American Mineralogist, Volume 92, pages 10311037, 2007 0003-004X/07/00071031$05.00/DOI: 10.2138/am.2007.2445 1031 INTRODUCTION Magnesium vanadate spinel, MgV 2 O 4 , has been the subject of extensive studies in earth and planetary science because the distribution of V is thought to be controlled primarily by spi- nel-like oxides during magmatic differentiation and evolution (cf. Canil 1999; Lee et al. 2003; Papike et al. 2004; Karner et al. 2006). Since V can occur in four valence states (V 2+ , V 3+ , V 4+ , and V 5+ ) in nature, its redox system has been used as an oxybarometer for terrestrial or planetary interiors (cf. Sutton et al. 2005). Natural spinel may contain a signicant amount of vanadium. For example, a suite of vanadian magnesiochromites, Mg(Cr 2x V x )O 4 , found in the Sludynaka metamorphic complex in Russia contains a spinel with x = 0.95, nearly 50% of the octahedral site (Lavina et al. 2003a). Structurally, vanadium spinel (AV 2 O 4 , where A = Mg, Zn, Cd, etc.) represents one of the most typical geometrically mag- netic-frustrated systems, due to its unlled d-orbitals and pos- sible presence of different valence states. At low temperatures, a cubic-to-tetragonal phase transition, attributed possibly to the Jahn-Teller effect and spin ordering, has been observed in vana- dium spinel, and the relationship between magnetic properties and its crystal structure has been discussed (Mamiya et al. 1997; Motome and Tsunetsugu 2005; Di Matteo et al. 2005). The spinel unit cell (space group Fd3m, no. 227) consists of a slightly distorted cubic close-packed array (CCP) of oxygen atoms with 1/8 of the tetrahedral sites (T) and 1/2 of the octa- hedral sites (M) occupied by various cations (Hill et al. 1979). A variety of cations can be accommodated in the T and M sites. There are two extreme cation distribution states: the normal spi- nel, T X M (Y 2 )O 4 , and the inverse one, T Y M (XY)O 4 , where X and Y represent divalent and trivalent cations, respectively. Most natural spinels exhibit an intermediate conguration between these two states, T (X 1-i Y i ) M (X i Y 2-i )O 4 , where 0 ≤ i ≤ 1. Many physical prop- erties of spinel, such as magnetism, electrical conductivity, bulk modulus, thermal expansion, and compressibility, are largely inuenced by the cation distribution (order-disorder) (Hazen and Yang 1999). Materials with the spinel structure are also radia- tion-resistant due to the ease of exchange between the two cation sites (cf. Bordes et al. 1995; Devanathan et al. 1996). The presence of a small amount of cations in the interstitial sites of the spinel structure has been observed previously. For example, Fleet (1981) studied a natural, end-member magnetite with single-crystal X-ray diffraction and observed very weak but signicant residual electron density on the difference Fourier maps at the interstitial tetrahedral site (8b). He suggested that * E-mail: uchidah@email.arizona.edu Investigation of synthetic Mg 1.3 V 1.7 O 4 spinel with MgO inclusions: Case study of a spinel with an apparently occupied interstitial site HINAKO UCHIDA, 1, * KEVIN RIGHTER, 2 BARBARA LAVINA, 3,4 MATTHEW M. NOWELL, 5 STUART I. WRIGHT, 5 ROBERT T. DOWNS, 1 AND HEXIONG YANG 1 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. ABSTRACT A magnesium vanadate spinel crystal, ideally MgV 2 O 4 , synthesized at 1 bar, 1200 C and equilibrated under FMQ + 1.3 log f O 2 condition, was investigated using single-crystal X-ray diffraction, electron microprobe, and electron backscatter diffraction (EBSD). The initial X-ray structure renements gave tetrahedral and octahedral site occupancies of T (Mg 0.966 ■ ■ 0.034 ) and M (V 3+ 0.711 V 4+ 0.109 Mg 0.180 ), respectively, along with the presence of 0.053 apfu Mg at an interstitial octahedral site (16c). Back-scattered elec- tron (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 nal structure renements were optimized by subtracting the X-ray intensity contributions (~9%) of periclase reections, which eliminated the interstitial Mg, yielding a structural formula for spinel T Mg M (V 3+ 1.368 V 4+ 0.316 Mg 0.316 )O 4 . This study provides insight into possible origins of rened interstitial cations reported in the literature for spinel, and points to the difculty of using only X-ray diffraction data to distinguish a spinel with interstitial cations from one with coherently oriented MgO inclusions. Keywords: Spinel, crystal chemistry, XRD, inclusion, periclase, electron backscatter diffraction