minerals

Review Spinels in : Observation Using Mössbauer Spectroscopy

Alevtina A. Maksimova 1, Andrey V. Chukin 1, Israel Felner 2 and Michael I. Oshtrakh 1,* 1 Department of Experimental Physics, Institute of Physics and Technology, Ural Federal University, Ekaterinburg 620002, Russia; [email protected] (A.A.M.); [email protected] (A.V.C.) 2 Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel; [email protected] * Correspondence: [email protected]; Tel.: +7-912-283-7337

 Received: 5 September 2018; Accepted: 9 January 2019; Published: 13 January 2019 

Abstract: In this mini-review, we consider the results of various studies using Mössbauer spectroscopy with a high velocity resolution in order to reveal the minor spectral components related to spinels such as chromite, hercynite, magnesiochromite, magnesioferrite and daubréelite in bulk meteorite matter or in some extracted phases. Spinels observation in the Mössbauer spectra is supported by characterization of the studied samples by means of optical and scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and magnetization measurements. Mössbauer parameters obtained for extraterrestrial spinels are compared with those obtained for terrestrial analogs published in the literature.

Keywords: Spinels; Meteorites; Mössbauer spectroscopy

1. Introduction Meteorites are space messengers reaching the Earth and bringing information about solar system formation. These rocks are the result of their parent bodies’ ( and planets) collisions in space. A simple classification of meteorites permits us to consider three groups: stony, stony-iron and iron meteorites (more detailed meteorite classification can be found in [1] and references therein). The basic information about chemical and mineral composition of various meteorites can be found in reviews [2,3]. Almost all meteoritic minerals can be found on Earth. However, terrestrial minerals were formed in significantly different conditions in comparison with extraterrestrial minerals, which were affected by various extreme factors in space (very slow cooling, reheating, impact melting, etc.). Therefore, the phase composition of meteorites and the physical properties of their minerals are of interest for a complex investigation. All meteorites consist of iron-bearing minerals represented by Fe-Ni-Co alloy in the forms of α-Fe(Ni, Co), α2-Fe(Ni, Co), γ-Fe(Ni, Co) and γ-FeNi phases, olivine (Fe, Mg)2SiO4, orthopyroxene (Fe, Mg)SiO3, clinopyroxene (Fe, Mg, Ca)SiO3, troilite FeS and some other minerals. Iron-bearing spinels can also be found in meteorites as the minor accessory minerals. Some of them, for example daubréelite (FeCr2S4) and chromite (FeCr2O4), were formed with meteorite matter in space. Other spinels such as magnetite (Fe3O4) or magnesioferrite (MgFe2O4) can be a result of meteorites weathering (oxidation) in the terrestrial conditions. Since all these minerals contain iron, it is possible to use 57Fe Mössbauer spectroscopy for studying meteorites. About 55 years of experience demonstrate significant progress in the development of Mössbauer spectroscopy of various meteorites from the first review [4] until the modern studies (see, for instance, [5,6]). However, the Mössbauer spectra of meteorites are very complex and consist of various numbers of major and minor components related to different phases and mineral compositions of rocks. Therefore, revealing of the minor phases, for instance, spinels, appears to be very difficult in the Mössbauer spectra of bulk meteorite samples. Therefore, we used Mössbauer spectrometers with a high velocity

Minerals 2019, 9, 42; doi:10.3390/min9010042 www.mdpi.com/journal/minerals Minerals 2019, 9, 42 2 of 16 resolution. The velocity driving system in these spectrometers has the higher discretization of the velocity reference signal (212 versus 29 in conventional spectrometers). This leads to the smaller Doppler modulation step for resonant γ-quanta energy, that is why the high velocity resolution Mössbauer spectroscopy is a useful technique for excavating the minor components in the complex spectra due to much higher sensitivity, precision and accuracy then those in conventional Mössbauer spectrometers. Some advances of this method in meteorites study have been considered in [7–9]. Therefore, in this mini-review, we consider our results related to the observation of various spinel phases in meteorites using the high velocity resolution Mössbauer spectroscopy. Additional information obtained by other techniques such as optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and magnetization measurements is used for supporting observation of spinels.

2. Materials and Methods We studied several fragments of different ordinary (Chelyabinsk LL5 No 1a and No 2, Northwest Africa (NWA) 6286 LL6 and NWA 7857 LL6, Tsarev L5 and Annama H5), Seymchan main group (PMG) and troilite inclusion extracted from Sikhote-Alin IIAB . Polished sections of these meteorite fragments were prepared by the standard method for samples characterization by optical microscopy and SEM with EDS. Powdered samples were then prepared from the polished surfaces for XRD, magnetization measurements and Mössbauer spectroscopy. Additionally, the fusion crust from Chelyabinsk LL5 fragment No 1a and the massive troilite inclusion from the Sikhote-Alin iron meteorite were removed and prepared as a powder for the study. Details of different samples preparation and characterization were described in [9–15]. Meteorites characterization was done using an Axiovert 40 MAT optical microscope (Carl Zeiss, Oberkochen, Germany), a ΣIGMA VP electron microscope (Carl Zeiss, Oberkochen, Germany) with an X-max 80 (Oxford Instruments, Abingdon, Oxfordshire, England) energy dispersive spectroscopy device, an AMRAY 1830 scanning electron microscope equipped with EDAX PV9800 energy dispersive spectrometer, X-ray diffractometers Shimadzu XRD-7000 and Panalytical X’Pert Pro MPD and commercial SQUID magnetometer MPMS-5S (Quantum Design, San Diego, CA, USA). Mössbauer spectra were measured using an automated precision Mössbauer spectrometric system built on the base of the SM-2201 spectrometer with a saw-tooth shape velocity reference signal formed by the digital-analog converter using discretization of 212 (quantification of the velocity reference signal using 4096 steps) and a liquid nitrogen cryostat with moving absorber. The high level of the velocity scale discretization provides much better adjustment to resonance, and significantly increases the spectra quality and analytical possibilities of Mössbauer spectroscopy. On the other hand, this increases the measurement time. Registration of γ-rays was done using scintillator detector with NaI(Tl) crystal with a thickness of 0.1 mm. Details and characteristics of this spectrometer and the system as well as this method’s features are described in [16,17]. The (1.8–1.0) × 109 Bq 57Co in rhodium matrix sources (Ritverc GmbH, St. Petersburg, Russian Federation) were at room temperature. The Mössbauer spectra were measured in transmission geometry with moving absorber in the cryostat and recorded in 4096 channels. To increase the signal-to-noise ratio in the complex spectra with the minor components, they were converted into 1024 channels by consequent summation of four neighboring channels to reach higher statistics and larger signal-to-noise ratio (details for each sample study are given in [9–15]). Mössbauer spectra were computer-fitted with a UNIVEM-MS program using the least squares procedure with a Lorentzian line shape. This procedure uses the usual perturbation of the first order method for magnetically split components. Therefore, the spectral component of troilite, which requires the full static Hamiltonian for the fit, cannot be fitted correctly. To overcome this problem, the Mössbauer spectra containing troilite component were fitted using a simulation of the full static Hamiltonian by means of the method described in detail in [18,19]. The results obtained are very close to parameters for the minor spectral components obtained from the ordinary chondrites Mössbauer spectra fits using both the full static Hamiltonian and above-mentioned simulation method. Minerals 2018, 8, x FOR PEER REVIEW 1 of 12

