Shear viscosities of CaO-AL(2)O(3)-SiO2, and MgO-Al2O3-SiO2 liquids: Implications for the structural role of aluminium and the degree of polymerisation of synthetic and natural aluminosilicate melts

摘要

The shear viscosity of 66 liquids in the systems CaO-Al2O3-SiO2, (CAS) and MgO-Al2O3-SiO2 (MAS) have been measured in the ranges 1-10(4) Pa s and 10(8)-10(12) Pa s. Liquids belong to series, nominally at 50, 67, and 75 mol.% SiO2 with atomic M2+/(M2++2Al) typically in the range 0.60 to 0.40 for each isopleth. In the system CAS at 1600degreesC. viscosity passes through a maximum at ail silica contents. The Maxima are clearly centered in the peraluminous field, but the exact composition at which viscosity is a maximum is poorly defined. Similar features are observed at 900degreesC. In contrast. data for the system MAS at 1600degreesC show that viscosity decreases with decreasing Mg/(Mg + 2Al) at all silica contents. but that a maximum in viscosity must occur in the field where Mg/2Al > 1. On the other hand, the viscosity at 850degreesC increases with decresing Mg/(Mg + 2Al and shows no sign of reaching a maximum, even for the most peraluminous composition studied. The data from both systems at 1600degreesC have been analysed assuming that shear viscosity is proportional to average bond strength and considering the equilibrium:Al-[4]-(Mg, Ca)(0.5) double left right arrow (Mg, Ca)(0.5)-NBO divided by Al-XSwhere Al-[4]-(Mg.Ca)(0.5) represents a charge-balanced tetrahedrally coordinated Al; (Mg, Ca)(0.5)-NBO represents a nonbridging oxygen (NBO) associated with Ca or Mg, and Al-XS represents any structural role of Al that does not require a charge-balancing cation. The viscosity data were fitted using two adjustible variables: i) the equilibrium constant of the above reaction, and ii) the relative bond strength of Al-XS. The values of these parameters in the system CAS suggest that Al,, remains in tetrahedral coordination. its charge deficit being satisfied by association with a three-coordinate oxygen in a structure called a tricluster. In contrast. fits to the MAS data at 1600degreesC infer the presence of high-coordinate Al. These interpretation S are found to be consistent with independent spectroscopic and theoretical data. Furthermore, the fitted value of the equilibrium constant in the system MAS is close to zero. implying that Al has no thermodynamic preference to be charge-balanced by Mg. In light of these results, it is shown that values of the ratio 'NBO/T' (the ratio of NBO to tetrahedrally coordinated network formers) will not be a true reflection of the polymerisation state of the liquid if ii is assumed that all Al is char-e-balanced by metal cations. This will be important to take into account when considering the compositional dependence of physical and thermodynamic parameters and may have direct relevance to certain liquids produced during partial melting of the mantle. Copyright (C) 2004 Elsevier Ltd.