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3.3 c. Iron crystal chemistry in (Mg,Fe)SiO3 majorite (C.A. McCammon, in collaboration with N.L. Ross/London)

Results from high-pressure experiments combined with geophysical data have indicated that majorite is likely a dominant phase in the transition zone. Due to its variable oxidation state, iron is expected to play a major role in determining the physical and chemical properties of majorite. As a first step to characterize the effect of iron on majorite, we have examined the system (Mg,Fe)SiO3. Samples were synthesised using a multianvil press at varying oxygen fugacity conditions. All samples were examined using the electron microprobe, Mössbauer spectroscopy, powder X-ray diffraction and optical spectroscopy. Results show that Fe3+/Fe in (Mg,Fe)SiO3 majorite increases with both increasing oxygen fugacity and total iron content. Rapid electron transfer occurs between Fe2+ - Fe3+, likely across the shared edge between the octahedral and dodecahedral sites. Fe3+ appears to occupy the octahedral site primarily, and the amount of Fe2+ in the octahedral site decreases with increasing Fe3+ content, coupled with an increase in unit cell volume. We are using this data to construct plausible defect models that balance the charge deficit caused by the substitution of Fe3+ for Si4+.

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