60 巻 (2018) 1 号 p. 2-8
Microstructures and symmetries of synthetic and natural（Mg, Fe）SiO3 garnet（majorite）have been investigated by transmission electron microscopy. Modulated and twin structures commonly observed in the synthetic majorite suggest a cubic-tetragonal phase transition due to cation ordering in its octahedral sites during quenching. Natural tetragonal majorite, which was newly discovered in a shock meteorite, has less-ordered octahedral cations than those in the synthetic samples. The order-disorder state of majorite provides important constraints not only on shock metamorphism of meteorites, but also on the deep Earth's mineralogy.