Open Collections

The Fe-Mg solution properties of olivine, enstatite, anthophyllite and talc, from ion-exchange experiments with aqueous chloride solutions

University of British Columbia

Ion-exchange equilibria between a 2molal aqueous Mg-Fe chloride solution and synthetic olivines, orthopyroxenes, orthoamphiboles and talcs have been experimentally bracketed between 450° and 800°C and between 1 and 4kb. Additional experiments at lower chloride molalities reveal that the distribution coefficient measured is dependent upon chloride concentration. Thermodynamic modelling of the chloride solution, assuming ideal mixing of Mg and Fe species, qualitatively reproduces this behavior if the Mg and Fe species have contrasting dissociation constants. Internally consistent thermodynamic properties of all four minerals and the 'delta' properties of a hypothetical associated electrolyte solution are calculated through a combination of linear programming and least-squares optimization while simultaneously considering constraints provided by calorimetry, net-transfer equilibria and other ion-exchange data. Adequate correspondence between thermodynamic model and the data cannot be attained without allowing the mixing of Mg and Fe in the chloride solution to be non-ideal. Qualitative constraints on the chloride 'excess' properties allow a preliminary estimate of the solid solution properties to be made. All solid solutions are modelled with a symmetric Margules formulation. The resulting olivine WG function has a value near 7kJ at 400°C and drops asymptotically toward zero at high temperatures. The orthopyroxene WG is approximately zero at 400°C and drops toward -3kJ at high temperatures. A 'microscopic' orthopyroxene solution model is formulated to be simultaneously consistent with this bulk excess free energy and with measured M1-M2 site distributions. The single WG value (0.58kJ) calculated for the anthophyllite solid solution is qualitatively consistent with fractionation of Fe into M4. The talc solid solution cannot be constrained to be different from ideal by the data presented here. At present, ion-exchange experiments with Mg-Fe chloride solutions can only be used to compare mineral properties due to uncertainties in dissociation constants and 'excess' properties for the aqueous solution. The role of such uncertainties in the data analysis indicates that solid solution properties derived from previous aqueous chloride ion-exchange experiments must be considered suspect until the properties of each pertinent aqueous electrolyte solution have been examined.

Text

2010-06-11

For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.

http://hdl.handle.net/2429/25558

Geological Sciences

University of British Columbia

Graduate