Experiments have been carried out to determine the nature and origin of the spots growing on silica glass, surfaces in contact with liquid silicon during CZ-Si crystal growth. Silica glass ampoules were filled with silicon and tempered. between 5 min and 40 h at a temperature (1693 K) slightly above the melting point of. silicon. Cross sections of the ampoules with solidified silicon have been examined by scanning electron microscopy and optical polarization microscopy. In addition cross sections from commercial silica glass crucibles used in the Czochralski process or dipped into the silicon melt were investigated with the same methods. At the silicon/silica glass interface different reaction zone morphologies were detected. A solution-precipitation mechanism is suggested for the fast lateral growth of the reaction zone, which is proposed to consist of small cristobalite crystals embedded in a silica glass matrix. (C) 2003 Elsevier Science B.V. All rights reserved.