Structure and properties of magmatic liquids: From haplobasalt to haploandesite

As a step toward structural characterization of magmatic liquids and to cor relate their structure with melt properties, in-situ, high-temperature stru ctural data have been obtained along the joins N2Si3O7-Na-2(NaAl)(3)O-7 and Na2Si4O9-Na-2(NaAl)(4)O-9. A total of 211 data points containing anionic s peciation information have been determined and combined with published stru cture data from other alkali aluminosilicate melt systems. Numerical descri ption of the relationship between abundance of structural units, X-Qi, temp erature, and bulk composition was derived by stepwise regression of express ions such as: X-Qi = a + b . (NBO/T) + c . T + d/T + e . [Al/(Al + Si)] + f . [Al/(Al + S i)](2). The equilibrium constants, K, for the two relevant anionic equilibria, 2Q(3 ) double left right arrow Q(2) + Q(4) (1), and 2Q(2) double left right arro w Q(1) + Q(3) (2), were determined at temperatures above the glass transiti on. The enthalpy of reaction for the anionic equilibria was extracted from the linear relationships, In K = a + b/T. The enthalpy values for reaction (1) are in the range -30-70 kJ/mol and are systematic functions of Al/(Al + Si) and ionization potential of the metal cation. The enthalpy is not sensitiv e to bulk melt polymerization. The configurational heat capacity of individ ual anionic species were derived by combining structure data for metal oxid e silicate melts with published thermodynamic data. The configurational entropy and heat capacity of other aluminosilicate melt s were then estimated from those data combined with the data on abundance o f structural units. The topological and mixing contributions to those param eters were also evaluated. The topological contribution to both heal capaci ty and entropy represents more than 90% of the total value. Copyright (C) 1 999 Elsevier Science Ltd.