The R2* relaxation in brain's white matter(WM) exhibits a dependency on WM fibre orientation relative to the external magnetic field, B0. Here, we introduce a computational model based on first principles derived from magnetic field inhomogeneities generated by ferritin and myelin. We investigate their effects on the multi gradient-echo signal by fitting simulated R2*, over angles 0–90o, to experimental R2*(R2=0.94). By comparing different myelin concentrations, we present how ferritin is required to complement myelin in describing the orientation dependency in R2*. Additionally, we propose a model for mapping R2* as a function of fibre orientation, myelin and iron concentration.