Experimental, in situ, high-temperature studies of properties and structure of silicate melts relevant to magmatic processes
European Journal of Mineralogy Volume 7 Number 4 (1995), p. 745 - 766
published: Aug 1, 1995
manuscript accepted: Dec 21, 1994
manuscript received: Sep 12, 1994
ArtNo. ESP147050704004, Price: 29.00 €
Abstract Important aspects of structure and properties of natural magmatic liquids may be examined in appropriate portions of the system MOn/2-Al2O3-SiO2, where M = Na, K, Mg, Fe2+, and Ca. Within this system physicochemical properties of binary metal oxide-silica (MOn/2-SiO2) melts are often simple functions of bulk composition. For example, activity coefficients of SiO2 are linear functions of the ionization potential of the metal cation (at constant electric charge), and heat capacities are linear functions of metal/silicon. In chemically more complex aluminosilicate melts, the distribution of Al3+ among structural units is non-random and the perturbation of the bridging oxygen bonds by Al3+Si4+ substitution affect melt properties in a nonlinear fashion. The anionic structure of silicate and aluminosilicate melts and glasses to temperatures at or exceeding those of natural magmatic liquids is described in terms of simple structural units (Q-species) characterized by their individual number of bridging oxygen per tetrahedrally coordinated cation (Si4+ and charge-balanced Al3+). These units are linked via their nonbridging oxygens between the metal cation(s). The equilibria among these units are simple functions of metal/silicon, electronic properties of the metal cation, Al/Si, and temperature. The relationships between bulk composition, temperature, and structure can be employed to describe activity-composition relations among the units, their relationships to viscous behavior, and their contribution to configurational properties (e.g., configurational entropy and heat capacity) of melts.