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From: Paul Foellbach Subject: How can I include oxide components in the basis ? I attempt to build reactant, that has got the composition of a basalt. I would like to constrain a initial system by the components of SiO2, Al2O3, Fe2O3, FeO, MnO, MgO, CaO, Na2O, K2Oand H2O, I like to use this components to define the Basalt composition. For example, basalt components: SiO2 = 50.61%, Al2O3 = 15.93%, Fe2O3 = 1.42% ..... I can constrain SiO2(aq) and H2O, but how can I include the oxides into the calculation and in the basis? From: Craig Bethke Subject: Re: How can I include oxide components in the basis ? You cannot constrain the composition of a fluid by equilibrium with a set of oxide components because oxide components are fictive. In other words, they are simply a mathematical tool for describing composition. They do not actually exist as species or phases, and we do not know their chemical potentials. There are two ways to formulate the problem you describe. First, you can constrain the initial system by equilibrium with the minerals that actually compose the basalt. Second, you can take a fluid (perhaps rainwater?) and titrate the oxide components into it until you reach an invariant point. In this case the oxide are simple reactants; they are listed at the end of the thermo dataset.