The heterogeneous nucleation rate of mu-cordierite at the surface of cordierite glass (2MgO . 2Al(2)O(3) . 5SiO(2)) is discussed in terms of classical nucleation theory (CNT). Calculations are based on experimental viscosity data and estimates of the melting temperature and melting enthalpy of mu-cordierite from calorimetric measurements. A virtual melting temperature of the metastable mu-cordierite, T-M(mu) was estimated, by means of X-ray measurements, from the primary occurrence of mu-cordierite after brief thermal treatment. T-M(mu) was found to lie in the range 1300-1467 degrees C. The molar enthalpy of melting, AH(M)(mu), was calculated from calorimetric measurements and literature data. Referring to the molar weight of the formula unit, M = 585 g mol(-1), Delta H-M(mu) could be narrowed to the range 170-190 kJ mol(-1). The heterogeneous volume nucleation rate, I*(T), was estimated from the apparent nucleation rate of mu-cordierite which was previously studied at fractured and polished surfaces of cordierite glass by means of transmitting light microscopy. I*(T) steadily increases up to 235 K above the glass transition temperature at 815 degrees C without reaching a maximum. In the present paper, this exceptional temperature-dependence is discussed in terms of CNT. The best fit of experimental data was obtained for T-M(mu) approximate to 1467 degrees C, a low liquid-crystal interfacial free energy, sigma, and a low activity of nucleating substrates, Phi, if sigma Phi(1/3) approximate to 100 mJ m(-2) is valid.