Experimental investigation of andradite and hedenbergite equilibria employing the hydrogen sensor technique, with revised estimates of Δ_fG⁰ _m,298 for andradite and hedenbergite

Hydrothermal experiments were carried out at 2 kbar, 600-860°C to resolve discrepancies among previous studies on f_O2-T stability relations for the following equilibria: 1) andradite (And) = hematite (Hm) + wollastonite (Wo), 2) And = magnetite (Mt) + Wo + O₂, 3) And + quartz (Qz) = hedenbergite (Hd) + Wo + O₂, and 4) And + Mt + Qz = Hd + O₂. A variety of O buffer and H₂ generation techniques were used to access a wide range of f_O2-T conditions. The results yielded an internally consistent set of experiments on andradite and hedenbergite stability. The Δ_fG⁰ _m,298 for andradite and hedenbergite calculated from the data are -5428.0 ± 5.9 and =2676.6 ± 5.3 kJ/mol, respectively. -from Authors