The nature of multicomponent aluminosilicate melts

Abstract The parental silicate melts of the most abundant igneous rocks in the earth's crust may be regarded as multicomponent solutions of as many as 15 chemically discrete, electrostatically neutral complexes or species. These species arecorrelated with thermodynamic components and, by adoption of a “quasi-crystalline” model, are identified, both in terms of chemical composition (stoichiometry) and their basic structural components, with the crystalline phases that form on the liquidus under equilibrium conditions. Among the approximately 10 end-member components (species) in any one of these igneous-rock melts, the three aluminosilicate (feldspar-like) components, NaAlSi3O8 (ab), CaAl2Si2O8 (an), and KAlSi3O8 (or), together comprise more than 50 mole % of the solution, hence they may be regarded as a multicomponent solvent in which the several other non-aluminosilicate species are dissolved. The three components of this multicomponent solvent, moreover, are found from the solution behavior of H2O to mix essentially ideally with each other. Similarly, the non-aluminosilicate component, Si4O8 (qz), is found to mix ideally with the aluminosilicates of the solvent. The thermodynamic behavior of the end-member components as a function of liquidus temperature, pressure, and composition (including H2O content), interpreted in accordance with the quasi-crystalline model, lead to the recognition of two basic types of homogeneous melt reactions that result in formation of other species from the end-member components. These end-member components—especially the feldspar-like aluminosilicates (a)—may undergo partial dissociation, or the aluminosilicates may interact with the non-aluminosilicate components. In either type, the reaction may involve no change in the coordination number of the Al atoms (IVAlO45− in the feldspar-like species), or it may involve a change to higher coordination numbers (VAlO57− and VIAlO69−) for at least part of the Al atoms. This tendency toward variable coordination of Al is regarded as the principal factor in determining the nature and extent of speciation reactions in aluminosilicate-rich melts, especially at high pressures where higher coordination numbers are favored. The basis for these interpretations is the difference between the mole fraction of end-member component i (Xiam), as obtained from published experimental liquidus data, and the activity of i (aiam), calculated from internally consistent sets of Gibbs free energies and volumes of melting for crystalline material of pure i composition. These differences for the end-member aluminosilicate components, Xaam − aaam, are either zero (no speciation reaction involving a) or greater than zero (speciation reaction consumes a) in solutions containing such chemically and structurally dissimilar species as Si4O8 (qz) and Mg4Si2O8 (fo), a forsterite-like species. The thermodynamic consequences of attributing the differences to speciation reactions are to return the aluminosilicate components to Raoultian behavior in melts where aluminosilicates crystallize on the liquidus. By the same procedures, Si4O8 and Mg4Si2O8 also become Raoultian in behavior when mixed with the aluminosilicates, but more experimental data are needed to evaluate the mixing properties of other geochemically important melt components. The relationships between Xim and aiam, when expressed in simple empirical equations of the form Xiam − aiam = Xiam ƒ(P, Xjam,Xwm), contain the information required to calculate liquidus equilibrium relations in hydrous (Xwm) and anhydrous (Xi,j,…am) melts of geochemical interest over a wide range of conditions. When interpreted in terms of the quasi-crystalline model, they also provide new insights into the nature of multicomponent aluminosilicate melts and into numerous magmatic phenomena.