Synthesis and recovery of bulk Fe4O5 from magnetite, Fe3O4. A ...

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Synthesis and recovery of bulk Fe4O5 from magnetite,. Fe3O4. A member of a ... the bulk Earth, constituting more than half of the ... E-mail: [email protected].
Mineralogical Magazine, 2014, Vol. 78(2), pp. 361–371

Synthesis and recovery of bulk Fe4O5 from magnetite, Fe3O4. A member of a self-similar series of structures for the lower mantle and transition zone J. GUIGNARD1,* 1 2

AND

W. A. CRICHTON1,2

ESRF, European Synchrotron Radiation Facility, 6 rue Jules Horowitz, 38043 Grenoble cedex, France Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK [Received 13 October 2013; Accepted 14 January 2014; Associate Editor: S.J. Mills] ABSTRACT

A multi-anvil press was combined with monochromatic synchrotron X-ray radiation to investigate the synthesis, at high-pressure high-temperature conditions, of a recoverable bulk sample of Fe4O5, from an initial magnetite, Fe3O4, sample. Angle-dispersive diffraction patterns show that magnetite firstly breaks down, into an assemblage of hematite (Fe2O3) + Fe4O5. By increasing temperature at constant load, hematite disappears progressively by either reduction or by melting, or a combination of both. In the final product only Fe4O5 remains and, in the absence of hematite, can be kept stable and be recovered at ambient conditions. Refinement of the diffraction patterns at standard conditions ˚, demonstrate that Fe4O5 has the Sr2Tl2O5-type-structure with space group Cmcm and a = 2.8964(2) A ˚ ˚ b = 9.8225(6) A and c = 12.5808(7) A. This structure-type and related members of a homologous series, offer the possibility that the general sequence of AB(2+n)X(4+n) chemistries could, under certain conditions, be extended to accommodate prevalent oxygen fugacity or, indeed, other ordered stoichiometries through extension of the c axis by the addition of FeO6 octahedral blocks. This structural series, as in other systems, offers possibilities of hosting charge-transfer, Jahn-Teller and other electronic phenomena as well as supporting metric distortions. Each of these possibilities is highlighted through illustration and extension to related structure-types, most notably from those of the spinels, post-spinels and post-perovskites. K EY WORDS : Fe4O5, magnetite, phase transition, spinel, Sr2Tl2O5 structures, HPHT, synchrotron.

Introduction IRON and O are the two most abundant elements of the bulk Earth, constituting more than half of the planet (Alle`gre et al., 1995). Iron oxides are therefore ubiquitous on Earth, being present as minor phases from the crust to the deep lower mantle. At ambient conditions, several different Fe oxides are observed with various Fe3+/Fe2+ ratios and different minerals; Fe2O3, (hematite, with the corundum structure), Fe3O4 (magnetite, a

* E-mail: [email protected] DOI: 10.1180/minmag.2014.078.2.09

# 2014 The Mineralogical Society

spinel) and FeO (wu¨stite, which shares its structure with periclase), as well as a host of synthetic and sub-stoichiometric forms. Under high-pressure, high-temperature (HPHT) conditions, several phase transitions have been observed in hematite (e.g. Rozenberg et al., 2002; Ito et al., 2009) and magnetite (see below), whereas wu¨stite remains stable up to the core-mantle boundary (Ozawa et al., 2010).

This paper is published as part of a special issue of Mineralogical Magazine, Volume 78(2), 2014, in celebration of the International Year of Crystallography.

J. GUIGNARD AND W. A. CRICHTON

Among these phases, magnetite has probably received the most attention because its chemistry (mixed valences) and structure (spinel structure, with A2+B3+ 2 O4 stoichiometry) are essential to constrain the redox state of the upper mantle and explain the stability field of spinel-structured ringwoodite (O’Neill et al., 1993). Moreover, magnetite appears as inclusions in diamonds; therefore, its HPHT behaviour could be used to constrain their conditions of formation (Kaminsky, 2012). For all these reasons, the understanding of magnetite behaviour at pressure and temperature conditions relevant to an uppermantle emplacement is essential and has been investigated for several decades. A high-pressure (room temperature) phase transition of magnetite was described at ~21 GPa (Mao et al., 1974), yet the structure of this phase (called h-Fe3O4) is still debated due to the unquenchable behaviour of this phase, which often renders unambiguous structure determinations difficult under the HPHT conditions reported. Therefore, successive authors have proposed: a monoclinic structure (Mao et al., 1974), an orthorhombic CaMn2O4type structure (Fei et al., 1999) and, more recently, a CaTi2O4-type structure (Haavik et al., 2000; Dubrovinsky et al., 2003). Studies at conditions more relevant to the upper mantle were performed more recently (e.g. Shollenbruch et al., 2011) and demonstrated a nearly isobaric (~10 GPa) transition in the range 700 1400ºC from Fe3O4 to a mixture of h-Fe3O4 + new unattributed reflections. These diffraction peaks were attributed to a new Fe oxide phase, with Fe4O5 chemistry, the structure of which was determined in 2011 (Lavina et al., 2011) and are of great interest to the Earth Sciences (upper mantle and transition zone redox state) and for technology (semiconductors, catalysts or biomedical uses). Fe4O5 was first synthesized from siderite, FeCO3 and is stable at the pressuretemperature conditions relevant to the upper mantle. Moreover Fe4O5 has a chemistry and a structure (Sr2Tl2O5-type, also known as the CaFe3O5-type) more favourable at these conditions than those of stoichiometric FeO or Fe3O4. Therefore, those authors proposed that Fe4O5 could be a minor phase of the upper mantle; yet siderite is not very abundant in the whole crust and/or the upper mantle. However, Fe4O5 has also been observed at similar conditions, but from the breakdown of magnetite to hematite + Fe4O5 (Woodland et al., 2012). This phase was probably also observed by Shollenbruch et al. (2011) but

attributed to the high-pressure polymorph of magnetite (h-Fe3O4). These latter studies also provide evidence that Fe4O5 is not recoverable at ambient conditions when mixed with hematite and recombines immediately into magnetite. A new in situ study under HPHT conditions has therefore been undertaken using a novel angledispersive X-ray powder diffraction setup coupled with a 20MN large-volume device, with the aim of obtaining recoverable pure Fe 4 O 5 from magnetite. Experimental procedure Starting material Magnetite was synthesized from commercial nanometric powder of hematite (Fe 2 O 3 ) (Aldrich, purity 99.9%,