Thermodynamic Optimization of the Li2O-Na2O-CaO-FeO-Fe2O3-Al2O3-SiO2-P2O5 System

English Abstract:

P₂O₅ is an important oxide component in the late stage products of igneous rocks such as granites and anorthosites, for example. Most of the time, P₂O₅ combines with CaO and crystallizes in the form of apatite, while in volatile-free conditions, Ca-whitlockite is formed. In spite of their interest, the thermodynamic properties and phase diagrams of P₂O₅-bearing systems are still not well known yet. As a result, we critically reviewed and optimized the structural, thermodynamic and phase diagram data belonging to the system Li₂O-Na₂O-CaO-FeO-Fe₂O₃-Al₂O₃-SiO₂-P₂O₅. This system contains numerous compounds including the phosphate minerals lithiophosphate (γ-Li₃PO₄), Ca-whitlockite (β-Ca₃(PO₄)₂), silicocarnotite (Ca₅SiP₂O₁₂), nagelschmidtite (Ca₇Si₂P₂O₁₆), grattarolaite (α-Fe₃(PO₄)O₃), and rodolicolite (α-FePO₄). The liquid phase was parametrized using the Modified Quasichemical Model while solid solutions such as spinel and FeO were modelled using the Compound Energy Formalism and the Regular Solution Model, respectively. All available thermodynamic and phase equilibrium data were simultaneously reproduced and discrepancies in the available data resolved in order to obtain one set of model equations for the Gibbs energies of all phases as functions of temperature and composition at 1 atm. The CaO-SiO₂-P₂O₅ liquidus projection is shown below as an example. This is part of a larger research project to develop the thermodynamic database of the Li₂O-Na₂O-K₂O-CaO-MgO-MnO-ZnO-FeO-Fe₂O₃-Al₂O₃-SiO₂-P₂O₅ system at room pressure.