Relationships between silicate melt structure and petrologic processes

Abstract

Structure and property data for silicate melts at 1 bar in unary, ternary, quaternary and complex natural compositions have been reviewed. From vibrational and NMR spectroscopic information, the data are consistent with an anionic structure of depolymerized (NBO/T>0) melts that can be described in terms of a small number of stoichiometrically simple anionic units. Aluminum in aluminosilicate melts generally is distributed between these units with a very strong preference for the most polymerized unit. Ferric iron generally is in tetrahedral coordination, but when Fe 3/ΣFe is less than about 0.3, octahedral coordination dominates. Phosphorus and titanium most probably are tetrahedrally coordinated, but these cations form separate complexes instead of substitution for Si 4+ in the anionic network. Expressions that relate proportions of anionic units to overall melt polymerization and Al/(Al + Si) have been derived from the simple system data. These and other information are then employed to estimate the anionic structure of natural silicate melts at 1 bar. The nonbridging oxygens per tetrahedrally coordinated cations in natural magmatic liquids generally fall in the range between 0 and 1 with increasing degree of depolymerization (increasing NBO/T) as a melt becomes more mafic. The principal anionic units in magmatic liquids are three-dimensional network units, units withNBO/T= 1, 2, and 4. The principal reservoir of nonbridging oxygens is found in the units withNBO/T= 2. The numerically dominant network-modifying cation is natural magmatic liquids in magnesium, followed be ferrous iron. Alkali metals usually are needed to charge-balance tetrahedrally coordinated Al 3+, and play a subordinate role as network-modifier in most magmatic liquids. Many properties of silicate melts can be rationalized in terms of mixing of these simple anionic units. These properties include viscosity, volume properties, thermochemical properties and liquidus phase relations.