Sequential deposition of Mn on a polycrystalline Al layer at 523-573 K was found to result in an anomalous icosahedral (i) Al-Mn phase, characterized by alterations of intensity- and linewidth ratios. The anomalies suggest a non-equilibrium atom distribution, related to the limited diffusion of Al through the already-formed layer of i-AlMn. The activation energy of Al diffusion was inferred from X-ray diffraction (XRD) data taken for different deposition temperatures.