Diffusion along Crystal Imperfections

Experiments demonstrate that along crystal imperfections such as dislocations, internal interfaces, and free surfaces, diffusion rates can be orders of magnitude faster than in crystals containing only point defects. These line and planar defects provide short-circuit diffusion paths, analogous to high-conductivity paths in electrical systems. Short-circuit diffusion paths can provide the dominant contribution to diffusion in a crystalline material under conditions described in this chapter. 

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Volume diffusion refers to the transport of atoms through the body of a solid. It is also called lattice or bulk diffusion. In amorphous or glassy solids and in cubic crystals, the speed of diffusion in all directions is the same and is said to be isotropic. In all other crystals, the rate of volume diffusion depends upon the direction taken and is anisotropic. Volume diffusion is usually much slower than short-circuit diffusion, which refers to diffusion along two- and three-dimensional imperfections in the material. 

The short-term potential for solid solution formation is low for minerals with small solubility products, such as aluminum oxides and aluminosilicates, because spontaneous dissolution and recrystallization is very slow in these minerals. Without recrystallization, trace metals cannot be incorporated into the mineral structures. Movement of metal ions into these mineral crystals by solid diffusion is not possible on the time scale of adsorption experiments ionic diffusion into most crystalline solids is negligibly slow at all but extremely high temperatures. Nevertheless, metals could diffuse into imperfect solids along interstices, pores, or other structural defects. 

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