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

• Published in: Chemical geology 2013-05, Vol.346, p.81-92
• Main Authors: Ottonello, G., Attene, M., Ameglio, D., Belmonte, D., Zuccolini, M. Vetuschi, Natali, M.
• Format: Article
• Language: English
• Subjects: calcium silicate; CAS system; Convex hull; Gibbs free energy; loci; melting; oxides; Polymer chemistry; polymers; thermal properties; Thermodynamics; topology
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2012.09.018

The system CaO–Al2O3–SiO2 (CAS) is explored in the pressure–temperature (P, T) range 0–2GPa and 1000–3000K 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=1bar (105Pa) 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≥1GPa. [Display omitted] ► The liquidus of the CaO–Al2O3–SiO2 system at 1 and 2GPa ► The nature of high order interactions in the CaO–Al2O3–SiO2 liquid ► Pressure induced congruent to incongruent melting behavior of anorthite ► Grossular stability induced by pressure in the CaO–Al2O3–SiO2 system ► Convex hull topology of the CaO–Al2O3–SiO2 liquidus at high pressure