Review Spinels in Meteorites: Observation Using Mössbauer Spectroscopy

Alevtina A. Maksimova 1, Andrey V. Chukin 1, Israel Felner 2 and Michael I. Oshtrakh 1,* Minerals 2019, 9, 42 3 of 16 1 Department of Experimental Physics, Institute of Physics and Technology, Ural Federal University, Ekaterinburg, 620002, Russian Federation; [email protected] (A.A.M.); [email protected] (A.V.C.) δ The2 Racah spectral Institute parameters, of Physics, such The asHebrew isomer University shift, ,, Jerusalem, quadrupole 91904 splitting, Israel; [email protected]∆EQ, quadrupole shift for magnetically* Correspondence: split spectra, [email protected];ε (∆EQ = 2ε ),Tel.: hyperfine +7-912-283-7337 magnetic field, Heff, line width (a full width at a half maximum), Γ, relative subspectrum area, A, and statistical quality of the fit, χ2, were determined. Received: 5 September 2018; Accepted: 9 January 2019; Published: 13 January 2019 Calibration of the velocity scale was made using the reference absorber of α-Fe foil with a thickness of 7 µm. The line shapes were pure Lorentzian with the first and the sixth, the second and the fifth, Abstract: In this mini-review, we consider the results of various meteorite studies using Mössbauer and the third and the fourth line widths values of Γ = 0.238 ± 0.008 mm/s, Γ = 0.232 ± 0.008 mm/s spectroscopy with a high velocity resolution in1,6 order to reveal the minor2,5 spectral components and Γ = 0.223 ± 0.008 mm/s for the α-Fe spectrum recorded in 4096 channels. The velocity range related3,4 to spinels such as chromite, hercynite, magnesiochromite, magnesioferrite and daubréelite was about ±(10–7) mm/s depending on the studied sample. The instrumental (systematic) error for in bulk meteorite matter or in some extracted phases. Spinels observation in the Mössbauer spectra each spectrum point was ±0.5 channel (the velocity scale). The instrumental (systematic) error for the is supported by characterization of the studied samples by means of optical and scanning electron hyperfine parameters was ±1 channel. If an error calculated with the fitting procedure (fitting error) microscopy, energy dispersive spectroscopy, X-ray diffraction and magnetization measurements. for these parameters exceeded the instrumental (systematic) error, we used the larger error instead. Mössbauer parameters obtained for extraterrestrial spinels are compared with those obtained for Relative error for A did not usually exceed 10%. Criteria for the best fits were differential spectrum, terrestrial analogs published in the literature. χ2 and physical meaning of the spectral parameters. Isomer shifts are given relative to α-Fe at 295 K. Keywords:To demonstrate Spinels; the Meteorites; difference Mössbauer in the quality spectroscopy of the Mössbauer spectra measured with conventional velocity resolution and with a high velocity resolution, we show a comparison of the Mössbauer spectra of Mount Tazerzait L5 ordinary samples in Figure1. This comparison could be good evidence of the effect of increasing the velocity resolution (discretization of the velocity reference signal) in Mössbauer spectrometers and spectra.

Figure 1. Mössbauer spectra of Mount Tazerzait L5 samples measured: (left) using conventional Figure 1. Mössbauer spectra of Mount Tazerzait L5 samples measured: (left) using conventional Mössbauer spectrometer with a low velocity resolution and folding (in 256 channels) from Ref. [8] Mössbauer spectrometer with a low velocity resolution and folding (in 256 channels) from Ref. [8] cited in [17]; and (right) using Mössbauer spectrometer SM-2201 with a high velocity resolution cited in [17]; and (right) using Mössbauer spectrometer SM-2201 with a high velocity resolution (4096 (4096 channels) without folding (this spectrum was further converted into 1024-channel spectrum to channels) without folding (this spectrum was further converted into 1024-channel spectrum to increase signal-to-noise ratio for the minor spectral components); the differential spectrum is shown increase signal-to-noise ratio for the minor spectral components); the differential spectrum is shown below. T = 295 K. Adopted from [17]. below. T = 295 K. Adopted from [17]. 3. Results and Discussion 3. Results and Discussion Spinel phases were observed in all studied samples. However, these spinels were different for different types of meteorites. Therefore, we consider these results for stony (ordinary chondrites), stony-iron (main group pallasite) and iron meteorites separately.

3.1. Chromite in Ordinary Chondrites Optical microscopy of polished sections of Chelyabinsk LL5, NWA 6286 LL6, NWA 7857 LL6, Tsarev L5 and Annama H5 fragments demonstrated the presence of silicate phases with small metallic Fe-Ni-Co grains, troilite and chromite inclusions. A representative optical microphotograph of the Minerals 2018, 8, x FOR PEER REVIEW 2 of 12

Spinel phases were observed in all studied samples. However, these spinels were different for different types of meteorites. Therefore, we consider these results for stony (ordinary chondrites), stony-iron (main group pallasite) and iron meteorites separately.

3.1. Chromite in Ordinary Chondrites Minerals 2019, 9, 42 4 of 16 Optical microscopy of polished sections of Chelyabinsk LL5, NWA 6286 LL6, NWA 7857 LL6, Tsarev L5 and Annama H5 fragments demonstrated the presence of silicate phases with small NWAmetallic 6286 Fe-Ni-Co polished grains, section troilite is shown and in chromite Figure2a. in SEMclusions. with A EDS representative demonstrated optical the presence microphotograph of olivine, pyroxenes,of the NWA troilite, 6286 polishedα-Fe(Ni, section Co) and isγ shown-Fe(Ni, in Co) Figure phases 2a. andSEM chromite with EDS (representative demonstrated SEM the imagepresence of NWAof olivine, 6286 pyroxenes, is shown in troilite, Figure2 b).α-Fe(Ni, Co) and γ-Fe(Ni, Co) phases and chromite (representative SEM image of NWA 6286 is shown in Figure 2b).

(a) (b)

