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American Journal of Science


Chemical components for use in the description of a system involved in a diffusion process may be selected according to the same criteria followed in the selec­tion of components for an equilibrium system. The various possible sets of components are not independent and may be interrelated through a component transformation. Actual components are preferred over nonvariable components, since they simplify descriptions and avoid theoretically nonmeasureable quantities. Two systems involved in an interdiffusion process may always be described using components that are actual components of both systems. For example, ternary reciprocal exchange of alkalis be­ tween alkali feldspars and dioctahedral alkali micas may be described using binary equations in terms of the exchange component, KNa_1, which is an actual component of both minerals. Small quantities of additional components in approximately binary systems may have significant effects on measured diffusion coefficients, if neglected. Nevertheless, binary solutions to the continuity equation may be used, if the system is constrained in any of several ways to some binary or to a single set of boundary con­ditions.



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