SpinelS RenaiSSance: The paST, pReSenT, and fuTuRe of ThoSe ubiquiTouS mineRalS and maTeRialS Pressure-volume equation of state for chromite and magnesiochromite: A single-crystal X-ray diffraction investigation† fabRizio neSTola 1, *, benedeTTa peRioTTo 1 , Giovanni b. andReozzi 2 , enRico bRuSchini 2 and feRdinando boSi 2 1 Dipartimento di Geoscienze, Università di Padova, Via Gradenigo 6, I-35131, Padova, Italy 2 Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro 5, I-00185, Roma, Italy abSTRacT The pressure-volume equation of state for the two spinel end-member compositions chromite Fe- Cr 2 O 4 and magnesiochromite MgCr 2 O 4 was determined for fux-grown synthetic single crystals at room temperature up to 8.2 and 9.2 GPa, respectively, by single-crystal X-ray diffraction using a diamond- anvil cell. The pressure-volume data show that the linear volume compressibility (here used only for purpose of comparison), calculated as β V = |[(ΔV/V 0 )/ΔP]|, is 0.00468 and 0.00470 GPa –1 , for chromite and magnesiochromite, respectively, with a negligible difference below 0.5%. The experimental data were ftted to a third-order Birch-Murnaghan equation of state (BM3) allowing a simultaneous refning of the following coeffcients: V 0 = 588.47(4) Å 3 , K T0 = 184.8(1.7) GPa, and K′ = 6.1(5) for chromite and V 0 = 579.30(4) Å 3 , K T0 = 182.5(1.4) GPa, and K′ = 5.8(4) for magnesiochromite. The difference in K T0 is reduced to <1.5% going from Fe to Mg end-member composition, whereas the frst pressure derivative seems not to be affected by the chemical variability. The limited difference in the equation of state coeffcients recorded for FeCr 2 O 4 and MgCr 2 O 4 allowed us to ft the pressure- volume data of both to a single BM3 equation, resulting in a K T0 = 184.4(2.2) GPa and K′ = 5.7(6). Keywords: Magnesiochromite, chromite, high-pressure, diamond, equation of state inTRoducTion Spinels belonging to the chromite-magnesiochromite (FeCr 2 O 4 -MgCr 2 O 4 ) solid-solution series are among the most common inclusions found in diamonds (Stachel and Harris 2008), and because of that this work represents a contribution to a wider project focused on minerals found as inclusions in diamonds (ERC Starting Grant 2012, 307322). As demonstrated by Nestola et al. (2011a) studying olivine micro-crystals included in diamond, it is crucial to obtain accurate and precise thermo- elastic parameters to apply the so called “elastic method” capable to determine the pressure of formation of the diamond-inclusion pair. Thus, this study aims at being an important starting point in terms of available thermodynamic data to help in determining the pressure of spinel formation in diamond. Spinel minerals present a wide range of solid solutions with a chemistry reflecting igneous processes and superimposed meta- morphic effects (Perinelli et al. 2012 and references therein). In particular, chromite-magnesiochromite spinels are frequent in ultramafic and mafic rocks originated from primitive, mantle- derived magmas (Barnes and Roeder 2001). Chromium-spinels are typically only accessories phases but widely recognized as important petrogenetic indicators (e.g., Irvine 1965, 1967; Ev- ans and Frost 1975; Sack and Ghiorso 1991; Bosi et al. 2008) because they present compositional variations and order-disorder modifications related to the petrologic processes in which they are involved (e.g., Perinelli et al. 2014). Chromium-spinels are stable over a wide range of temperatures and pressures in the Earth upper mantle and therefore the determination of their thermoelastic properties becomes fundamental to better define the thermodynamic modeling of Cr-bearing minerals in mantle assemblages. Besides Cr, the equilibrium inter-crystalline ex- change of Mg and Fe 2+ cations has been intensely analyzed, and revealed its great potential to be used as geothermometer in rocks containing the paragenesis olivine-spinel (Fabries 1979; Ballhaus et al. 1990), as geobarometer in rocks containing olivine, garnet, and pyroxene (O’Neill 1981; O’Neill and Wall 1987), and as oxy- gen barometer in rocks containing olivine-orthopyroxene-spinel (Ballhaus et al. 1991). Finally, adopting the “elastic method” as in Nestola et al. (2011a), spinels can represent an important potential geobarometer for the diamond formation. Spinels have general formula AB 2 O 4 , and their cation distri- bution is represented as IV (A 1–x B x ) VI (B 2–x A x )O 4, where commonly A = Mg, Fe 2+ , Zn, Mn 2+ and B = Al, Fe 3+ , Cr 3+ , and IV and VI represents tetrahedrally coordinated T sites and octahedrally coordinated M sites, respectively. Normal spinels have x = 0 and inverse spinels have x = 1; however, intermediate (0 < x < 1) disordered cation distributions are possible between the two extremes (e.g., O’Neill and Navrotsky 1983; Andreozzi and Lucchesi 2002; Martignago et al. 2006; Hålenius et al. 2011; Bosi et al. 2012). The chromite-magnesiochromite series exhibits an ordered IV A VI B 2 O 4 cation distribution at room temperature American Mineralogist, Volume 99, pages 1248–1253, 2014 0003-004X/14/0007–1248$05.00/DOI: http://dx.doi.org/10.2138/am.2014.4765 1248 * E-mail: fabrizio.nestola@unipd.it † Special collection papers can be found on GSW at http://ammin. geoscienceworld.org/site/misc/specialissuelist.xhtml.