Figure 2. Microphotographs of NWA 6286 polished section obtained using: (a) optical microscopy; Figure 2. Microphotographs of NWA 6286 polished section obtained using: (a) optical microscopy; and (b) scanning electron microscopy. Adopted from [13]. and (b) scanning electron microscopy. Adopted from [13]. Chemical analysis of selected chromite inclusions in Chelyabinsk LL5, NWA 6286 LL6, NWA 7857 LL6,Chemical Tsarev L5 analysis and Annama of selected H5 fragments chromite carriedinclusions out within Chelyabinsk EDS showed LL5, some NWA variations 6286 LL6, in NWA metal content7857 LL6, (see Tsarev Table 1L5). Theseand Annama inclusions H5 contain fragments Cr and carried Fe as out the with main EDS metals. showed However, some itvariations was found in metal content (see Table 1). These inclusions contain Cr and Fe as the main metals. However, it was the presence of Al as the third metal, except chromite in Tsarev L5. In chromite inclusions of the found the presence of Al as the third metal, except chromite in Tsarev L5. In chromite inclusions of latter Mg and Al were presented with a similar content as the third and the the latter ordinary chondrite Mg and Al were presented with a similar content as the third and the fourth metals. The presence of Al as the third metal in chromite inclusions indicates the formation of fourth metals. The presence of Al as the third metal in chromite inclusions indicates the formation of additional spinels such as hercynite FeAl2O4 or mixed Fe(Al1–xCrx)2O4 spinel. Therefore, XRD patterns additional spinels such as hercynite FeAl2O4 or mixed Fe(Al1–xCrx)2O4 spinel. Therefore, XRD of Chelyabinsk LL5, NWA 6286 LL6, NWA 7857 LL6, Tsarev L5 and Annama H5 fragments were patterns of Chelyabinsk LL5, NWA 6286 LL6, NWA 7857 LL6, Tsarev L5 and Annama H5 fragments fitted using the Rietveld full profile analysis without and with accounting for the minor spinel phases were fitted using the Rietveld full profile analysis without and with accounting for the minor spinel of chromite and hercynite. A comparison of both fits demonstrated that accounting for two minor phases of chromite and hercynite. A comparison of both fits demonstrated that accounting for two spinels led to a better fitting quality (representative XRD pattern for the powdered bulk NWA 6286 minor spinels led to a better fitting quality (representative XRD pattern for the powdered bulk NWA matter is shown in Figure3a). The phase composition for studied meteorites is presented in Table2. 6286 matter is shown in Figure 3a). The phase composition for studied meteorites is presented in It should be noted that the presence of Mg in chromite in Tsarev L5 is comparable with Al content and Table 2. It should be noted that the presence of Mg in chromite in Tsarev L5 is comparable with Al therefore indicates the presence of Mg-bearing spinels. We have so far been unable to reveal these content and therefore indicates the presence of Mg-bearing spinels. We have so far been unable to minor spinels from this XRD pattern. However, measurements of the zero-field-cooled (ZFC) and reveal these minor spinels from this XRD pattern. However, measurements of the zero-field-cooled field-cooled (FC) magnetization curves for NWA 6286 LL6, NWA 7857 LL6, Tsarev L5 and Annama H5 (ZFC) and field-cooled (FC) magnetization curves for NWA 6286 LL6, NWA 7857 LL6, Tsarev L5 fragments demonstrated the phase transition in the temperature range of 48–60 K (see representative and Annama H5 fragments demonstrated the phase transition in the temperature range of 48–60 K ZFC/FC curves in Figure3b for NWA 6286). This temperature range is in agreement with the range for (see representative ZFC/FC curves in Figure 3b for NWA 6286). This temperature range is in Curie temperature of 40–80 K for the chromite ferrimagnetic-paramagnetic phase transition in various agreement with the range for Curie temperature of 40–80 K for the chromite ordinary chondrites obtained in [20]. ferrimagnetic-paramagnetic phase transition in various ordinary chondrites obtained in [20].

Table 1. Average values and ranges of the content (in at. %) of some metals determined in selected chromite inclusions in ordinary chondrites by energy dispersive spectroscopy.

Chelyabinsk LL5 Chelyabinsk LL5 NWA 6286 NWA 7857 Tsarev Annama Metal No 1a No 2 LL6 LL6 L5 H5

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Table 1. Average values and ranges of the content (in at. %) of some metals determined in selected Mineralschromite 2018, 8 inclusions, x FOR PEER in REVIEW ordinary chondrites by energy dispersive spectroscopy. 3 of 12

Chelyabinsk Chelyabinsk NWA 6286 NWA 7857 Tsarev Metal Annama H5 Fe 13.2LL5 No 1a LL5 No 13.1 2 LL6 12.2–13.5LL6 10.2–16.1L5 10.7 8.0–8.4 Cr 21.2 21.0 19.9–21.6 17.5–28.4 21.1 15.2–16.1 Fe 13.2 13.1 12.2–13.5 10.2–16.1 10.7 8.0–8.4 Al Cr3.5 21.2 21.0 3.9 19.9–21.63.1–3.8 17.5–28.4 3.3–4.8 21.1 3.2 15.2–16.1 2.8–3.0 Mg Al1.5 3.5 3.9 1.3 3.1–3.8 1.5–1.9 3.3–4.8 1.2–2.0 3.2 3.7 2.8–3.0 1.8–1.4 Ti Mg1.0 1.5 1.3 0.6 1.5–1.90.9–1.6 1.2–2.0 0.2–1.1 3.7 1.0 1.8–1.4 0.4 Ti 1.0 0.6 0.9–1.6 0.2–1.1 1.0 0.4

NWA 6286 Ol 20000 Ol Ol OPy 15000 OPy 10000 Ol Hc Ol Ol OPy Tr Ol Ol OPy Tr γ CPy Ch Tr Ol 5000 CPy Hc α INTENSITY, counts INTENSITY,

0 Ch 22 27 32 37 42 Θ ° 2 , (a) (b)

Figure 3. Characterization of the powdered bulk NWA 6286 matter: (a) X-ray diffraction pattern Figure 3. Characterization of the powdered bulk NWA 6286 matter: (a) X-ray diffraction pattern with with indication of some reflexes of the iron-bearing phases: Ol is olivine, OPy is orthopyroxene, indication of some reflexes of the iron-bearing phases: Ol is olivine, OPy is orthopyroxene, CPy is CPy is clinopyroxene, Tr is troilite, Ch is chromite, Hc is hercynite, α is α-phase, and γ is γ-phase. clinopyroxene, Tr is troilite, Ch is chromite, Hc is hercynite, α is α-phase, and γ is γ-phase. (b) (b) Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves with inset that shows an Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves with inset that shows an enlarged part of ZFC curve indicated the phase transition in chromite. Adopted from [13]. enlarged part of ZFC curve indicated the phase transition in chromite. Adopted from [13]. Table 2. Phase composition (in wt.%) of some ordinary chondrites determined by X-ray diffraction. Table 2. Phase composition (in wt.%) of some ordinary chondrites determined by X-ray diffraction. Chelyabinsk Chelyabinsk NWA 6286 NWA 7857 4 5 Phase/Mineral Chelyabinsk1 Chelyabinsk2 NWA3 NWA3 7857Tsarev Tsarev L5 AnnamaAnnama H5 Phase/mineral LL5 No 1a LL5 No 2 LL6 LL6 LL5 No 1a 1 LL5 No 2 2 6286 LL6 3 LL6 3 L5 4 H5 5 Olivine 50.6 48.6 57.3 59.2 43.1 38.6 AnorthiteOlivine 8.250.6 8.2 48.6 11.857.3 9.459.2 9.743.1 38.6 4.7 OrthopyroxeneAnorthite 31.98.2 25.2 8.2 18.911.8 19.99.4 28.69.7 4.7 36.6 ClinopyroxeneOrthopyroxene 5.531.9 6.9 25.2 3.7 18.9 3.6 19.9 6.228.6 36.6 1.4 ClinopyroxeneTroilite 6.75.5 6.2 6.9 4.8 3.7 3.6 3.6 7.26.2 1.4 5.6 α-Fe(Ni, Co) 1.4 1.8 0.2 1.8 9.0 Troilite 6.7 6.2 4.8 3.6 0.67.2 5.6 γ-Fe(Ni, Co) 0.9 0.8 1.2 0.5 1.3 α-Fe(Ni, Co) 1.4 1.8 0.2 1.8 9.0 Chromite 1.5 1.5 1.7 1.7 3.50.6 2.7 γHercynite-Fe(Ni, Co) 0.4 0.9 0.8 0.8 0.4 1.2 0.3 0.5 0.7 1.3 0.2 Chromite 1.5 1 [9]; 2 submitted 1.5 for publication; 3 1.7[13]; 4 [14]; 5 [12].1.7 3.5 2.7 Hercynite 0.4 0.8 0.4 0.3 0.7 0.2 Two representative Mössbauer1 [9]; 2 submitted spectra for ofpublication; non-weathered 3 [13]; 4LL [14]; ordinary 5 [12]. chondrites are shown in Figure4. Two representative Mössbauer spectra of non-weathered LL ordinary chondrites are shown in Figure 4.

