Osumilite-bearing Granulites in the Eastern Grenville Province, Eastern Labrador, Canada: Mineral Parageneses and Metamorphic Conditions

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Abstract
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-XCO2 equilibria involving meionite in the CaO-Al2O3-SiO2-CO2-H2O (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 reaction

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