbonatites and related rocks are the primary source of rare earth elements, with hydrothermal assemblages near them indicating aqueous transport of REEs. The authors performed high‑temperature, high‑pressure experiments (1200 °C–200 °C, 1.5–0.2 GPa) on La and Dy, producing fluorapatite intergrown with calcite, dolomite, and ankerite. REEs are only highly soluble when alkalis are present; Dy is more soluble than La under alkali complexation, while silica addition either sequesters REEs in apatite or immobilizes alkalis, demonstrating that alkali complexation—not anionic species alone—is essential for REE mobility around carbonatites.

Abstract

Carbonatites and associated rocks are the main source of rare earth elements (REEs), metals essential to modern technologies. REE mineralization occurs in hydrothermal assemblages within or near carbonatites, suggesting aqueous transport of REE. We conducted experiments from 1200°C and 1.5 GPa to 200°C and 0.2 GPa using light (La) and heavy (Dy) REE, crystallizing fluorapatite intergrown with calcite through dolomite to ankerite. All experiments contained solutions with anions previously thought to mobilize REE (chloride, fluoride, and carbonate), but REEs were extensively soluble only when alkalis were present. Dysprosium was more soluble than lanthanum when alkali complexed. Addition of silica either traps REE in early crystallizing apatite or negates solubility increases by immobilizing alkalis in silicates. Anionic species such as halogens and carbonates are not sufficient for REE mobility. Additional complexing with alkalis is required for substantial REE transport in and around carbonatites as a precursor for economic grade-mineralization.