Modification of silicon by sodium in aluminum silicon eutectic alloy has been examined in detail by optical, SEM, and TEM methods. The aluminum phase is not significantly affected but the silicon becomes very heavily twinned. Modification by quenching does not involve an increase in twin density. Consideration of the atomic positions which attend the formation of growth twins on {111} planes suggests that adsorbed impurity atoms of suitable size, on the solid-liquid interface, could be responsible for changing the {111} stacking sequence, so promoting ‘impurity induced twinning