Thermodynamic investigation of the CaO-Al2O3-SiO2 system at high P and T through polymer chemistry and convex-hull techniques

The system CaO-Al2O3-SiO2 (CAS) is explored in the pressure-temperature (P, T) range 0-2 GPa and 1000-3000 K with the aim of defining the complex topology of the liquidus surface at various isobaric conditions and assessing the role of P on the stability fields and melting behavior of the various solids nucleating in the system. Calculations are carried out by coupling a generalized polymeric approach that reduces the system to two interacting sub-lattices (Network Formers and Network Modifiers; NF, NM) with an improved and generalized convex-hull procedure that conforms mathematically the equipotential loci at the various T, P conditions. The thermodynamic model operates through a Toop's asymmetric deconvolution (interactions within the NM sub-lattice unaffected by NF; interactions within the NF sub-lattice affected by NM) of the bulk Gibbs free energy of mixing. Mixing energies (chemical and strain) are calculated with a polymeric model where the individual properties of the oxides composing the NF and NM matrixes are determined by their Lux-Flood acid-base properties and weighted on the basis of their electrical equivalent fractions. The convex-hull procedure locates points on the liquidus by lifted Delaunay triangulation. The isobaric liquidus at P = 1 bar (105 Pa) is in reasonable agreement with the experimental observations. As far as we know isobaric sections at higher P conditions based on calculus have never been produced in literature. Our results indicate that the primary phase fields of anorthite and gehlenite shrink progressively with increasing P and a primary phase field of grossular appears at high P predating on the fields of the neighboring phases (gehlenite, rankinite, anorthite and wollastonite). Increasing P also causes the progressive disappearance of the liquid miscibility gap at high SiO2 content. Moreover the congruent melting of anorthite becomes incongruent. The fields of rankinite (Ca3Si2O7), tri-calcium aluminate (Ca3Al2O6) and grossite (CaAl4O7) disappear at P >= 1 GPa.