Maohokite, a post‐spinel polymorph of MgFe2O4 in shocked gneiss from the Xiuyan crater in China

Maohokite, a post‐spinel polymorph of MgFe2O4, was found in shocked gneiss from the Xiuyan crater in China. Maohokite in shocked gneiss coexists with diamond, reidite, TiO2‐II, as well as diaplectic glasses of quartz and feldspar. Maohokite occurs as nano‐sized crystallites. The empirical formula is (Mg0.62Fe0.35Mn0.03)2+Fe3+2O4. In situ synchrotron X‐ray microdiffraction established maohokite to be orthorhombic with the CaFe2O4‐type structure. The cell parameters are a = 8.907 (1) Å, b = 9.937(8) Å, c = 2.981(1) Å; V = 263.8 (3) Å3; space group Pnma. The calculated density of maohokite is 5.33 g cm−3. Maohokite was formed from subsolidus decomposition of ankerite Ca(Fe2+,Mg)(CO3)2 via a self‐oxidation‐reduction reaction at impact pressure and temperature of 25–45 GPa and 800–900 °C. The formation of maohokite provides a unique example for decomposition of Fe‐Mg carbonate under shock‐induced high pressure and high temperature. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2017‐047). The mineral was named maohokite after Hokwang Mao, a staff scientist at the Geophysical Laboratory, Carnegie Institution of Washington, for his great contribution to high pressure research.


INTRODUCTION
Ferrous oxide contributes about 8% to the mass of the bulk of Earth's primitive mantle (Lyubetskaya and Korenaga 2007). It was indicated that a significant amount of ferric iron (Fe 3+ ) might exist in the Earth's deep mantle due to the disproportionation reaction of ferrous iron (Fe 2+ ) at high-pressure and hightemperature conditions (Mao 1974;Mao and Bell 1975;Frost et al. 2004;McCammon 2005;Fialin et al. 2009;Rohrbach et al. 2011). Ferrous iron can also be oxidized by other oxidizing agents, such as CO 2 , at mantle pressure-temperature conditions (Boulard et al. 2012). The concentration of ferric iron in the lower mantle should be higher than that in the upper mantle even though oxygen fugacity decreases with depth (McCammon 2005). The redox state and physical properties of the mantle should have been influenced by the existence of ferric iron in minerals. Then, what would be the most possible carriers of ferric iron in the mantle? The post-spinel phases of MgFe 2 O 4 ÀFeFe 2 O 4 were suggested to be the potential host minerals of ferric iron in the lower mantle Mao and Bell 1975). Some other high-pressure iron oxides that could occur in the deep Earth, such as HP phases of Fe 2 O 3 (Shim et al. 2009;Bykova et al. 2016) and Fe 3 O 4 (Fei et al. 1999;Dubrovinsky et al. 2003), as well as HP phases with mixed-valence of Fe 4 O 5 (Lavina et al. 2011) and Fe 5 O 7 and Fe 25 O 32 (Bykova et al. 2016) have also been synthesized. Magnesioferrite, MgFe 2 O 4 , is a Fe 3+ -rich member of the spinel group, which has been found in kimberlites and carbonatites as well as volcanic fumaroles. The post-spinel transformation in MgFe 2 O 4 under high-pressure and high-temperature conditions has been observed experimentally in the laser-heated diamond anvil cell above 20 GPa (Mao and Bell 1975;Andrault and Bolfan-Casanova 2001;Winell et al. 2006). However, no post-spinel phase of MgFe 2 O 4 has been previously found in nature.
The deep mantle contains abundant high-pressure minerals but most of them cannot be preserved during the long journey to the surface. The naturally impacted terrestrial rocks or meteorites provide the major sources of high-pressure minerals that may predominate in the Earth's deep mantle. A lot of high-pressure minerals that may predominate in the deep mantle, such as coesite and stishovite (Chao et al. 1962), ringwoodite (Binns et al. 1969), bridgmanite (Tschauner et al. 2014) and others, have been found in naturally impacted terrestrial rocks and meteorites, which provide a window to look into the mineral constituents of the mantle rocks (e.g., St€ offler 1997;Akaogi 2000).
The Xiuyan crater is a bowl-shaped structure 1.8 km in diameter of where the crystalline rocks are composed of the Early Proterozoic gneiss-amphibolitemarble complexes. This crater was confirmed as a meteorite impact site through the discovery of a series of shock-metamorphic features of minerals in impacted rocks (Chen et al. 2010a(Chen et al. , 2010b(Chen et al. , 2011(Chen et al. , 2013a(Chen et al. , 2013b. In a study of shocked ankerite from impact breccia of the crater, we discovered a shock-induced post-spinel polymorph of MgFe 2 O 4 . This mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2017-047). The mineral has been named maohokite after Hokwang Mao of the Geophysical Laboratory, Carnegie Institution of Washington, in honor of his great contribution to high-pressure research.

