A magnesium vanadate spinel crystal, ideally MgV2O4, synthesized at 1 bar, 1200 °C and equilibrated under FMQ + 1.3 log fO2 condition, was investigated using single-crystal X-ray diffraction, electron microprobe, and electron backscatter diffraction (EBSD). The initial X-ray structure refinements gave tetrahedral and octahedral site occupancies of T(Mg0.9660.034) and M(V3+0.711V4+0.109Mg0.180), respectively, along with the presence of 0.053 apfu Mg at an interstitial octahedral site (16c). Back-scattered electron (BSE) images and electron microprobe analyses revealed the existence of an Mg-rich phase in the spinel matrix, which was too small (≥ 3 μm) for an accurate chemical determination. The EBSD analysis combined with X-ray energy dispersive spectroscopy (XEDS) suggested that the Mg-rich inclusions are periclase oriented coherently with the spinel matrix. The final structure refinements were optimized by subtracting the X-ray intensity contributions (~9%) of periclase reflections, which eliminated the interstitial Mg, yielding a structural formula for spinel TMgM(V3+1.368V4+0.316Mg0.316)O4. This study provides insight into possible origins of refined interstitial cations reported in the literature for spinel, and points to the difficulty of using only X-ray diffraction data to distinguish a spinel with interstitial cations from one with coherently oriented MgO inclusions.

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