To understand how deformation and the chemical processes of metamorphism interrelate, we need the concept of chemical potential. The state of stress of any material involved is likely to be nonhydrostatic; hence, to define chemical potential, we need the idea of an associated equilibrium state and need to recognize that the associated equilibrium state changes with the orientation of the plane considered. Along these lines, chemical potential appears as an attribute of a plane through a point, like the normal component of the stress.Given these ideas, a potential surface is envisaged; it can be represented in three dimensions if we use only one axis for spatial position and one axis for the inclination of a plane of interest to the principal stress directions. Diagrams of this type, while obviously limited, can show (i) the potential surface for simple diffusion (without deformation) and (ii) the potential surface for simple deformation (with no concurrent chemical change) and various states where deformation and chemical change go on concurrently.When a megacryst that is stiffer than its surroundings sits in a nonhydrostatic stress field, it creates stress shadows, and mobile material tends to migrate toward the shadows, through the megacryst and through the intergranular phase. In two alkali-feldspar gneisses, the potassium distribution is nonhomogeneous in ways possibly related to the foregoing theory.

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