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Fe  L  2,3 -edge XAS and XMCD studies have been used to unravel structural trends in the MgAl 2 O 4 –Fe 3 O 4  solid solution where thermodynamic modeling has presented a challenge due to the complex ordering arrangements of the end-members. Partitioning of Fe 3+  and Fe 2+  between tetrahedral (Td) and octahedral (Oh) sites has been established. In the most Fe-rich samples, despite rapid quenching from a disordered state,Fe    Td    2  +${\text{Fe}}_{{\text{Td}}}^{2 + }$    is not present, which matches the ordered, inverse spinel nature of end-member magnetite (Mgt) at room temperature. However, in intermediate compositions Al and Mg substantially replace Fe and small amounts ofFe    Td    2  +${\text{Fe}}_{{\text{Td}}}^{2 + }$    are found, stabilized, or trapped by decreasing occurrence of the continuous nearest neighbor Fe–Fe interactions that facilitate charge redistribution by electron transfer. Furthermore, in the composition range ~Mgt 0.4–0.9 , XAS and XMCD bonding and site occupancy data suggest that nanoscale, magnetite-like Fe clusters are present. By contrast, at the spinel-rich end of the series, Mgt 0.17  and Mgt 0.23  have a homogeneous long-range distribution of Fe, Mg, and Al. These relationships are consistent with the intermediate and Fe-rich samples falling within a wide solvus in this system such that the Fe-clusters occur as proto-nuclei for phases that would exsolve following development of long-range crystalline order during slow cooling.

american mineralogist

An X-ray magnetic circular dichroism (XMCD) study of Fe ordering in a synthetic MgAl 2 O 4 -Fe 3 O 4  (spinel-magnetite) solid-solution series: Implications for magnetic properties and cation site ordering