Neutron powder diffraction patterns are calculated from a random cluster model (RCM) for icosahedral phase AlMnSi for comparison with experimental intensity data. The diffuse scattering seen experimentally under the groups of strong peaks is revealed selectively in the calculated partial intensities for AlAl, MnMn and AlMn correlations. This diffuse intensity is known to result from the enhancement of the dense background of weak peaks by the structure factor of a single atomic cluster, rather than from substitutional chemical disorder or orientational disorder. A single crystal diffraction pattern normal to a two-fold symmetry axis is also calculated to determine the distribution of diffuse intensity in reciprocal space. This intensity is found to be in agreement with experimental transmission electron diffraction patterns in that the diffuse scattering is centered around the expected positions for the most intense superlattice reflections found in fci quasicrystals.