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American Journal of Science


Petrologic problems may be analyzed using any of a variety of possible choices of units. Although gram-formula units (moles) are commonly used, other units of quantity arc more appropriate for many applications. Gram-atom units offer the numerical simplicity of gram-formula units and have the advantage of being conserva­tive. A conservative unit of quantity is one for which the sum of the units of reactants equals the sum of the units of products in a chemical reaction. For many graphical problems, a conservative unit of quantity, such as gram-atom or mass units, should be used. Since gram-formula units are not conservative, the lever rule can lead to incorrect results when applied to composition axes based on units of gram-formula percent. Similarly, the graphical evaluation of Gibbs energy relationships and chemical poten­tials on energy-composition diagrams is facilitated by the use of a conservative unit of quantity. Thermochemical data show consistent patterns when normalized on a gram­atom basis, whereas with gram-formula units they do not. Mean atomic entropies of formation arc so similar for all complex oxides that they may be predicted with pre­cision. This is partly due to the dominance of the entropy term for gaseous oxygen, when the elements in their stable form are chosen as the standard state. In detail, mean atomic entropies of formation show a positive correlation with mean atomic weight for all silicates, a relationship that can be used to correct entropy estimates for mass effects.



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