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(a) (b)

Figure 4. Mössbauer spectra of LL ordinary chondrites: (a) NWA 6286; and (b) Chelyabinsk, Figure 4. Mössbauer spectra of LL ordinary chondrites: (a) NWA 6286; and (b) Chelyabinsk, fragment No 2. Indicated components are the results of the best fits. The differential spectra are fragment No 2. Indicated components are the results of the best fits. The differential spectra are shown below. T = 295 K. Adopted from [13,21], respectively. shown below. T = 295 K. Adopted from [13] and [21], respectively. These spectra demonstrate a very complex composition of different magnetic and paramagnetic These spectra demonstrate a very complex composition of different magnetic and paramagnetic components. The best fits of the Chelyabinsk LL5 (fragments No 1a and No 2), NWA 6286 LL6 components. The best fits of the Chelyabinsk LL5 (fragments No 1a and No 2), NWA 6286 LL6 and and NWA 7857 LL6 Mössbauer spectra revealed components which were related to the M1 and NWA 7857 LL6 Mössbauer spectra revealed components which were related to the M1 and M2 sites M2 sites in silicate phases (olivine, orthopyroxene and clinopyroxene), ferromagnetic α2-Fe(Ni, in silicate phases (olivine, orthopyroxene and clinopyroxene), ferromagnetic α2-Fe(Ni, Co), α-Fe(Ni, Co), α-Fe(Ni, Co) and γ-Fe(Ni, Co) phases and a paramagnetic γ-Fe(Ni, Co) phase, troilite and Co) and γ-Fe(Ni, Co) phases and a paramagnetic γ-Fe(Ni, Co) phase, troilite and non-stoichiometric non-stoichiometric troilite Fe1–xS, chromite and hercynite and/or mixed Fe(Al1–xCrx)2O4 spinel on troilite Fe1–xS, chromite and hercynite and/or mixed Fe(Al1–xCrx)2O4 spinel on the basis of Mössbauer the basis of Mössbauer hyperfine parameters. It is well known that the Mössbauer spectra of normal hyperfine parameters. It is well known that the Mössbauer spectra of normal chromites measured at chromites measured at room temperature demonstrate single-peak shapes, which were fitted as a room temperature demonstrate single-peak shapes, which were fitted as a quadrupole doublet with quadrupole doublet with a very small value of quadrupole splitting: ∆EQ = 0.15 mm/s in [22] and a very small value of quadrupole splitting: ΔEQ = 0.15 mm/s in [22] and ΔEQ = 0.06 mm/s (δ = 0.92 ∆EQ = 0.06 mm/s (δ = 0.92 mm/s) in [23], or as a single line with δ = 0.90 mm/s in [24], δ = 0.92 mm/s mm/s) in [23], or as a single line with δ = 0.90 mm/s in [24], δ = 0.92 mm/s in [25] and δ = 0.93 mm/s in in [25] and δ = 0.93 mm/s in [26]. However, experimental observation of ∆EQ value, which is smaller [26]. However, experimental observation of ΔEQ value, which is smaller than the 57Fe natural line than the 57Fe natural line width (0.19 mm/s), is doubtful because, in fact, any Lorentzian line can be width (0.19 mm/s), is doubtful because, in fact, any Lorentzian line can be decomposed into two decomposed into two equal Lorentzian lines with slightly different peak positions. Therefore, it is equal Lorentzian lines with slightly different peak positions. Therefore, it is reasonable to consider a reasonable to consider a paramagnetic single line for the chromite Mössbauer spectrum. In contrast, paramagnetic single line for the chromite Mössbauer spectrum. In contrast, the Mössbauer spectra of the Mössbauer spectra of hercynite and mixed Fe(Al1–xCrx)2O4 spinel demonstrated a quadrupole hercynite and mixed Fe(Al1–xCrx)2O4 spinel demonstrated a quadrupole doublet with the following doublet with the following hyperfine parameters: δ = 0.91 mm/s and ∆EQ = 1.57 mm/s obtained for hyperfine parameters: δ = 0.91 mm/s and ΔEQ = 1.57 mm/s obtained for hercynite in [24], while ΔEQ hercynite in [24], while ∆EQ value for the mixed Fe(Al1–xCrx)2O4 spinel may vary depending on x value for the mixed Fe(Al1–xCrx)2O4 spinel may vary depending on x (see [27]). Suggesting small x (see [27]). Suggesting small x values, we can consider similar Mössbauer hyperfine parameters for values, we can consider similar Mössbauer hyperfine parameters for these spinels and use one these spinels and use one quadrupole doublet to fit component assigned to hercynite and/or mixed quadrupole doublet to fit component assigned to hercynite and/or mixed Fe(Al1–xCrx)2O4 spinel. Fe(Al xCrx) O spinel. Revealing1– 2 4the chromite component in the Mössbauer spectra of weathered ordinary chondrites Revealing the chromite component in the Mössbauer spectra of weathered ordinary chondrites is is very complex because the spectral component related to the paramagnetic ferric compounds very complex because the spectral component related to the paramagnetic ferric compounds overlaps overlaps with chromite single peak (see [19,21]). Nevertheless, measurement of the Mössbauer with chromite single peak (see [19,21]). Nevertheless, measurement of the Mössbauer spectrum of spectrum of the weathered Tsarev L5 new fragment with better quality than in a previous study [19] the weathered Tsarev L5 new fragment with better quality than in a previous study [19] permitted us permitted us to reveal spectral components associated with chromite and hercynite (Figure 5a). to reveal spectral components associated with chromite and hercynite (Figure5a). However, it was However, it was not possible to find spectral components related to Mg-bearing spinels. The latter not possible to find spectral components related to Mg-bearing spinels. The latter spinels can be spinels can be presented at least by magnesioferrite MgFe2O4 or by magnesiochromite presented at least by magnesioferrite MgFe2O4 or by magnesiochromite (Fe1–xMgx)Cr2O4. The room (Fe1–xMgx)Cr2O4. The room temperature Mössbauer spectrum of bulk magnesioferrite demonstrates temperature Mössbauer spectrum of bulk magnesioferrite demonstrates magnetic ordering with two magnetic ordering with two six-line patterns related to the 57Fe in tetrahedral (A) and octahedral [B] six-line patterns related to the 57Fe in tetrahedral (A) and octahedral [B] positions with hyperfine positions with hyperfine parameters: δA = ~0.26 mm/s, HeffA = 464 kOe; δB = ~0.35 mm/s, HeffB = 496 parameters: δ = ~0.26 mm/s, H A = 464 kOe; δ = ~0.35 mm/s, H B = 496 kOe, respectively kOe, respectivelyA (see, for instance,eff [28,29]). Therefore,B it is not possibleeff to observe a very small

