부산대학교 도서관

Experiments at high pressures and temperatures reveal the stability of a Fe 4O 5-type structured phase in several simple chemical systems. On the one hand, the Fe 4O 5 end-member is stable in the presence of SiO 2-rich phases, including stishovite, but contains ≤0.01 Si cations per formula unit. This indicates that Si is essentially excluded from this phase. On the other hand, the Fe 4O 5 phase can form solid solutions with Mg and Cr and can coexist with silicate phases at the high P– T conditions expected in the transition zone of the mantle (i.e. >~9 GPa). It can coexist with both wadsleyite and Mg-rich ringwoodite and can contain at least 25 mol% Mg 2Fe 2O 5 component. The Fe 4O 5 phase always contains the least amount of Mg in any given mineral assemblage. Cr-bearing Fe 4O 5 has been synthesised with up to 46 mol% Fe 2Cr 2O 5 component and can coexist with spinel and/or hematite-eskolatite solid solutions. Substitution of Mg and Cr for Fe 2+ and Fe 3+, respectively, leads to variations in Fe 3+/∑Fe from the ideal value of 0.5 for the Fe 4O 5 end-member composition, which can influence its redox stability. These cations also have contrasting effects on the unit-cell parameters, which indicate that they substitute into different sites. This initial study suggests that Fe 4O 5-type structured phases may be stable over a range of P– T– fO 2 conditions and bulk compositions, and can be important in understanding the post-spinel phase relations in a number of chemical systems relevant to the Earth’s transition zone. Thus, the presence of even small amounts of Fe 3+ could alter the expected phase relations in peridotitic bulk compositions by stabilising this additional phase. [ABSTRACT FROM AUTHOR]