Dissolved water has significant effects on the physical and chemical properties of silicate melts. Some of the different approaches towards understanding these effects are reviewed here. Spectroscopic measurements on hydrous glasses quenched from melts provide good models for the structure of the melts at the glass transition temperature (Tg). Such measurements suggest that the mechanism of dissolution of water in silicate melts varies strongly with the bulk composition. In particular framework aluminosilicate compositions seem to have very different dissolution mechanisms from Al-free compositions. The water speciation reactions are temperature dependent, with some of the molecular water which is present in glasses at room temperature being converted to hydroxyl at high temperatures. This conversion probably occurs only above Tg. Data on the kinetics of the speciation reaction and the dynamics of microscopic processes in hydrous silicate melts are also discussed. Finally some important aims of future work on hydrous silicate melts are suggested

The commissioning and operation of apparatus for neutron diffraction at simultaneous high temperatures and pressures is reported. The basic design is based on the Paris-Edinburgh cell using opposed anvils, with internal heating. Temperature is measured using neutron radiography. The apparatus has been shown in both on-line and off-line tests to operate to a pressure of 7 GPa and temperature of 1700°C. The apparatus has been used in a neutron diffraction study of the crystal structure of deuterated brucite, and results for 520°C and 5.15 GPa are presented. The diffraction data that can be obtained from the apparatus are of comparable quality to previous high-pressure studies at ambient temperatures, and are clearly good enough for Rietveld refinement analysis to give structural data of reasonable quality.

The thermodynamics and kinetics of CO2 speciation in silicate melts have been studied by measuring the concentration of CO2mol and carbonate in jadeite glass annealed at 575, 450 and 400°C. Assuming that the reaction is

1

where CO2mol..Obr represents a CO2 molecule weakly bonded to a bridging oxygen in the network and CO3 represents a bridging carbonate group with no net negative charge, ΔH for the reaction is –17 (+4/–8) kJ mol−1 and ΔS is –24 (+6/–9) J K−1 mol−1. The rate of equilibration of the species was measured at each temperature and the rate constants were deduced. The temperature dependence of the rate constants was used to determine the activation energy of the forward and reverse reactions which are 68 (+3/–31) kJ mol−1 and 86 (+1/–69) kJ mol−1 respectively. The data suggest that CO2mol may be much more abundant in silicate melts than previously assumed on the basis of studies of CO2-bearing glasses. Models of solubility, diffusion, and isotope fractionation should take this into account.

The structure of a synthetic end-member wairakite (CaAl2Si4O12·2H2O) has been determined using Rietveld analysis of high-resolution, synchrotron X-ray powder diffraction data, and 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy. The framework in the synthetic sample is more disordered than that in natural wairakite. Ca is distributed over the cavity cation sites M2, M12A, M12B in the approximate proportions 0.8:0.1:0.1, respectively, with M11 being vacant. 29Si MAS NMR data are consistent with about 80% of the Si occupying tetrahedral T11 and T12 sites linked to two Al atoms [Q4(2Al) silicons]. Tetrahedral and cavity cation site disorder are coupled so that Al mainly occupies T2 sites, with Ca in M12A and M12B being balanced by Al in T12A and T12B; T11A and T11B sites appear to only contain Si, in agreement with the M11 site being vacant. The crystal chemistries of the wide range of stoichiometries which crystallize with the leucite/pollucite structure-type are also reviewed, with particular attention being paid to the tetrahedral ordering configurations present in these phases, and the implications to crystallographic phase transitions.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Muon diffusion can be studied in metals without nuclear moments by the addition of paramagnetic impurities. We describe the theory and report measurements of muon depolarization in gold and silver doped with gadolinium and erbium. As well as diffusion rates, strength of the magnetic interaction and spin lattice relaxation rate of the paramagnetic ion are inferred from the data.