Microstructures and structural phase transition in (Mg,Fe)SiO₃ majorite

(Mg_1-x,Fe_x)SiO₃ majorites with the compositions of X=0.0-0.20 were synthesized at 20 GPa and 1950-2200 °C in a uniaxial split-sphere apparatus, and the recovered samples were examined by analytical transmission electron microscopy. Although the diffraction patterns of synthetic majorites in all the recovered specimens show a tetragonal (I4₁/a) symmetry, specimens quenched from temperatures above ~1950 °C show frequent pseudomerohedral twins and modulated "tweed" structures, while specimens quenched from temperatures below ~1950 °C show few or no twins. Right-angled pseudomerohedral twins domains occur without dislocations to relax strain at the domain corners. These observations suggest that twin and tweed structures in tetragonal majorite form by the cubic-tetragonal phase transition during quenching and that cubic majorite is stable at temperatures above ~1950 °C at 20 GPa. Natural majorites in shocked chondrites appear to be cubic. The difference between natural cubic majorites in shocked meteorites and synthetic tetragonal majorites is attributed to the faster cooling rate in natural samples.