SAMPLES AND METHODS
The investigated samples of impact breccia were collected from the drill cores into the depth interval of 260-295 m at the center of the Xiuyan crater. The impact breccias are composed of fragments of gneiss, amphibolite, marble, and fine-grained matrix.
Gneiss fragments picked up from the impact breccias were cut into slices and then polished into 30 lm thick for phase identifications. The bulk compositions of shocked ankerite in gneiss were analyzed by a JEOL JXA-8100 electron microprobe with the wavelength dispersive technique at 15 kV accelerating voltage and 10 nA beam current. Raman spectra of shocked ankerite were recorded with a Renishaw R-2000 instrument. A microscope was used to focus the excitation beams (514.5 nm and 638 nm lasers) to 1 lm wide spots and to collect the Raman signal.
X-ray diffraction analyses of shocked ankerite were conducted in situ on the polished thin sections of gneiss by synchrotron X-ray microdiffraction analysis on the Beamline 12.3.2 at the Advance Light Source of the Lawrence Berkeley National Laboratory, USA. The X-ray energy was 10.0001 KeV (k = 1.23986 A). The synchrotron X-ray microdiffraction analysis with a beam size of <1 lm was used to scan the sample spot by spot, to obtain diffraction signals. The diffraction patterns were collected with aMAR133CCD detector at a distance of 137.714 mm from the sample. The experimental setup was calibrated by an Alumina standard, Al 2 O 3 . The data collection time for each pattern was kept unchanged as 300 s through the whole experiment. The diffraction data were processed by using the program XMAS V.6 (IDL Virtual Machine, X-ray diffraction analysis software).
In preparation for the study with STEM and TEM, flakes of shocked ankerite were extracted by the focused ion beam (FIB) milling technique with a FEI Versa 3D Dual Beam FIB/SEM system at the Center for High Pressure Science and Technology Advanced Research, Shanghai, China. The samples were thinned to 10 9 5 lm flakes with a thickness of 90 nm by using a Ga + FIB operating at 30 kV and currents from 0.5 nA down to 48 pA for final surfacing. The phase identifications were performed with a FEI Talos F200s scanning/transmission electron microscope operating at 200 kV, equipped with a field emission gun, at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China. Bright-field, highresolution bright-field, and STEM images of minerals as well as selected area electron diffractions in TEM were obtained from the samples. Compositional analysis of nano-sized crystallites of maohokite and magnesioferrite was conducted by using EDXS with an accelerating voltage of 200 kV, a beam current of 1 nA, and a beam size of~5 nm in STEM mode. Accumulation time of X-ray signals was 5 s per analysis.

Occurrence
The investigated gneiss from impact breccia of the Xiuyan crater is composed of about 55 vol% feldspar, 38 vol% quartz, and 5 vol% ankerite (Fig. 1a). We have identified three kinds of gneiss fragments according to shock-metamorphic features, i.e., slightly, moderately, and strongly shocked gneisses. In the slightly shocked gneiss, most of the minerals remain intact or display some planar deformation features in quartz and feldspars (Chen et al. 2011). The strongly shocked gneiss was shock-melted and contains abundant coesite that had crystallized from shock-induced silica melt (Chen et al. 2010b). The moderately shocked gneiss is characterized by extensive solid-state phase transformation of minerals. Quartz has been completely transformed into diaplectic glass that has retained the previous morphological character of a quartz crystal, and feldspar partially transformed into diaplectic glass. Accessory minerals of rutile and zircon have been partially transformed into high-pressure polymorph of TiO 2 -II (Chen et al. 2013a) and reidite (Chen et al. 2013b), respectively. About 30% of the rutile in the gneiss transformed to TiO 2 -II. Maohokite occurs in the moderately shocked gneiss.
Most grains of ankerite in the moderately shocked gneiss still preserve their original crystal morphologies (Fig. 1b), but are now decomposed into a multiphase mixture composed of magnesioferrite, maohokite, calcite, and diamond ( shocked ankerite are about 8% and about 20%, respectively. The crystal size of diamond varies from 2 to 5 nm (Chen et al. 2018). Calcite occurs as crystallite aggregates with individual crystal size ranging from 5 to 300 nm. Some irregular bubbles up to 150 nm in size can be observed within the shocked ankerite.

