As determined by quantitative differential thermal analysis, the enthalpies of fusion of forsterite and spinel, are ΔHf=142¿14 and 107¿11 kJ/mol at 2174 and 2408 K, respectively. Significant premelting has been observed for forsterite, beginning about 100 K below the congruent melting point. When heated in graphite crucibles, forsterite reacts with carbon and the products of reaction vaporize with an overall enthalpy of reaction of the order of 1600 kJ/mol. The configurational entropy of Mg2SiO4 liquid decreases so rapidly with decreasing temperature that it would be almost zero at its nominal glass-transition temperature as estimated by extrapolating viscosity data for magnesium aluminosilicate melts. Extreme cooling rates are thus needed in order to quench actual Mg2SiO4 glasses having significant configurational entropies. ¿ American Geophysical Union 1993