The rare earth elements (REEs) are indispensable metals for manufacturing permanent magnets - key components in renewable energy technologies such as wind turbines and electric vehicles. However, the growing global demand for REEs, coupled with supply chain vulnerabilities, has raised the need for increasing REE production and diversifying REE resources. Many nations have thus classified the REEs as critical metals, and are incentivizing both the discovery of new REE deposits and the development of innovative approaches to REE recovery from less conventional sources. Florencite (REEsal3(PO4)2.6H2O) - a secondary REE phosphate mineral - has traditionally been perceived to have limited economic value in conventional REE deposits such as hydrothermal and igneous ores. However, florencite is now emerging as a potential resource in unconventional deposit types like coal seams, bauxite residues and regolith-hosted deposits, where its presence may permit economical REE recovery. Understanding the nature of florencite formation may therefore be crucial for facilitating REE exploration across various geological settings and ore systems, though despite this the mechanisms of florencite formation have been overlooked relative to other REE minerals. Our work studies the impact of fluid composition (eg aqueous aluminium and cerium activities) on the formation of florencite through the hydrothermal conversion of apatite at 140-220C. Results reveal the critical roles of temperature and fluid composition on the mechanisms of florencite formation, outlining temperature-dependent reaction pathways and the influence of al/Ce ratios on florencite stabilisation relative to other phosphate minerals. These findings may be extrapolated to natural hydrothermal systems and supergene deposits, to offer exploration criteria for identifying: 1. Florencite-dominated zones over monazite or rhabdophane rich assemblages in conventional deposits. 2. Zones of economic florencite precipitation in unconventional deposits.