Chemical Composition and Crystallography
The energy dispersive X-ray microanalyses of nanosized crystallites of maohokite and magnesioferrite clearly indicate the compositions of magnesium-iron oxides (Fig. 2b) Synchrotron-based micro-X-ray diffraction analyses were carried out to determine the crystalline phases present in the shocked ankerite. Four different phases including maohokite, magnesioferrite, calcite, and diamond have been identified (Chen et al. 2018). Figure 3 is an X-ray diffraction pattern from which 23 diffraction lines can be assigned to maohokite, and indexed to an orthorhombic CaFe 2 O 4 -type structure with space group Pnma (Table 1). The cell parameters of maohokite are: a = 8.907 (1) A, b = 9.937 (8) A, c = 2.981(1) A, V = 263.8 (3) A 3 . The calculated density of maohokite is 5.33 g cm À3 . The density of magnesioferrite is 4.6-4.7 g cm À3 . Greater density of maohokite than magnesioferrite demonstrates that maohokite is a high-pressure polymorph of magnesioferrite.
The nanocrystallites of maohokite and magnesioferrite have been further characterized by transmission electron microscopy (TEM) and electron diffraction analysis. planes, respectively, have been observed by highresolution TEM (Fig. 2d).

Raman Spectroscopy
The crystalline phases in shocked ankerite have been characterized by Raman spectroscopy (Fig. 4). The strong band at 1086 cm À1 and weak band at 279 cm À1 can be assigned to calcite (Sun et al. 2014). The strong band at 686 cm À1 and some weak bands at 554, 478, 328, and 210 cm À1 are attributed to magnesioferrite (Winell et al. 2006), and the strong band at 602 cm À1 to maohokite, in comparison to the Raman spectrum of post-spinel polymorph of FeCr 2 O 4 (Chen et al. 2003). Raman spectra of both magnesioferrite and magnetite usually display a strong and well-defined A 1 g band at approximately 670 cm À1 and 700 cm À1 , respectively (D'Ippolito et al. 2015). The observed strong band of magnesioferrite at 686 cm À1 for our sample, which falls in between 670 cm À1 (magnetite) and 700 cm À1 (magnesioferrite), is in agreement with the composition of magnesioferrite close to 2:1 solid solution of magnesioferrite : magnetite.

