Equilibrium density and expansivity of silicate melts in the glass transition range

Equilibrium volumes and expansivities of three liquids in the system anorth ite (CaAl2Si2O8)-diopside (CaMgSi2O6) have been derived from dilatometric m easurements of the equilibrium length of samples in the glass transition ra nge. The typical temperature range of 40 K for the measurements is limited at low temperature by the very long times necessary to reach structural equ ilibrium and at high temperature by the penetration of the rod used to meas ure sample dilatation. Despite such narrow intervals, the expansivities are determined to better than 3% thanks to the high precision with which lengt h changes are measured. The coefficient of volume thermal expansion (1/V dV /dT) of the fully relaxed liquid just above the glass transition is found t o decrease linearly from diopside composition (139 +/- 4 x 10(-6) K-1) to a northite composition (59 +/- 2 x 10(-6) K-1). These values are greater than those determined for the same liquids at superliquidus temperatures, demon strating that expansivities of silicate melts may decrease markedly with in creasing temperature. A predictive model based upon partial molar volumes w hich vary as a linear function of the logarithm of temperature is proposed.