Grossular activity-composition relationships in ternary garnets determined by reversed displaced-equilibrium experiments |
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| Authors: | Andrea M Koziol Robert C Newton |
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| Institution: | 1. Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Ave, 60637, Chicago, IL, USA
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| Abstract: | Activity-composition relationships of Ca3Al2Si3O12 (grs) in ternary Ca-Mg-Fe garnets of various compositions have been determined by reversed displaced equilibrium experiments at 1000° C and 900° C and pressures of 8 to 17 kbar. The mixing of grs in garnet is nearly ideal at 30 mol% grs, with positive deviations from ideality at lower grs contents. Models of garnet mixing currently in the literature do not predict this trend. Analysis of the present reversals, in conjunction with a garnet mixing model based solely on calorimetry measurements on the binary joins, indicates that a ternary interaction constant for a ternary asymmetric Margules model (Wohl 1953) cannot be constrained. Apparently, some aspects of the garnet binary joins are still not well-known. An alternative asymmetric empirical model, based on analysis of pseudobinary joins of constant Mg/Mg + Fe(Mg #), reproduces the data well and is able to predict grs activity coefficients for garnets with grs contents between 3 and 40 mol% and Mg numbers between 0 and 0.60. The grossular activity coefficient,
grs, is given by:
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![$$RT\ln \gamma _{grs} = (1 - X_{grs} )^2 W_{Ca} + 2X_{grs} (W_{FM} - W_{Ca} )]$$](/content/GV450VR0N8465231/410_2005_Article_BF01041750_TeX2GIFE1.gif)