Phase equilibria for calcic scapolite, and implications of variable Al-Si disorder for P-T, T-X_co2, and a-X relations

Carbonate scapolite is a potentially powerful mineral for calculating CO₂ activities in non-calcareous rocks, but an analysis of the thermodynamics and phase equilibria of carbonate scapolite is first necessary. This includes an evaluation of Al-Si disorder in meionite, as this has the greatest effect on derived phase relations. Available experimental data on meionite stability, X-ray diffraction refinements and nuclear magnetic resonance spectra for calcic scapolite do not uniquely constrain the Al-Si ordering state of synthetic meionite. However, the data are most consistent with a high degree of Al-Si disorder and inconsistent with long-range Al-Si order. An internally consistent thermodynamic data set was derived and used to calculate P-T and T-X_co2 equilibria involving meionite in the CaO-Al₂O₃-SiO₂-CO₂-H ₂O (CASCH) system. The effect of Al-Si disorder is illustrated by calculating the phase equilibria using an ordered, an arbitrary intermediate disordered, and a completely Al-Si disordered standard state for meionite. The Gibbs free energy of meionite was calculated from reversals (at 790-815°C, 2-15 kb) on the reaction3 Anorthite +Calcite =Meionite. The Δ_fG°_{m, 298} for each of the standard states is -13 146·6, -13128·8, and -130930kJ/mol, respectively. Because of the steep slope of reaction (1) and limited temperature range over which it breaks down, meionite used in the experiments to constrain reaction (1) must possess a limited range of Al-Si disorder. The P-T slope of reaction (1) increases, and the slope of meionite decarbonation equilibria changes from positive to negative in T-X_co2 and P-T space, as a function of increasing Al-Si disorder. Meionite has a wide stability field at high T in T-X space at 5 and 10 kb (P_Total=P_Fluid), being stable to X_co2=0·06. Meionite alone breaks down to undersaturated gehlenite and/or corundum-bearing assemblages at 5 kb, and to clinozoisite at 10 kb. The effect of solid solutions on the T-X stability of meionite is similar to that of increasing pressure, stabilizing meionite to lower temperature. Variable Al-Si disorder does not significantly affect the upper limit of meionite stability in T-X_co2 space. Activity-composition relations for meionite in carbonate scapolite were calculated relative to reaction (1) from data on natural scapolite-plagioclase-calcite assemblages. The extent of departure from ideality varies as a function of Al-Si disorder. Negative deviations from ideality are indicated for natural scapolite solid solutions at T<750°C, based on a disordered Al-Si standard state for meionite. This is likely to reflect a more ordered Al-Si distribution in natural scapolites compared with the synthetic endmember standard state.