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Lattice defects in lawsonite: a TEM investigation

F. Cámara^1,*,, J. C. Doukhan¹ and M. A. Carpenter²

¹ Laboratoire Structure et Propriétés de l’Etat Solide Université Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq-Cedex, France
² Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK

^* E-mail: camara{at}crystal.unipv.it

Lattice defects in lawsonite have been studied by transmission electron microscopy. It is proposed that twinning and easy glide systems are 1/2[110]·{110}; the easy glide planes are coincident with twin planes. This mineral displays high sensitivity to the electron beam, even at low temperatures. In situ precipitates appear as a consequence of beam irradiation. The precipitation takes places first on dislocations, then on twin boundaries and then in the matrix, causing ‘coffee-bean’ contrast features typical of precipitates. The studies were performed at low temperature (~110 K) in order to investigate the low temperature displacive transitions from space group Cmcm to Pmcn and P2₁cn and elucidate their microscopic character. No characteristic microstructural texture, such as antiphase domains associated with the transition, were observed, however. This is probably due to the high mobility of protons under the electron beam. The development of regularly spaced dislocations along twin planes is hypothesized as the only evidence that a phase transition takes place at a nanoscale.

KEYWORDS: lawsonite, transmission electron microscopy, lattice defects, phase transition, twins

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