Dislocations in meteoritic and synthetic majorite garnets

Abstract

Shock veins in the Acfer 90072, L5/6 (S6) chondrite contain polycrystalline fragments constituted of ringwoodite and majorite. The dislocation microstructures of the majorite crystals have been characterised by transmission electron microscopy and large-angle convergent-beam electron diffraction. Most of the dislocations form sub-grain boundaries organised in cells. Some free dislocations can be found in the cells. Dislocations with 1/2 and Burgers vectors have been characterised. These microstructures compare well with those observed in majorite synthesised from a glass in multi-anvil experiments. These observations suggest that the dislocation microstructures observed in both synthetic and meteoritic majorite result from imperfect growth.