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(see, for instance, [28,29]). Therefore, it is not possible to observe a very small contribution of Minerals 2018, 8, x FOR PEER REVIEW 5 of 12 possible magnesioferrite magnetic sextets beyond the spectral noise. The room temperature Mössbauer δ spectrumcontribution of magnesiochromite of possible magnesioferrite is similar to chromitemagnetic and sextets demonstrates beyond the the paramagneticspectral noise. singlet The room with valuetemperature of about Mössbauer 0.9 mm/s (see spectrum [23]). In of this magnesiochromi case, it is veryte difficult is similar to extractto chromite correctly and a demonstrates very small singlet the componentparamagnetic in additionsinglet with to a singletδ value related of about to chromite0.9 mm/s with (see relatively[23]). In this larger case, area, it is when very both difficult singlets to overlapextract correctly with a doublet a very relatedsmall singlet to ferric component compounds in addition with much to a larger singlet relative related area. to chromite For example, with whenrelatively we introducedlarger area, anwhen additional both singlets small overlap singlet linewith intoa doublet the fitting related model, to ferric we compounds obtained slightly with bettermuch fit larger with relative two singlets area. For with example, the following when parameters:we introducedδ = an 0.855 additional± 0.015 small mm/s, singlet A ≈ line1.3(1) into % the and δfitting= 1.198 model,± 0.019 we mm/s obtained,A ≈ slightly0.7(1) %.better The fit first wi singletth two cansinglets be related with the to chromitefollowing whileparameters: the second δ = one0.855 can ± be0.015 assigned mm/s, toA magnesiochromite≈ 1.3(1) % and δ = 1.198 with a± larger0.019 mm/s,δ value A than ≈ 0.7(1) that %. obtained The first in singlet [23]. However, can be therelated reliability to chromite of this while result the can second be confirmed one can be in assigned the case to of magnesiochromite the study of non-weathered with a larger ordinaryδ value chondritesthan that obtained with similar in [23]. chemical However, composition the reliability of chromite of this result inclusions. can be confirmed As for quadrupole in the case doublet of the associatedstudy of withnon-weathered hercynite and/or ordinary mixed chondrites Fe(Al1–x Crwix)th2O 4similarspinel, chemical its presence composition can be found of bychromite analysis ofinclusions. the small As peak for at quadrupole about +1.7 doublet mm/s, whichassociated is related with hercynite mainly to and/or the fourth mixed peak Fe(Al in1– thexCr troilitex)2O4 spinel, sextet. Theits presence six-line pattern can be forfound troilite by analysis should beof the fitted small with peak the at constrained about +1.7 peakmm/s, areas which ratio is related A1,6:A2,5 mainly:A3,4 = 3:2:1.to the In fourth the fit peak without in the a quadrupole troilite sextet. doublet The associatedsix-line pattern with hercynitefor troilite and should free variationbe fitted with of troilite the sextetconstrained areas, thepeak value areas of ratio A3,4 isA larger1,6:A2,5:A to3,4 keep = 3:2:1. the requiredIn the fit constraint,without a quadrupole while the fit doublet with this associated constraint showswith hercynite a misfit at and the differentialfree variation spectrum of troilite for thesextet peak areas, at about the ±value1.7 mm/s. of A3,4 Adding is larger the to quadrupole keep the doublet,required which constraint can be related to hercynite, improves the fit, while Mössbauer parameters of this doubletin areaddition suitable to the to be overlapped associated fourth with hercynite peaks of and/orseven magnetic mixed Fe(Al sextets1–xCr relatedx)2O4 spinel.to the Fe-Ni-Co A similar approachphases; and was (ii) used additional for the above-mentioned quadrupole doublet non-weathered is needed for ordinary the better chondrites. fit of the fourth peak of troilite sextet at about +1.7 mm/s similar to those described for the Tsarev L5 Mössbauer spectrum.

(a) (b)

FigureFigure 5.5. MössbauerMössbauer spectra of L andand H ordinary chondrites: (a (a) )Tsarev Tsarev L5; L5; and and (b (b) )Annama Annama H5. H5. IndicatedIndicated components components are are the the results results of theof the best best fits. fits. The The differential differential spectra spectra are shownare shown below. below. T = 295 T = K. Adopted295 K. Adopted from [12 from,14], [14] respectively. and [12], respectively.

TheMössbauer Mössbauer parameters spectrum obtained of non-weathered for chromite Annama and hercynite H5 (Figure and/or5b) mixed shows Fe(Al a huge1–xCr contributionx)2O4 spinel offrom components the Mössbauer related spectra to Fe-Ni-Co of ordinary alloy. chondrites In this spectrum, are presented there in isTable also 3 a in problem comparison with with revealing data thefor minorsynthetic components chromite and related hercynite. to spinels, which overlap with other spectral components. However, in this spectrum, it was also possible to find spinel components because: (i) an envelope peak at about3.2. Chromite +0.9 mm/s in Seymchan demonstrates Main Group some Pallasite features, which can be better fitted using a minor singlet peak in additionCharacterization to the overlapped of the fourthstony part peaks of ofa sevenslightly magnetic weathered sextets Seymchan related toPMG the Fe-Ni-Cofragment phases;using andoptical (ii) additionalmicroscopy quadrupole showed the doublet presence is needed of olivine for thewith better inclusions fit of the of fourthtroilite peakand ofchromite troilite (Figure sextet at about6a). SEM +1.7 with mm/s EDS similar confirmed to those the described presence of for troilite the Tsarev and chromite L5 Mössbauer inclusions spectrum. in olivine while stony fragments were imbedded in Fe-Ni-Co alloy matrix (Figure 6b). Chemical analysis of chromite inclusions demonstrated the presence of ~26–28 at.% of Cr, ~9–10 at.% of Fe, ~5–6 at.% of Mg and ~0.7–1 at.% of Al. In contrast to chromite in the studied ordinary chondrites, chromite inclusions in

Minerals 2019, 9, 42 8 of 16

Mössbauer parameters obtained for chromite and hercynite and/or mixed Fe(Al1–xCrx)2O4 spinel from the Mössbauer spectra of ordinary chondrites are presented in Table3 in comparison with data for synthetic chromite and hercynite.

Table 3. Mössbauer parameters for spinels found in the bulk ordinary chondrites in comparison with data obtained for synthetic spinels.

