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

¹ Department of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, U.S.A.
² Mailcode KT, NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas 77058, U.S.A.
³ High Pressure Science and Engineering Center, University of Nevada Las Vegas, Las Vegas, Nevada 89154-4002, U.S.A.
⁴ GSECARS, University of Chicago, 5734 S. Ellis Avenue, Chicago, Illinois 60637, U.S.A.
⁵ EDAX-TSL, Draper, Utah 84020, U.S.A.

Correspondence: ^* E-mail: uchidah{at}email.arizona.edu

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_0.9660.034) and ^M(V³⁺_0.711V⁴⁺_0.109Mg_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(V³⁺_1.368V⁴⁺_0.316Mg_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.

Key Words: Spinel • crystal chemistry • XRD • inclusion • periclase • electron backscatter diffraction