" /> " /> " /> atom clusters with icosahedral symmetry (a Mackay icosahedron and a double-Mackay atom clusters with icosahedral symmetry (a Mackay icosahedron and a double-Mackay atom clusters with icosahedral symmetry (a Mackay icosahedron and a double-Mackay icosahedron). Si and Mn atoms are proposed icosahedron). Si and Mn atoms are proposed icosahedron). Si and Mn atoms are proposed to constitute icosahedral arrangements in the atom clusters. to constitute icosahedral arrangements in the atom clusters. to constitute icosahedral arrangements in the atom clusters.