Investigation of synthetic Mg_1.3V_1.7O₄ spinel with MgO inclusions: Case study of a spinel with an apparently occupied interstitial site

A magnesium vanadate spinel crystal, ideally MgV₂O₄, synthesized at 1 bar, 1200 °C and equilibrated under FMQ + 1.3 log f_o2 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(Mg_0.966□_0.034) and ^M(V_0.711³⁺ V_0.109⁴⁺Mg_0.180), 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 ^TMg^M(V_1.368³⁺ V_0.316⁴⁺Mg_0.316)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.