Chelyabinsk Chelyabinsk NWA 6286 NWA 7857 Synthetic Parameter Tsarev L5 3 Annama H5 5 LL5 No 1a 1 LL5 No 2 2 LL6 3 LL6 4 Spinels 6 Chromite Γ, mm/s 0.776 ± 0.107 0.776 ± 0.034 0.700 ± 0.028 0.776 ± 0.028 0.568 ± 0.030 0.498 ± 0.028 0.33 δ, mm/s 0.855 ± 0.026 0.777 ± 0.017 0.776 ± 0.014 0.662 ± 0.014 0.909 ± 0.015 0.748 ± 0.014 0.90 A, % ~1.6(2) ~2.7(3) ~3.1(3) ~2.9(3) ~2.3(2) ~2.4(2) 100 Minerals 2018, 8, x FOR PEER REVIEW 6 of 12 Hercynite and/or mixed Fe(Al1–xCrx)2O4 spinel Γ, mm/s 0.234 ± 0.033 0.238 ± 0.033 0.235 ± 0.028 0.237 ± 0.028 0.246 ± 0.030 0.260 ± 0.028 0.75 the stonyδ, mm/s part 0.883of Seymchan± 0.027 0.997 PMG± 0.017 contain 0.987 Mg± 0.014 as the 0.959 third± 0.014 metal, 0.843 while± 0.015 Al content 0.852 ± 0.014 is significantly 0.91 ∆EQ, mm/s 1.499 ± 0.469 1.480 ± 0.017 1.434 ± 0.014 1.504 ± 0.014 1.414 ± 0.018 1.465 ± 0.014 1.57 smaller.A, % Therefore, ~0.7(1) chromite ~1.7(2) inclusions ~0.9(1) can also ~1.6(2)contain Mg-bearing ~1.0(1) spinels. ~0.9(1) For example, 100 magnesiochromite may be a result1 of[9 ];Fe2 [ 21substitu]; 3 [13]; tion4 [14]; by5 [12 Mg.]; 6 [ 24However,]. EDS cannot distinguish the presence of magnesiochromite or magnesioferrite in chromite. 3.2. ChromiteThe results in Seymchan of further Main characterization Group Pallasite of the stony part extracted from Seymchan PMG by XRD and magnetizationCharacterization measurements of the stony part are ofshown a slightly in Figure weathered 7. The Seymchan fit of the PMGXRD fragmentpattern demonstrates using optical microscopythat there are showed positions the of presence minor reflexes of olivine corresponding with inclusions to chromite of troilite and magnesiochromite and chromite (Figure instead6a). SEMof MgFe with2O EDS4. The confirmed phase composition the presence of ofthe troilite stony andpart chromitefrom Seymchan inclusions PMG in olivinewas determined while stony as fragmentsfollows: olivine were (~95.5 imbedded wt.%), in clinopyroxene Fe-Ni-Co alloy (~2.3 matrix wt.%), (Figure chromite6b). Chemical(~1.1 wt.%), analysis troilite of (~0.3 chromite wt.%) inclusionsand mixed demonstratediron-magnesium the chromite presence (~0.8 of ~26–28 wt.%). at.% of Cr, ~9–10 at.% of Fe, ~5–6 at.% of Mg and ~0.7–1 at.% of Al. In contrast to chromite in the studied ordinary chondrites, chromite inclusions in the stonyTable part 3. of Mössbauer Seymchan parameters PMG contain for spinels Mg as found the third in the metal, bulk ordinary while Al chondrites content isin significantlycomparison with smaller. Therefore, chromite inclusions candata also obtained contain for Mg-bearing synthetic spinels. spinels. For example, magnesiochromite may be a result of Fe substitution1 [9]; by 2 [21]; Mg. 3 [13]; However, 4 [14]; 5 [12]; EDS 6 [24]. cannot distinguish the presence of magnesiochromite or magnesioferrite in chromite.

(a) (b)

Figure 6. Characterization of the stony part of Seymchan main group pallasite: (a) optical Figure 6. Characterization of the stony part of Seymchan main group pallasite: (a) optical microphotograph; and (b) scanning electron microscopy image with the results of energy dispersive microphotograph; and (b) scanning electron microscopy image with the results of energy dispersive spectroscopy. Adopted from [15]. spectroscopy. Adopted from [15]. The results of further characterization of the stony part extracted from Seymchan PMG by XRD and magnetization measurements are shown in Figure7. The fit of the XRD pattern demonstrates that there are positions of minor reflexes corresponding to chromite and magnesiochromite instead of MgFe2O4. The phase composition of the stony part from Seymchan PMG was determined as follows: olivine (~95.5 wt.%), clinopyroxene (~2.3 wt.%), chromite (~1.1 wt.%), troilite (~0.3 wt.%) and mixed iron-magnesium chromite (~0.8 wt.%).

