We present a discussion of the effect of chemical substitution on the stability of Mackay icosahedra in Al-Mn alloys based on a microscopic model of the total energy of metallic alloys. In particular, we demonstrate why the addition of Si stabilizes both the crystalline α-(Al-Mn-Si) structure and the icosahedral phase of Al-Mn-Si. We predict specific A1 sites which are favourable for Si substitution.