Abstract 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.

Abstract The study of the Earth’s interior is based upon the comparison of laboratory data on longitudinal and shear wave speeds of minerals with the seismic wave speeds from the Earth (see Fig. 1 ). This requires (1) laboratory measurements of the temperature- and pressure-dependence of single-crystal elastic moduli to be recast in terms of wave speeds and densities of polycrystalline materials of possible mantle compositions and mineralogies; together with (2) highly accurate information on seismic wave speeds as a function of depth in the mantle, together with (3) jumps in wave speeds due to phase transitions, (4) a temperature profile of the Earth, (5) a density profile of the Earth, (6) a pressure profile of the Earth, together with (7) a petrological model of the Earth as a function of depth. While seismologists and petrologists have been acquiring their data, mineral physicists have been working on new, varied and imaginative methods of measuring wave speed first in single crystals, and more recently in polycrystalline materials at the temperature and pressure conditions of the Earth. There are a range of different methods used to determine the speed at which stress waves travel through materials at high-pressure and temperature conditions (see Fig. 2 ). These methods include the following: Shock wave measurements involve shooting a projectile at the sample of interest. The resulting collision creates high temperature, high pressure conditions within the sample, and the speed at which the shock wave travels through the sample is measured (e.g. Jackson & Ahrens, 1979 ; Watt & Ahrens, 1986 ; Luo et al., 2002 ; Panero et al., 2003 ; Langenhorst & Hornemann, 2005 ).