Minerals 2018, 8, x FOR PEER REVIEW 7 of 12 Minerals 2019, 9, 42 9 of 16

84000 Ol

72000 Ol

60000 Ol Ol 48000 Ol Ol Ol Ol 36000

INTENSITY, countsINTENSITY, Tr Tr Ol Tr

INTENSITY, counts Ol CPy Tr Tr 24000 Ol

12000 CPy Ch Ch Ch MCh MCh MCh 0 12 22 32 42 52 62 72 82 92 22 THETA, THETA, ° ° (a) (b)

Figure 7. Stony part of Seymchan main group pallasite: (a) X-ray diffraction pattern with indication of Figure 7. Stony part of Seymchan main group pallasite: (a) X-ray diffraction pattern with indication some reflexes of the iron-bearing phases: Ol is olivine, CPy is clinopyroxene, Tr is troilite, Ch is chromite, of some reflexes of the iron-bearing phases: Ol is olivine, CPy is clinopyroxene, Tr is troilite, Ch is and MCh is magnesiochromite. (b) Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves chromite, and MCh is magnesiochromite. (b) Zero-field-cooled (ZFC) and field-cooled (FC) with inset which shows enlarged part of ZFC/FC curves indicated a small bulge, probably related to magnetization curves with inset which shows enlarged part of ZFC/FC curves indicated a small the phase transition in chromite. Adopted from [15]. bulge, probably related to the phase transition in chromite. Adopted from [15]. Magnetization measurements showed a very weak bulge at 56 K, which is from the range of Curie Magnetization measurements showed a very weak bulge at 56 K, which is from the range of temperature for chromite [20]. The reason for so small bulge in comparison with that for ordinary Curie temperature for chromite [20]. The reason for so small bulge in comparison with that for chondrites (see Figure3b) might be explained as follows. Chromite should be randomly distributed in ordinary chondrites (see Figure 3b) might be explained as follows. Chromite should be randomly the bulk material, that is why very low chromite content appeared to be in the sample of few mg that distributed in the bulk material, that is why very low chromite content appeared to be in the sample was taken for magnetization measurements from the bulk powder. of few mg that was taken for magnetization measurements from the bulk powder. TheThe Mössbauer Mössbauer spectra spectra of of the the stony stony part part extracted extracted from from Seymchan Seymchan PMG PMG were were measured measured in large in andlarge small and velocity small velocity ranges (Figureranges8 (Figure) to check 8) theto check presence the of presence magnetically of magnetically split components split components (troilite and magnesioferrite(troilite and magnesioferrite demonstrate magnetically demonstrate split magnetically spectra at room split temperature). spectra at room The Mössbauertemperature). spectrum The measuredMössbauer in spectrum a large velocity measured range in showeda large velocity only one ra magneticnge showed sextet only related one magnetic to troilite. sextet This related spectrum to wastroilite. decomposed This spectrum also in severalwas decomposed quadrupole also doublets in several related quadrupole to silicate phasesdoublets and related unknown to silicate ferrous andphases ferric and compounds unknown and ferrous one singlet and ferric which compounds was attributed and to one chromite. singlet The which presence was ofattributed a weak ferric to spectralchromite. component The presence did notof a prevent weak ferric us revealing spectral acomponent singlet subspectrum. did not prevent To increase us revealing resolution a singlet in the spectrum,subspectrum. we measuredTo increase the resolution same sample in the in aspectrum, small velocity we measured range. The the same same spectral sample components in a small werevelocity used range. to fit this The spectrum same spectral (for troilite components subspectrum, were we used know to peak fit positionsthis spectrum for the (for second troilite and thesubspectrum, fifth, and the we thirdknow and peak the positions fourth linesfor the in second the sextet). and the However, fifth, and to the reach third the and best the fit, fourth we had lines to addin the an sextet). additional However, minor to single reach line the to best fit thefit, envelopewe had tospectrum add an additional feature in minor the range single +0.5–1.2 line to fit mm/s. the Mössbauerenvelope spectrum parameters feature for these in the two range singlets +0.5–1.2 are asmm/s. follows: MössbauerΓ = 0.776 parameters± 0.016 mm/s, for theseδ = 0.886two singlets± 0.009 mm/s,are as follows: and A = Γ ~3.4(3) = 0.776% ± and0.016Γ mm/s,= 0.213 δ ±= 0.8860.016 ± mm/s, 0.009 mm/s,δ = 0.796 and± A 0.021= ~3.4(3) mm/s, % and and Γ A= 0.213 = ~0.20(2) ± 0.016 %. Amm/s, singlet δ with = 0.796 relatively ± 0.021 larger mm/s, area and was associatedA = ~0.20(2) with %. chromite A singlet while with the relatively second singlet larger was area assigned was toassociated magnesiochromite. with chromite The whileδ value the for second magnesiochromite singlet was assigned determined to magnesiochromite. in the stony part ofThe Seymchan δ value PMGfor magnesiochromite is slightly smaller thandetermined the δ = 0.92in the mm/s stony value part obtainedof Seymchan for synthetic PMG is slightly magnesiochromite smaller than samples the δ in= [230.92]. Themm/sδ value value for obtained chromite for in thesynthetic stony partmagnesiochromite extracted from Seymchansamples in PMG [23]. is inThe agreement δ value withfor thechromite range ofinδ thevalues stony shown part forextracted chromites from in TableSeymchan3. PMG is in agreement with the range of δ values shown for chromites in Table 3.

Minerals 2019, 9, 42 10 of 16 Minerals 2018, 8, x FOR PEER REVIEW 8 of 12

(a) (b)

FigureFigure 8.8.Mössbauer Mössbauer spectra spectra of of the the stony stony part part of Seymchanof Seymchan main main group group pallasite pallasite measured measured in: (a )in: large (a) velocitylarge velocity range; range; and ( band) small (b) small velocity velocity range. range. Indicated Indicated components components are theare resultsthe results of the of bestthe best fits. Thefits. differentialThe differential spectra spectra are shown are shown below. below. T = 295 T = K. 295 Adopted K. Adopted from from [15]. [15].

3.3.3.3. DaubrDaubréeliteéelite inin TroiliteTroilite ExtractedExtracted fromfrom thethe Sikhote-AlinSikhote-Alin Iron Iron Meteorite Meteorite AA massivemassive troilite troilite inclusion inclusion was was found found in the in polished the polished section ofsection one fragment of one of fragment the Sikhote-Alin of the IIABSikhote-Alin iron meteorite IIAB iron (Figure meteorite9a). This (Figure troilite, 9a). extracted This troilite, from the extractedα-Fe(Ni, from Co) the matrix, α-Fe(Ni, was analyzedCo) matrix, by SEMwas withanalyzed EDS (Figureby SEM9b). with Chemical EDS analysis(Figure of9b). several Chemical troilite analysis particles of demonstrated several troilite the presenceparticles ofdemonstrated ~34 wt.% of the S, presence ~65 wt.% of of ~34 Fe wt.% and of ~1 S, wt.% ~65 wt.% of Cr of (averagedFe and ~1 wt.% values). of Cr The (averaged latter can values). indicate The that there is a small amount of daubréelite FeCr S in troilite extracted from the Sikhote-Alin iron latter can indicate that there is a small amount of2 4daubréelite FeCr2S4 in troilite extracted from the meteorite.Sikhote-Alin The iron XRD meteorite. pattern The of the XRD troilite pattern inclusion of the troilite was measured inclusion and was fitted measured using and the fitted full profile using Rietveldthe full profile analysis Rietveld (Figure analysis 10a). The (Figure results 10a). showed The theresults presence showed of ~93the presence wt.% of troilite of ~93 andwt.% ~7 of wt.% troilite of daubrand ~7éelite. wt.% The of unitdaubréelite. cell of this The daubr unitéelite cell (cubic of this spinel daubréelite structure, (cubic space spinel group structure,Fd3m) with space parameters group MineralsaFd= b3m= 2018c) with= 9.98(5), 8, xparameters FOR Å PEER is shown REVIEW a = b in = Figurec = 9.98(5) 10b. Å is shown in Figure 10b. 9 of 12 Magnetization measurements of the troilite inclusion extracted from the Sikhote-Alin iron meteorite demonstrated two features in the ZFC/FC curves (Figure 11a): (i) a distinguished peak at 74 K; and (ii) a sharp magnetic transition at 168 K. It is well known that daubréelite has the ferrimagnetic–paramagnetic phase transition around 177 K (see, for instance, [30] and references therein) while recently a transition temperature of 166.5 K was found in [31]. Therefore, the magnetic phase transition at 168 K can be assigned to the ferrimagnetic–paramagnetic phase transition in daubréelite in the troilite inclusion. Based on the data about the cubic to triclinic phase transition in daubréelite at ~60 K [32], the peak around 74 K can be assigned also to another phase transition in daubréelite found in the troilite inclusion. Similar magnetization behavior was observed earlier for troilite extracted from the Nantan iron meteorite, and phase transitions at 70 and 169 K were related to the phase transitions in daubréelite presented in troilite [33]. Therefore, both features in the ZFC/FC curves demonstrate the presence of daubréelite in the troilite inclusion extracted from the

Sikhote-Alin iron meteorite. (a) (b)

Figure 9. TroiliteTroilite inclusion in Sikhote-Alin IIAB iron meteorite: ( (aa)) photograph photograph of of massive troilite inclusioninclusion in in α-Fe(Ni,-Fe(Ni, Co) Co) matrix; matrix; and and ( (bb)) scanning scanning electron electron microscopy microscopy image image of of troilite particle extractedextracted from Sikhote-Alin iron meteorite. Adopted from [11]. [11].

