EXPERIMENTAL, IN-SITU, HIGH-TEMPERATURE STUDIES OF PROPERTIES AND STRUCTURE OF SILICATE MELTS RELEVANT TO MAGMATIC PROCESSES

Important aspects of structure and properties of natural magmatic liqu ids may be examined in appropriate portions of the system MO(n/2)-Al2O 3-SiO2, where M = Na, K, Mg, Fe2+, and Ca. Within this system physicoc hemical properties of binary metal oxide-silica (MO(n/2)-SiO2) melts a re often simple functions of bulk composition. For example, activity c oefficients of SiO2 are linear functions of the ionization potential o f the metal cation (at constant electric charge), and heat capacities are linear functions of metal/silicon. In chemically more complex alum inosilicate melts, the distribution of Al3+ among structural units is non-random and the perturbation of the bridging oxygen bonds by Al(3+) double left right arrow Si4+ substitution affect melt properties in a nonlinear fashion. The anionic structure of silicate and aluminosilica te melts and glasses to temperatures at or exceeding those of natural magmatic liquids is described in terms of simple structural units (Q-s pecies) characterized by their individual number of bridging oxygen pe r tetrahedrally coordinated cation (Si4+ and charge-balanced Al3+). Th ese units are linked via their nonbridging oxygens between the metal c ation(s). The equilibria among these units are simple functions of met al/silicon, electronic properties of the metal cation, Al/Si, and temp erature. The relationships between bulk composition, temperature, and structure can be employed to describe activity-composition relations a mong the units, their relationships to viscous behavior, and their con tribution to configurational properties (e, g., configurational entrop y and heat capacity) of melts.