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Cation distribution and valence in synthetic Al-Mn-O and Fe-Mn-O spinels under varying fO2 conditions

DOI: 10.1180/mgm.2018.109 DOI Help

Authors: T. N. Stokes (University of Edinburgh) , G. D. Bromiley (Università degli Studi di Milano) , G. D. Gatta (Università degli Studi di Milano) , N. Rotiroti (Università degli Studi di Milano) , N. J. Potts (University of Edinburgh) , K. Saunders (University of Edinburgh)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Mineralogical Magazine

State: Published (Approved)
Published: May 2018
Diamond Proposal Number(s): 10070

Abstract: The 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 redox conditions of parental magmas. Naturally occurring spinel can contain varying quantities of Mn, an element which occurs terrestrially and extraterrestrially as Mn2+, Mn3+, Mn4+ and Mn5+. However, a lack of information on the effects of oxygen fugacity (fO2) 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 fO2 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 fO2 conditions in parental melts due to changes to the octahedral site under more reducing conditions. 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 fO2, implying that crystal chemistry and cation site geometry may significantly influence cation distribution, and by inference, crystal-melt partitioning, in spinel group minerals.

Journal Keywords: spinel; manganese; XANES; oxygen fugacity; jacobsite; galaxite

Subject Areas: Earth Science, Chemistry

Instruments: I18-Microfocus Spectroscopy