Lattice defects in lawsonite: a TEM investigation

Lattice defects in lawsonite have been studied by transmission electron mic roscopy. 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 d isplays high sensitivity to the electron beam, even at low temperatures. In situ precipitates appear as a consequence of beam irradiation. The precipi tation takes places first on dislocations, then on twin boundaries and then in the matrix, causing 'coffee-bean' contrast features typical of precipit ates. The studies were performed at low temperature (similar to 110 K) in o rder to investigate the low temperature displacive transitions from space g roup Cmcm to Pmcn and P2(1)cn and elucidate their microscopic character. No characteristic microstructural texture, such as antiphase domains associat ed with the transition, were observed, however. This is probably due to the high mobility of protons under the electron beam. The development of regul arly spaced dislocations along twin planes is hypothesized as the only evid ence that a phase transition takes place at a nanoscale.