Journal of Conference Abstracts

Influence of Structural Phase Transitions on Reactivity

A. Aird (aa224@esc.cam.ac.uk)

Dept. of Earth Sciences, University of Cambridge.

How does structure affect chemical reactivity? This is a question of great importance to the chemical industry in the synthesis of materials, but as yet, little research has been done on the topic.

The effects of structure can be investigated by studying temperature controlled structural phase transitions. For example, quartz undergoes a transition from a trigonal (alpha-quartz) phase to a hexagonal (beta-quartz) phase at 573°C. The effect of temperature alone on the chemical reactivity can be predicted by thermodynamics, and by comparing this to the observed reactivity across the transition, we can see the effect of the structural change.

Here, the reactivity of quartz is studied across the alpha-beta phase transition, firstly by reaction with NiO using in-situ high temperature x-ray diffraction; and also by studying the diffusion of silver into a quartz crystal with a temperature gradient along its length. The aim is to correlate the reactivity as a function of temperature with the change in structure.

Cation Diffusion in Diopside: Experimental and Computer Modelling Studies

Feridoon Azough¹ (Feridoon.Azough@UMIST.ac.uk), Robert Freer¹ (R.Freer@fs2.mt.umist.ac.uk), C. R. A. Catlow² (Richard@RI.ac.uk) & R. A. Jackson³ (Fax + 44 - 1782 - 715944)

¹ Materials Science Centre, University of Manchester/UMIST, Manchester, M1 7HS.

² The Royal Institution of Great Britain, Albermarle Street, London, W1X 4BS.

³ Department of Chemistry, University of Keele, Keele, Staffordshire, ST5 5BG.

A number of important mineral thermometers and barometers are based on pyroxenes. Their operation depends critically upon the exchange behaviour of the major cations including Ca, Mg, Fe and Al. It is generally assumed that the diffusion rates of such species in pyroxenes are slow, but there is little direct evidence. We have undertaken experimental and computer modelling studies of cation diffusion in diopside to aid the understanding of transport processes in pyroxene minerals.

The computer simulation employed an atomistic approach; standard Mott-Littleton methods were used to simulate first the ideal, perfect lattice configuration and then the defective lattice in various configurations. In diopside the existence of three, non-equivalent oxygen sites complicates the assessment of defect formation energies, but overall the most favourable defects (with the lowest formation energies) are Ca vacancies(~350 kJ mol^-1) and Mg vacancies (~380 kJ mol^-1). Interstitial defects, both Mg and Ca have much higher formation energies (>500 kJ mol^-1). Due to the chain structure of diopside, cation diffusion is anisotropic, with the c-direction being easiest for Mg and Ca transport. The lowest (most favourable) calculated migration energies for Ca and Mg are ~190 kJ mol^-1.

Experimental studies of calcium and magnesium diffusion in single crystal diopside have been undertaken using tracers ⁴²Ca and ²⁶Mg. Thin films of individual tracers were applied to prepared [001] faces of diopside and evaporated to dryness. Samples were annealed at temperatures in the range 1000-1250°C at 1 bar total pressure. The resulting sub-micron diffusion profiles were determined by ion microprobe at Edinburgh University. Typical experiments at 1100°C yielded diffusion coefficients of 1.3x10^-19 m²s^-1 for Mg and 4x10^-20 m²s^-1 for Ca.

A detailed comparison of the experimental and simulation studies will be presented. The implications of the results for thermobarometry will be discussed.