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3.6 i. Viscosities of XAlSi3O8 (X=Li,Na,K) melts with added water: implications for water solution mechanisms (C. Romano, V. Mincione, K.-U. Hess and D.B. Dingwell)

The role of alkali exchange in influencing the structure and properties of silicate melts has been the subject of considerable research in recent years. The possibility that alkalies are involved in the coordination of hydrous species dissolved in silicate melts and therefore exercise a significant control on the solubility of water in feldspar and granitic melts is still greatly disputed. Despite the clear differences in water solubility that we have determined in XAlSi3O8 (X=Li,Na,K) melts, the homogeneous equilibrium between molecular water and hydroxyl groups appears to be relatively insensitive to the exchange of Na for K in feldspar stoichiometry melts.

Property-structure relationships for these melt systems can contribute important constraints on the role of alkali exchange in controlling water solubility and therefore activity in hydrous feldspar melts. Thus we have engaged in an investigation of the influence of water on the viscosity of XAlSi3O8 (X=Li,Na,K) melts under conditions of equal pressure, temperature and water content. The melts were synthesized dry by high temperature fusion and homogenisation by stirring and later hydrated via piston cylinder synthesis. The quenched hydrous glasses were recovered and prepared for micropenetration viscometry. The data were obtained in the high viscosity range and are compared at the 1011.4 Pa s isokom in Fig. 3.6-10.

Fig. 3.6-10: 
Temperature of the 1011.4 (Pa s) viscosity isokom as a function of water content for XAlSi3O8 melts (X=Li, Na, K). Note that the viscosities for the stoichiometric melt compositions (NaAlSi3O8 melt is slightly peralkaline) are the same beyond 0.5 wt% water.

The viscosities of the melts exhibit similar trends for all three XAlSi3O8 (X=Li,Na,K) melts. The viscosities of the hydrous LiAlSi3O8 and KAlSi3O8 melts are the same beyond 0.5 wt% water. The viscosities of the hydrous NaAlSi3O8 melts are slightly lower but this is likely due to a slight nonstoichiometry (peralkalinity) of the nominally NaAlSi3O8 melts. Additional experiments are underway to confirm this suspicion.

Comparison of the hydrous LiAlSi3O8 and KAlSi3O8 melt data with the trend previously observed for hydrous HPG8 (haplogranitic melt) viscosities reveals a remarkable coincidence of the data. We must infer from the present comparison that the viscosities of hydrous calcalkaline rhyolitic to trachytic melts will be very insensitive to compositional changes due to fractionation.

The similar viscosities of the hydrous LiAlSi3O8 and KAlSi3O8 melts would appear to be consistent with the observation that the homogeneous equilibrium between molecular water and hydroxyl groups is at most weakly dependent on the identity of the alkali cation. It is difficult to envisage a melt structural model based on alkali coordination of dissolved water that could explain the present trends. One could however postulate that the exchange of alkali for hydrogen as network stabilizer role for an aluminate tetrahedron is the determining factor in controlling the melt viscosity and that the equivalent viscosities might imply that this exchange is nearly complete in both hydrous LiAlSi3O8 and KAlSi3O8 melts.

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