DISCUSSION
According to the shock-metamorphic features of minerals in the moderately shocked gneiss in the Xiuyan impact breccia, we estimate the pressure and temperature conditions for the formation of maohokite. The transformation of quartz and feldspar to diaplectic glasses constrains shock pressure and temperature to 35-45 GPa and 300-900°C (Grieve et al. 1996). The moderately shocked gneiss contains abundant TiO 2 -II phase transformed from rutile. The transformation of rutile to TiO 2 -II can be produced by high-pressure shockloading above 20 GPa (McQueen et al. 1967), and the highest yield of TiO 2 -II was achieved at a peak pressure of~45 GPa (Kusaba et al. 1988). However, the transition from rutile to TiO 2 -II is sluggish at temperature below 800°C (Akaogi et al. 1992). Laboratory experiments revealed that magnesioferrite transforms to a highpressure form at 20 GPa after laser heating (Mao and Bell 1975). The spinel/post-spinel transition boundaries for the endmembers of magnesioferrite at 25 GPa (Andrault and Bolfan-Casanova 2001) and magnetite at 23.6 GPa (Fei et al. 1999) are within the above shock P-T constraints. Therefore, we suggest that maohokite crystallized at 25-45 GPa and 800-900°C.
The decomposition of ankerite corresponds to a subsolidus self-redox reaction of 6Ca(Fe,Mg) (CO 3 ) 2 ? 6CaCO 3 + 2MgFe 2 O 4 + C + 5CO 2 at high pressure and high temperature (Chen et al. 2018). In this reaction, part of the Fe 2+ of ankerite was oxidized to Fe 3+ by concurrent reduction of part of the C 4+ to   (St€ offler et al. 1991). The coexistence of maohokite and magnesioferrite in the samples indicates a rapid change in pressure over a relatively short time. Maohokite crystallized in the highpressure regime only occurs as tiny nanocrystallites, whereas magnesioferrite crystallized after pressure release has relatively large sizes of crystallites. The decomposition of Fe-rich carbonates has also been observed in terrestrial metamorphic rocks (Zuilen et al. 2003), high-pressure/high-temperature experiments (Tao et al. 2013;Kang et al. 2015), in shockmetamorphosed meteorites (Brearley 2003), and in shock experiments (Bell 2007), respectively. The decomposition of Fe-rich carbonates usually involves an oxidationreduction reaction resulting in the conversion of Fe 2+ to Fe 3+ and C 4+ to C 0 (Brearley 2003;Zuilen et al. 2003;Bell 2007;Kang et al. 2015). The decomposition of Ferich carbonates leads to the formation of magnetite and graphite, and has been generally attributed to a thermal effect (Brearley 2003;Zuilen et al. 2003;Bell 2007). The formation of maohokite provides a unique example for decomposition of Fe-Mg carbonate under shock-induced high pressure and high temperature.
The crystal structures related to the tunnel structures of CaFe 2 O 4 (CF), CaTi 2 O 4 (CT), and CaMn 2 O 4 (CM) are considered as post-spinel phases (Irifune et al. 1991;Kirby et al. 1996;Galuskin et al. 2016). So far, only three ferric iron oxide minerals with post-spinel structures, which include maohokite (CF) in this study, CaFe 3+ 2 O 4 harmunite (CF) (Galuskina et al. 2014) and CaFe 3+ 2 Mn 4+ O 4 wernerkrauseite (CF) (Galuskin et al. 2016), have been identified in nature. Maohokite is a high-pressure mineral that can be stable at lower mantle conditions. Harmunite and wernerkrauseite were formed in pyrometamorphic rocks of a volcano and belong to the group of low-pressure minerals. The natural occurrence of maohokite puts it into the category of potential mantle minerals and provides significant insights into the host phase of ferric iron in the lower mantle.
Magnesiow€ ustite (Mg,Fe)O is believed to be an important constituent of the lower mantle (Zhang and Herzberg 1994;Agee et al. 1995). A previous investigation of (Mg 0.35 Fe 0.65 )O inclusions in diamond Fig. 4. Raman spectra of shocked ankerite collected with the 514 nm laser. The bands at 686, 554, 478, and 328 cm À1 are attributed to magnesioferrite, whereas the strong band at 602 cm À1 corresponds to maohokite. The strong band at 1086 cm À1 and the weak band at 279 cm À1 are assigned to calcite. The spectrum (a) characterizes two major phases of magnesioferrite plus calcite, whereas the spectrum (b) characterizes two major phases of maohokite plus calcite and minor magnesioferrite. (Color figure can be viewed at wileyonlinelibrary.com.) from kimberlite reported the finding of magnesioferrite nanocrystallites that is embedded in a magnesiow€ ustite matrix, and the formation of magnesioferrite is attributed to the reversion from its high-pressure polymorph (Wirth et al. 2014). Fe 3+ -rich clusters (Mg 1+x Fe 2Àx O 4Àx/2 ) have been also identified from magnesiow€ ustite inclusions in kimberlitic lower mantle diamonds (Kaminsky et al. 2015).
The natural occurrence of Fe 3+ -rich ferromagnesian oxides from mantle rocks gives evidence for the existence of ferric iron in the Earth's deep interior. If a significant amount of ferrous iron in the deep mantle would have been converted to ferric iron via chemical reactions at high temperature and high pressure, maohokite might behave as an important carrier of ferric iron in the ferromagnesian mantle.
High-pressure mineral evidence for shock-induced high-pressure phase transformation of carbonates has so far never been found in terrestrial impact rocks and shock-metamorphic meteorites. The finding of maohokite and diamond (Chen et al. 2018) decomposed from ferromagnesian carbonate in the Xiuyan crater provides new evidence on the shock effects of high pressure and high temperature in carbonate.