The study of the physical properties of silicate melts is now at an exciting point. Given that a large amount of data exists for average melts at average conditions, we can now build on this knowledge to investigate melts at extreme conditions, to observe the unusual behaviour of melts. The combination of the average data that already exist and newer observations from extreme conditions illustrates how challenging the understanding of silicate melts is. Melts at extreme conditions do not show the physical properties extrapolated from the measurements at average conditions.
There is a range of extreme conditions for silicate melts:
Structure: structure varies with temperature, pressure and composition (T, P, X) and controls the physical properties of melts.
Composition: both Si-rich and Si-poor melts are yet to be investigated thoroughly, as well as Al-rich and Al-poor melts.
Temperature: both high- and low-temperature conditions - this means low and high viscosities, respectively.
Pressure: viscosity will either increase or decrease with pressure depending upon composition.
Time: the investigation of the change in physical properties with time as the melt structure equilibrates with the change in applied stress or temperature.
Although the physical properties of melts at these different conditions will be discussed separately, they are inter-related. The physical properties are a function of structure, which in turn is a function of composition, temperature, pressure and time.
In studying silicate melts the properties density, viscosity, surface tension, compressibility, electrical conductivity, and their dependence on pressure, temperature and composition are determined.