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Activity-Composition Relationships in Multicomponent Amphiboles: An Application of Darken's Quadratic Formalism

Thomas M. Will
Roger Powell
Journal / Anthology

American Mineralogist
Year: 1992
Volume: 77
Page range: 954-966

Activity-composition (a-x) relationships in multicomponent amphibole solid solutions are modeled using Darken's quadratic formalism (DQF). In this, the thermodynamic behavior is expressed in terms of ideal mixing between real and fictive end-members, with the properties of the fictive end-members being constrained by the compositions of naturally coexisting amphibole pairs. The approach has been used to determine a-x relationships for calcic amphibole, sodic amphibole, cummingtonite, and othoamphibole solid solutions in the system Na2O-CaO-MgO-FeO-Al2O3-SiO2-H2O. This has been used to compare experimentally determined P-T-x conditions of formation of tremolite with those calculated by using our results. The correspondence is good and supports the application of DQF in modeling the activity-composition relationships in amphibole solid solutions.

*Science > Chemistry
*Science > Geology and Geophysics