The optical absorption band induced by energetic heavy ions implanted into AgCl single crystals at 77°K must be connected with the formation of silver colloids. The experimental (primitization and E.P.R. spectra) and theoretical study, makes it possible to obtain an approximate value of cluster radius. The colloid concentration is correlated with nuclear stopping power. It is shown that the collision efficiency is modified by a chemical effect. Finally the conditions of cluster formation are analysed.