The stmctural role of Al3+ in quenched silicate melts at 1 atm pressure as a function of composition has been explored with Raman spectroscopy. The compositions studied have sufficient or excess alkali or alkaline earth contents to balance the charge of Al3+ in tetrahedral coordination.
Aluminum was added as CaAl2O4, NaAlO2 and Al2O3 to melts of Na2Si2O5 and CaSi2O5 composition. Published spectroscopic data on quenched melts of these compositions indicate that their anionic structures consist of mixtures of units that have, on the average, NBO/Si = 2 (chain), 1 (sheet) and 0 (three-dimensional network) forming in bulk NBO/Si = 1 (NBO/Si; nonbridging oxygens per silicon). Whenever there is sufficient Na+ in the melt to balance the charge of tetrahedral Al3+, sodium is the charge-balancing cation whether or not Ca2+ is also present. With up to about 10 mole % Al2O3 (about 6 wt.%) all Al3+ enters the three-dimensional network structural unit. In the compositional range between 10 and 20 mole % Al2O3, aluminum occurs in both the three-dimensional network and the sheet structural units with a preference for three-dimensional network > sheet > chain structure.
In systems with only Ca2+ present, only two compositional regions may be defined. With up to 10 mole % Al2O3, Al3+ enters both three-dimensional network structures and sheet structures. With more aluminum, Al3+ enters all structural units with the same preference as in the sodic system.
On the basis of the present data and other published structural data on Ti4+, P5+ and Fe3+ in quenched sihcate melts, the anionic constitution of common igneous melts has been calculated. Almost all natural magmas have a ratio of nonbridging oxygen per tetrahedral cation (NBO/T) less than 1. Basalts have NBO/T between 0.9 and 0.6, andesitic melts have about 0.3 and granitic melts, between 0.2 and 0.05. The anionic structure of these melts can be described as mixtures of unit with NBO/T = 2, 1 and 0. In all melts, the three-dimensional network unit is the most aluminous. In general, acidic melts have higher Si/(Si+Al) of the structural units than basic melts. The value of this ratio in melts of igneous rocks is positively correlated with geochemical indicators such as Mg/(Mg+Fe2+). The Si/(Si+Al) also is positively correlated with the activation energy of viscous flow of natural magma at 1 atm pressure.