Influence of pressure, temperature, and bulk composition on the structure of di- and tetra-aluminosilicate melts and quenched melts have been studied with Raman spectroscopy. The melt polymerization and Al(Al+Si) ranges represented are those typically found in quartz-normative basaltic magmatic liquids. The reaction describing the equilibrium among coexisting units in the melts is T2O5(2Q3)?TO3(Q4), where T=Si+Al, and the superscripts on the alternative Q- notations represent the number of bridging oxygen in the unit. The reaction shifts to the right with increasing temperature, pressure and Al(Al+Si). The ΔG for the reaction (derived from molar abundances under the asusumption of ideal mixing) decreases (becomes more negative), therefore, with increasing temperature, pressure, and bulk melt (Al(Al+Si). As the ionization potential of charge-balancing and network-modifying cations is increased at constant pressure, temperature, and Al(Al+Si), the ΔG increases. The Al3+ exhibits a preference for the most polymerized of the coexisting structural units (TO2 or Q4) probably because among these unit the TO2 units have the smallest average interterahedral angle.

It is proposed that the difference in intertetrahedral angle between the coexisting units exerts an important control on the magnitude of the Al- preference. The angle difference dereases as the network-modifying cation becomes smaller and the Al3+ preferences is less pronounced. The bulk compositional, temperature, and pressure effects leading to decreasing ΔG also weaken bridging oxygen bonds in the TO2 units. Transport properties (viscosity, diffusivity, and conductivity) are directly related to the strenght and abundance of the bridging oxygen bonds. Increasing pressure results in enhanced cation diffusivity and decreased viscosity, and these trends will be more pronounced the larger the charge-balancing and network-modifying metal cation and the more aluminous the melt. ¿American Geophysical Union 1990