Thermodynamic Properties of REE-Bearing Minerals in Hydrothermal Systems

To a large extent, the REE-bearing minerals coexisting with hydrothermal solutions control the transport and deposition of these elements through the process of saturation. This process can be modeled as the leaching of an element from the host mineral by an unsaturated fluid, transport of this element to a new locality, and saturation of the fluid with respect to a REE-dominant mineral due to changes in physicochemical conditions.

At any given set of conditions, the concentration of an element in an aqueous solution is limited by the solubility of the least soluble mineral containing this element. Thus, determination of the thermodynamic properties of REE minerals coexisting with or formed from hydrothermal solutions is essential for developing accurate models for predicting the transport and deposition of the REEs in hydrothermal systems.

The REEs occur in two classes of minerals. One of them is the REE ore minerals, including bastnäsite-(Ce), parasite-(Ce), synchysite-(Ce), monazite-(Ce), and xenotime-(Y), from which the most of the REEs are commercially extracted via operating mines. The other class is nominally non-REE minerals that contain low yet appreciable concentrations of REEs, frequently at the level of tens to hundreds of parts per million (ppm).

In both cases, the REEs can substitute readily for each other or replace other elements with similar characteristics (e.g. U and Th), forming solid solutions (e.g. LnPO₄-CaTh(PO₄)₂ (Ln = La, Ce) [59]). Depending on the ionic radii and oxidation states of the substituting cations, the extent of solid solution may vary from being very limited to complete. In fact, end-member REE minerals (containing a single REE) are rarely, if ever, observed.

The varying stoichiometry within a given mineral structure makes it extremely challenging to account for the mineralogical behavior of the REEs in hydrothermal systems, leading to considerable uncertainties in characterizing the structures and thermodynamic properties of REE minerals.

Thermodynamic models for most REE solid solutions are nonexistent; even for some end-members, there are still significant knowledge gaps. Yet these solid solutions control the formation and occurrence of the REEs in the earth’s crust and constrain the processes for REE mining, production, and purification. Thus, additional studies of REE minerals are critically needed.

Quantitative knowledge of the thermodynamic stability of REE-bearing minerals can be obtained either through solubility measurements (if the aqueous species are known) or calorimetric determinations of thermodynamic properties of these minerals. The two approaches are complementary. Solubility measurements quantify the concentration of the element that is in equilibrium with a particular mineral under specific aqueous conditions, and the results obtained can be used to calculate solubility products (K ) and, in turn, the Gibbs free energy of formation of the mineral.

Moreover, analysis of the temperature dependence of the Gibbs free energy yields the entropy of formation of the mineral. Calorimetric techniques are powerful means for determining the thermodynamic properties of a solid, via direct measurements of its heat capacity, and standard formation enthalpy, entropy, and Gibbs free energy.

It is particularly important to measure the enthalpy of mixing of the solid solutions, as they are frequently not zero, due to the complex interplay between local cation ordering, the occurrence of oxygen vacancies, and cation order-disorder or clustering.

For detailed information on the solubility and calorimetric data for REE phases, readers may refer to the recent reviews. Most solubility experiments have been performed at ambient temperature; only a limited number of studies have been conducted at hydrothermal conditions. The majority of the calorimetric data are for synthetic phases and limited to their enthalpies of formation. The following sections focus on bastnäsite-(Ce), parasite-(Ce), monazite-(Ce), and xenotime-(Y), i.e. the major REE ore minerals.