Energetics of glasses in the system diopside-anorthite-forsterite

Enthalpies of mixing in glasses in the experimentally accessible region of the system Di-An-Fo are generally less than 4 kJ/mol in magnitude. Enthalpies of mixing of liquids in this and in several other petrologically relevant melts are also small; thus, if magmas of different compositions mix isothermally, the heat released or absorbed can be neglected in any consideration of the thermal evolution of the magmas, unless melting or crystallization takes place. The enthalpy of vitrification of Mg₂SiO₄ (to form a hypothetical glass at 700°C) is estimated to be 61±4 kJ/mol, in contrast to the enthalpy of fusion at 1500°C, 89±12 kJ/mol (Navrotsky et al. 1989). This suggests an average difference in heat capacity (Δ C_p) between liquid and crystal of 35 J/K. mol, half that estimated by Ghiorso and Carmichael (1980, 1987).