Electron channelling spectroscopy of iron in majoritic garnet and silicate perovskite using a transmission electron microscope

Majoritic garnet and silicate perovskite assemblages synthesised from natural pyroxene in the multi-anvil press and diamond anvil cell were studied by energy dispersive X-ray spectroscopy (EDXS) and electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) to quantify the composition and oxidation state of iron. In these assemblages of the (Mg, Fe)SiO3–Al2O3 system, Fe3+ has a much stronger affinity to silicate perovskite than to majoritic garnet at the same pressure, temperature and oxygen fugacity. To determine the site occupancy of iron in majoritic garnet and silicate perovskite, we have also performed both spectroscopic techniques (EDXS and EELS) under channelling conditions. Electron channelling spectroscopy enables the octahedral sites of both phases to be distinguished from other cation sites by variations in the intensity ratios of the EDXS and EELS spectra acquired systematically at tilting conditions about the channelling-sensitive crystal plane and zone axis. In majorite almost all of ferric iron is partitioned into the octahedral site, whereas in silicate perovskite both ferrous and ferric iron substitute preferentially into the pseudo-dodecahedral site of the perovskite structure. The latter is charge balanced by aluminium in the octahedral site.