TEMPERATURE AND PRESSURE EFFECTS ON SILICATE MELT STRUCTURE: FROM “MICROSCOPIC TO MACROSCOPIC” (VIA THE “NANO-SCALE”)

Accurate data on the macroscopic properties of silicate melts, such as heat capacity, density, and viscosity, are critical for the quantitative understanding of magmatic processes. Those making such measurements have often realized that the complex behavior of such materials points to fundamental questions of how the short- to intermediate-scale structure of the liquid changes with temperature and pressure. Although answers to many of these questions remain unresolved, considerable progress has been made in recent years through the application of spectroscopic and scattering methods to quenched glasses and, in-situ, to high temperature and even high pressure liquids. One example is the increase in configurational disorder with temperature in aluminosilicate melts, involving complex interactions between the distribution of the network cations (Si, Al), proportions of bridging and non-bridging oxygens, and coordination changes (e.g. of Al). All contribute substantially to the entropy and thus must be part of calculations of free energy and phase equilibria. A second example is the large effect of the size and charge (field strength) of both network-former and network-modifier cations on the rate of increase of Al and Si coordination in melts with pressure, an important part of mechanisms of densification. Such effects are again complex and non-linear, suggesting that accurate models of compositional effects on melt density will also need to be complex. Between the short-range structure that is most readily observed by spectroscopy, and the macroscopic behavior that controls Earth processes, structures may emerge at the intermediate-range or “nano” scale that can, on the one hand, provide clues to energetics, and, on the other, sometimes be critical to bulk properties. Phase separation and crystal nucleation are the obvious examples from high-temperature melt systems. Thus, a final example of the micro- to macro- connection is a newly-applied NMR spectroscopic method for detecting liquid-liquid phase separation, and characterizing domain composition and structure.