Crystallization, replacement, and redox pathways governing rare earth carbonate and phosphate formation

Abstract

Rare earth elements are critical raw materials for modern technologies and the global shift toward sustainable energy. Yet, their extraction and separation remain environmentally challenging due to complex geochemical behavior and limited understanding of mineral formation mechanisms. This study integrates recent experimental advances to elucidate the fundamental processes controlling the crystallization and transformation of REE-bearing carbonates and phosphates. REE carbonate formation from aqueous solutions follows a non-classical pathway involving amorphous precursors and metastable intermediates that gradually transform into stable hydroxycarbonates such as kozoite and bastnäsite. The kinetics, polymorph selection, and resulting morphologies are governed by the ionic potentials of the REE³⁺ cations, temperature, and dehydration dynamics. Mineral replacement reactions between REE-bearing fluids and common host minerals like carbonates (calcite, aragonite, dolomite, siderite) and phosphates (vivianite), proceed through coupled dissolution–precipitation, producing pseudomorphic textures. The extent and texture of these transformations are controlled by epitaxial relationships, porosity generation, and local equilibrium conditions. Redox-driven processes, particularly Ce³⁺–Ce⁴⁺ and Fe²⁺–Fe³⁺ oxidation in siderite and vivianite, promote formation of secondary oxides (cerianite, hematite) that influence REE mobility and sequestration. In fluorine-bearing environments, transient fluocerite stabilizes and subsequently transforms epitaxially into bastnäsite, demonstrating the chemical and structural controls governing ore mineralization. Finally, we explore sustainable REE recovery using waste-derived sorbents such as eggshell calcite. Experiments show temperature-dependent REE uptake and complex phase transformations, offering insights for circular-economy approaches to resource recycling. Collectively, these results establish a unified mechanistic framework linking REE mineral formation, transformation, and recovery. By bridging natural ore-forming processes with green chemistry strategies, this study advances understanding of REE geochemistry and supports the development of environmentally sustainable extraction and recycling technologies.