Application of mixing models

Polymer models of the type described above have been used with considerable success to interpret the mixing properties of certain binary silicate melts. Althoiagh the polymerization constants for melts of geological complexity are as yet unknown and the 

The solubility of H2O in albite melts has been studied by Biirnham and Davis (1971 197 ) and some of their results are shown in Fig. 9. 

The results shown in Fig. 9 indicate that water solubility in albite melts increases linearly with the square root of water fugacity. This has been observed for a large number of different silicate melt compositions. The linear dependence of water solubility on the square root of fH20 implies that H2O dissociates in the melt to produce two independent species. Two equivalent models were proposed to explain this behaviour (Burnham and Davies, 197 Burnham, 1975). 

These reactions seem unlikely to be correct since melts containing 50 mol per cent H2O (cf. Fig. 9) would contain 50 mol per cent of molten NaOH in equilibrium with the acidic species AlSi30y0H, or alternatively the same proportion of free protons in an oxide melt. 

It is important to emphasize that no thermodynamic data can prove the validity of a structural theory. Thus in the present case, any mechanism involving the dissociation of H2O can account for the observed dependence of solubility on fH20 doing this is by adapting the Toop and Samis model. 

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