High temperature calorimetric studies of the heat of solution of La₂O₃ in silicate liquids

High temperature calorimetry at 1760 K has been used to measure the heat of solution of La₂O₃ in a series of simple alkali and alkaline earth silicate liquids. The heat of solution in these solvents is strongly exothermic and varies as a function of liquid composition. However, the variation of the heat of solution does not follow simple trends related to cation size or charge and varies little with La₂O₃ concentration. The variation of heat of solution with composition of the liquid reflects the ability of La(III) to perturb the transient silicate framework and compete with other cations for oxygen. This complex pattern of melt energetics is consistent with recent spectroscopic measurements which suggest extreme perturbation of the silicate framework by La(III), sufficient to isolate oxygen from silicon. This interpretation suggests the presence of phase-ordered regions rich in La(III) consistent with incipient liquid-liquid immiscibility suggested by previous calorimetric studies. Within error the heat capacity of La-bearing silicate liquids is the same over the super-cooled liquid range as in the stable liquid, with no evidence for the large heat capacities associated with melt restructuring. Thus the energetics of the liquid are dominated by the exothermic reactions which form La-clusters and these phase-ordered regions do not dissociate as temperature increases up to 1760 K.