HR: 14:10h
AN: MR33A-03    [Abstracts]
TI: The role of transformation twins on enhanced diffusion in minerals.
AU: * Redfern, S A
EM: satr@cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom
AB: Since the 1930's physical chemists have been aware that many solids show enhanced diffusion rates and reaction rates in the vicinity of a phase transition in one or other of the phases involved in a reaction or coupled diffusion process. This phenomenon has been termed the "Hedval effect", following Hedval's brief mention of it in one of his chemistry texts. The origins of this phenomenon have remained obscure, however. A number of recent examples now demonstrate that enhanced diffusion along transformation twin walls can be expected in certain silicate structures. Transformation twin walls in quartz show enhanced diffusion of cations such as Na+ and Li+, transport being limited only by charge balancing mechanisms. The density of such domain walls is known to be highly sensitive to temperature, showing a singularity as the transformation temperature is approached. In view of this, one origin of the Hedval effect, of enhanced transport close to the transition temperature of a mineral, can be attributed to the presence of domain walls and the increase in the number of these walls close to the transformation temperature.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3904 Defects
DE: 3999 General or miscellaneous
DE: 4465 Phase transitions
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting