Transient Epitaxial Growth of Rare Earth Carbonates during Low-Temperature Replacement of Calcite, Aragonite, and Dolomite

This study investigates transient epitaxial overgrowths of REE carbonates formed via fluid-mediated replacement reactions between La-, Ce-, Pr-, and Nd-bearing aqueous solutions and divalent Ca–Mg carbonate substrates (aragonite, calcite, and dolomite) under low hydrothermal conditions (80–165 °C). Experiments reveal that initial surface precipitation proceeds through the oriented nucleation of orthorhombic REE-kozoite (REECO3OH), exhibiting coherent epitaxy along the [100]koz direction on all host substrates. Crystallographic alignment is substrate-specific, with multidirectional epitaxy on aragonite due to pseudohexagonal twinning, and single-direction alignment on calcite and dolomite. Lattice misfit calculations confirm structural coherence within the <10–20% threshold for all REE-kozoite-substrate pairs, favoring transient epitaxial stabilization. However, subsequent transformation into hydroxylbastnäsite and cerianite phases occurs without substrate control, reflecting a regime shift from epitaxial interface-driven growth to supersaturation-controlled crystallization. The occurrence of epitaxial texture is constrained by three interdependent factors: crystallographic misfit, the ionic potential of REE3+ cations (affecting desolvation and nucleation kinetics), and the host mineral dissolution rate (controlling local supersaturation and interface reactivity). These results constrain the physicochemical and structural parameters governing REE-carbonate crystallization and provide mechanistic insight into the formation and breakdown of coherent interfacial relationships during coupled dissolution–reprecipitation processes. Implications extend to REE mobility in hydrothermal systems and the design of tailored sorbents and precipitates in REE recovery technologies.