Hydrothermal solubility of monazite rare earth element endmembers, The

Abstract

Monazite-(Ce), a light rare earth element (REE) phosphate, occurs as an accessory mineral in metamorphic, igneous, and sedimentary environments, and is a common ore mineral in hydrothermal REE deposits. The chemical composition of monazite has proven useful as a geochronometer and geothermometer, but currently there is no model describing the compositional variations of REE in monazite resulting from interactions with hydrothermal fluids. To develop such a model requires quantification of the chemical properties of the aqueous REE species and the solid solution properties of the mineral phase. The thermodynamic properties of monazite endmembers have been determined previously using calorimetric methods and low temperature (< 100 °C) solubility studies, but only a few solubility studies have been conducted at hydrothermal conditions. In this study the solubility products (Ks0) of LaPO4, PrPO4, NdPO4, and EuPO4 monazite endmembers have been measured at temperatures between 100 and 250 °C at saturated water vapor pressure (swvp), and are reported according to the reaction: REEPO4 = REE3+ + PO43-. The REE phosphates display retrograde solubility, with the measured Ks0 values varying four to five orders of magnitude over the experimental temperature range. A comparison of the solubility products determined in this study to those calculated from the available calorimetric data for monazite combined with the properties of the aqueous REE and phosphate species, show that there are discrepancies between the experimental and calculated values. Within the experimental temperature range, the differences between the calculated and measured Gibbs energy of reaction (ΔrG0) for PrPO4, NdPO4, and EuPO4 increase with higher temperatures (up to 15 kJ mol-1 at 250 °C), whereas for LaPO4 these differences in ΔrG0 increase at lower temperature (up to 8 kJ mol-1 at 100 °C). To reconcile these discrepancies, the enthalpy of formation of monazite was optimized by fitting the experimental solubility data and extrapolating these fits to reference conditions of 25 °C and 1 bar. The optimized thermodynamic data provides the first internally consistent dataset for all the monazite endmembers, and can be used to model REE partitioning between monazite and hydrothermal fluids. These data are implemented in a thermodynamic database available to a wide community and can be used to model monazite stability for a variety of geological and engineering applications.