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Structure of Glasses and Melts

Institute of Mathematical and Physical Sciences University of Wales Aberystwyth Ceredigion, SY23 3BZ, United Kingdom, e-mail: martin.wilding@aber.ac.uk

Chris J. Benmore

Intense Pulsed Neutron Source Argonne National Laboratory 9700 Cass Avenue Argonne, Illinois, 60439, U.S.A., e-mail: benmore@anl.gov

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Melting
The liquid state dominates terrestrial and planetary processes. The history of the early solar system for example involved the accretion of primarily molten bodies. In cooler temperature regimes, interplanetary dust, comets and the moons and planets in the outer part of the solar system are dominated by ices which may be present in liquid or amorphous forms (Cernicharo and Crovisier 2005; Porco et al. 2005a,b). The interior of the Earth and terrestrial planets are dominated by magnesium silicate minerals, a reflection of separation of iron dominated liquids from chondrite composition planetesimals (Poirier 2000). Subsequently planets evolved through segregation, crystallization and volcanic activity; all dominated by liquid state processes. The oceanic and continental crust, while compositionally distinct form the mantle is dominated by liquid state processes, the oceanic crust resulting from basaltic volcanism. Hydrothermal processes are important agents for geochemical processes in the Earth’s crust and also, according to recent surveys on Mars (Neukum et al. 2004; Bullock 2005; Newsom 2005).

Phase diagrams at constant pressure maps out stability domains of various crystalline and liquid phases as a function of composition and temperature. For example simple binary oxides MgO and SiO₂ (Wilding et al. 2004a) (Fig. 1) can show a variety of crystalline and liquid state structures which maybe further restricted in terms of important stoichiometric end-members such as Mg₂SiO₄ and MgSiO₃.

View larger version (18K):   Figure 1. Section of the earth and phase diagram of forsterite-SiO2 system at ambient pressure (adapted from Levin 1956). Fo; fosterite, En; enstatite, Cr; cristobalite, Tr; tridymite.

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