Effective diffusion coefficient of a species in a heterogeneous mixture
This greatly depends on macro structure. If the phases were say arranged in a sheet-like structure (like a diffusion front) then the low diffusivity phase would be the dominating determinate of the diffusion rate. For this macrostructure, if we assume steady state with no change in phase or length from diffusion or time, then mass transport of the bulk can be approximated by:

$$\frac{\mathrm dn}{\mathrm dt} = \frac{A \times \nabla C}{\sum\frac{L_i}{D_i}}$$

If the high diffusivity phase is continuous then that could be the dominating determinate of the diffusion rate. That does not mean it is easily calculable, just noting that dominating factors can change. If no phase is continuous or other phases are continuous then this also becomes convoluted. For non-sheet-like structures calculating diffusion requires a computer model of the grain structure and finite element analysis to resolve an approximate solution.