Cation distribution and valence in synthetic Al–Mn–O and Fe–Mn–O spinels under varying conditions

ABSTRACTThe spinel-group minerals, found in a range of igneous rocks, are resistant to weathering and can incorporate several multivalent elements, meaning they have the potential to provide insight into the redox conditions of parental magmas. Naturally occurring spinel can contain varying quantities of Mn, an element which occurs terrestrially and extra-terrestrially as Mn2+, Mn3+, Mn4+ and Mn5+. However, a lack of information on the effects of oxygen fugacity ($f_{{\rm O}_{\rm 2}}$) on: (1) Mn valence state and cation distribution; and (2) on spinel-melt partitioning means that the potential for a Mn-in-spinel oxy-barometer remains largely untested. Here, we use electron probe microanalysis, micro-focus X-ray Absorption Near Edge Structure (XANES) spectroscopy and single-crystal X-ray diffraction (SC-XRD) to investigate cation distribution and valence state in spinels in the Al–Mn–O and Fe–Mn–O systems synthesized at ambient pressure under varying $\hskip 2pt f_{{\rm O}_{\rm 2}}$ conditions. In contrast to previous studies, we find that the spectral resolution of the Mn K-edge XANES spectra is insufficient to provide quantitative data on Mn valence state and site occupancy, although it does verify that Mn is incorporated as both Mn2+ and Mn3+, distributed over tetrahedral and octahedral sites. Combination of data from XANES and SC-XRD refinements can, however, be used to model Mn, Al and Fe valence and site occupancy. It would be expected that Mn–Fe spinels have the potential to record $f_{{\rm O}_{\rm 2}}$ conditions in parental melts due to changes to the octahedral site under conditions that were more reducing. However, decoupling the effects of temperature and oxygen fugacity on the TFe3+–TMn2+ exchange in the Mn–Fe spinels remains challenging. In contrast, little variation is noted in Mn–Al spinels as a function of $\hskip 2pt f_{{\rm O}_{\rm 2}}$, implying that crystal chemistry and cation site geometry may significantly influence cation distribution, and by inference, crystal-melt partitioning, in spinel-group minerals.