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A study on the equilibrium grossular + clinochlore = 3 diopside + 2 spinel + 4 H₂O Wang, Xiaomin


The equilibrium grossular + clinochlore = 3 diopside + 2 spinel + 4 H₂0 was investigated using cold seal pressure vessels from 500°C to 700°C at 0.5 kilobar to 4.0 kilobars. All phases used in the experiments were synthesized from oxides. Good brackets with stable assemblages diopside + spinel or grossular + clinochlore were made at conditions far from the equilibrium. Assemblages diopside + clinochlore were found at conditions close to the equilibrium over the entire pressure range. Analysis of internal consistency by linear programming indicates that the experimental results from this study are fully consistent with the UBCDATABASE and Helgeson's database and the experiments, with UBCDATABASE may safely be used as an indication of the metamorphic conditions of metarodingites. Run product diopside, the only possible solid solution phase in this study was extensively examined. X-ray refinement demonstrates that the cell parameters of the diopside are well within the range for pure diopside and Ca-Tschermak pyroxene. Scanning electron microscope and electron microprobe analyses showed that these diopsides contain aluminum interpreted as substitution of both Ca-Tschermak and Mg-Tschermak pyroxene. This interpretation satisfies the mass-balance requirements of these assemblages. Theoretical thermodynamic prediction of the equilibrium allowing for solid solutions in pyroxene indicates that diopside should be the main component, which is consistent with the experiments in that the equilibrium curve according to the experimental brackets did not show measurable displacement caused by the low diopside activity. Thermodynamic calculation using diopside activity calculated from microprobe analysis data shows a significant shift in the equilibrium curve. Comparison of experimental results with natural minerals results in contradiction. Natural diopsides (clinopyroxene) found in similar assemblages at similar conditions contain much less aluminum. Undetected metastable zonation of aluminum or even aluminum-rich inclusions are probably the main causes for this.

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