The optical functions of metallic particles, resulting from the precipitation of alkali ions implanted in LiF crystals, have been calculated from optical absorption and reflectivity measurements in the photon energy range 0.4–6 eV. Optical data are analyzed in terms of local electrical field effects. The real part of the dielectric function remains positive in the whole energy spectrum, in opposition to the one calculated in the Maxwell-garnett approximation where enhanced reflection should correspond to negative values of the dielectric function. Additional resonances shown by the enery loss function are responsible for this discrepancy. An absorption peak associated to Li precipitates and a secondary peak of the plasma resonance are revealed, so as a loss structure at 5 eV due to F centres also detected in electron energy loss spectrometry (REELS). Implications for non linear optical properties are addressed.