Dislocations in meteoritic and synthetic majorite garnets

Shock veins in the Acfer 90072, L5/6 (S6) chondrite contain polycrystalline fragments constituted of ringwoodite and majorite. The dislocation microst ructures of the majorite crystals have been characterised by transmission e lectron microscopy and large-angle convergent-beam electron diffraction. Mo st of the dislocations form sub-grain boundaries organised in cells. Some f ree dislocations can be found in the cells. Dislocations with 1/2<111> and <100> Burgers vectors have been characterised. These microstructures compar e well with those observed in majorite synthesised from a glass in multi-an vil experiments. These observations suggest that the dislocation microstruc tures observed in both synthetic and meteoritic majorite result from imperf ect growth.