Tempera~re-dependent thermal expansivities of silicate melts: The system anorthite-diopside

Ah&act-The tem~ratu~-de~ndent thermal expansivities of melts along the join ano~hite-diopside have been determined on glassy and liquid samples using a combination of calorimetry, dilatometry, and Pt double bob Archimedean densitometry. Supercooled liquid volumes and molar thermal expansivities were determined using scanning calorimetric and dilatometric measurements of properties in the glass region and their behavior at the glass transition. The extraction of low-tem~mtu~ liquid molar ex~nsivities from ~latomet~/c~o~met~ is based on an assumed equivalence of the relaxation of volume and enthalpy at the glass transition using a method developed and tested by WEBB et al. ( 1992). This method corrects for transient effects at the glass transition which can lead to serious overestimates of liquid thermal expansivity from “peak” values. Superliquidus volumes were determined using double Pt bob Archimedean densitometry at temperatures up to 165O’C. The resulting data for liquid volumes near glass transition temperatures (8 lo-920°C) and at superliquidus temperatures ( 1400-1650°C) are combined to yield thermal expansivities over the entire supercooled and stable liquid range. The molar expansivities are, in general, temperature dependent. The temperaturedependence of thermal expansivity increases from anorthite to diopside commotion. The thermal expansivity of anorthite is essentially temperature independent, whereas that of diopside decreases by ~50% between 800 and 1500°C, with the consequence that the thermal expansivities of the liquids in the anorthite-diopside system converge at high temperature.