MineralsMinerals 2018 2018, 8, ,8 x, xFOR FOR PEER PEER REVIEW REVIEW 1111 of of 16 16

(a()a ) (b(b) )

FigureFigure 9. 9. Troilite Troilite inclusion inclusion in in Sikhote-Alin Sikhote-Alin IIAB IIAB iron iron meteorite: meteorite: ( a()a )photograph photograph of of massive massive troilite troilite Minerals 2018, 8, x FOR PEER REVIEW 10 of 12 inclusioninclusion in in α α-Fe(Ni,-Fe(Ni, Co) Co) matrix; matrix; and and ( b(b) )scanning scanning electron electron microscopy microscopy image image of of troilite troilite particle particle Minerals 2019, 9, 42 11 of 16 extracted extracted from from Sikhote-Alin Sikhote-Alin iron iron meteorite. meteorite. Adopted Adopted from from [11]. [11].

114114 114 2700027000 27000 110110 110 300300 112112 300 2200022000 112 22000

1700017000 224224 308308 17000 213213 222222 224 308 211211 213 222 118118 321321 203203 211 008008 118 321 104104 203 511511 008 311311 412412 INTENSITY, counts INTENSITY, INTENSITY, counts INTENSITY, 101101 311311 004 004 104 511 311 315315 405405 412 INTENSITY, counts INTENSITY, 101 311 004 400400 210210 116116 315 405 1200012000100100 220220 400 210 440440 116 12000 100 220 440

70007000 7000 1616 26 26 36 36 46 46 56 56 66 66 76 76 86 86 16 26 36 46 56 66 76 86 2 THETA,2 THETA, ° ° 2 THETA, ° ((a(a)a) ) ((b(bb)) )

FigureFigureFigureFigure 10. 10. 10.10.Characterization Characterization Characterization Characterization of of of of troilite troilite troilite troilite inclusion inclusion inclusion inclusion extracted extrac extrac extractedtedted from from from from Sikhote-Alin Sikhote-Alin Sikhote-Alin Sikhote-Alin iron iron iron iron meteorite: meteorite: meteorite: meteorite: ( a( ( a)(a)a X-ray) )X-ray X-ray X-ray diffractiondiffractiondiffractiondiffraction pattern pattern patternpattern with with withwith Miller Miller MillerMiller indices indices indicesindices for for forfor troilite troilite troilitetroilite reflections reflections reflectionsreflections (in (in (in(in bold) bold) bold)bold) and and andand for for forfor daubr daubréelite daubréelitedaubréeliteéelite reflections reflections reflectionsreflections (in(in(in(in bold bold bold bold italic);italic); italic); italic); andand and ( (( bb(bb))) )the thethe unit unit unitunit cell cell cell structure structure structure structure of of of ofdaubréelite daubréelite daubréelite daubréelite fo fo foundund foundund in in in troilite introilitetroilite troilite inclusion inclusion inclusion inclusion extracted extracted extracted extracted from from from  fromSikhote-AlinSikhote-AlinSikhote-Alin Sikhote-Alin iron ironiron iron meteorite; meteorite;meteorite; meteorite; a a,a , ,b b,b ,a ,and ,andandb, and c c c are arearec are the thethe the unit unitunit unit cell cellcellcell parameters; parameters;parameters; parameters;   – – —Fe,– Fe, Fe,Fe,   –—Cr, – – Cr, Cr,Cr, and andandand  —S. – – – S. S.S. AdoptedAdoptedAdoptedAdopted from from from from [ 11[11]. [11]. [11].]. Magnetization measurements of the troilite inclusion extracted from the Sikhote-Alin iron TheTheThe roomroomroom temperaturetemperaturetemperature MössbauerMössbauerMössbauer spectrumspectrumspectrum ofofof thethethe troilitetroilitetroilite inclusioninclusioninclusion extractedextractedextracted fromfromfrom thethethe meteorite demonstrated two features in the ZFC/FC curves (Figure 11a): (i) a distinguished peak Sikhote-AlinSikhote-AlinSikhote-Alin iron iron iron meteorite meteorite meteorite is is is shown shown shown in in in Figure Figure Figure 11b. 11b. 11b. Originally, Originally, Originally, in in in [11], [11], [11], this this this spectrum spectrum spectrum was was was fitted fitted fitted using using using at 74 K; and (ii) a sharp magnetic transition at 168 K. It is well known that daubréelite has the thethethe full fullfull static staticstatic Hamiltonian HamiltonianHamiltonian for forfor troilite troilitetroilite component. component.component. However, However,However, here herehere we wewe present presentpresent a a a simple simplesimple fit fitfit using usingusing a a a ferrimagnetic–paramagnetic phase transition around 177 K (see, for instance, [30] and references simulationsimulationsimulation of ofof the thethe full fullfull static staticstatic Hamiltonian HamiltonianHamiltonian for forfor troilit troilittroilitee e component componentcomponent and andand an anan additional additionaladditional four fourfour sextets sextetssextets for forfor therein)non-stoichiometricnon-stoichiometric while recently troilite troilite a transition Fe Fe1–1–xSxS for temperaturefor better better observation observation of 166.5 K of of was an an foundaddi additionaltional in [31 spectral ].spectral Therefore, component component the magnetic related related non-stoichiometric troilite Fe1–xS for better observation of an additional spectral component related phase transition at 168 K can be assigned to the ferrimagnetic–paramagnetic phase transitionΓ inΓ tototo daubréelite. daubréelite.daubréelite. Its ItsIts spectral spectralspectral co cocomponentmponentmponent has hashas a a a single-peak single-peaksingle-peak shape shapeshape with withwith the thethe following followingfollowing parameters: parameters:parameters: Γ = = = daubré±elite± in the troilite inclusion.±± Based on the data about the cubic to triclinic phase transition in 0.7760.7760.776 ± 0.008 0.0080.008 mm/s, mm/s,mm/s, δ δδ = = = 0.584 0.5840.584 ± 0.009 0.0090.009 mm/s, mm/s,mm/s, and andand A AA = = = 2.6(3) 2.6(3)2.6(3) %. %.%. This ThisThis single singlesingle peak peakpeak disappears disappearsdisappears in inin the thethe daubréelite at ~60 K [32], the peak around 74 K can be assigned also to another phase transition in spectrumspectrumspectrum measured measuredmeasured at atat 90 9090 K KK [11] [11][11] due duedue to toto the thethe magnetic magneticmagnetic phase phasephase transition transitiontransition and andand the thethe appearance appearanceappearance of ofof a a a very veryvery daubréelite found in the troilite inclusion. Similar magnetization behavior was observed earlier for smallsmallsmall magnetic magneticmagnetic sextet sextetsextet related relatedrelated to toto daubréelite, daubréelite,daubréelite, extraction extractionextraction of ofof which whichwhich is isis very veryvery difficult difficultdifficult in inin the thethe spectrum spectrumspectrum troilite extracted from the Nantan iron meteorite, and phase transitions at 70 and 169 K were related to withwithwith manymanymany overlappedoverlappedoverlapped sextets.sextets.sextets. ItItIt isisis wellwellwell knowknowknownnn thatthatthat thethethe MössbauerMössbauerMössbauer spectraspectraspectra ofofof daubréelitedaubréelitedaubréelite the phase transitions in daubréelite presented in troilite [33]. Therefore, both features in the ZFC/FC demonstratedemonstratedemonstrate a a a single singlesingle peak peakpeak compon componcomponententent only onlyonly above aboveabove Curie CurieCurie temperature temperaturetemperature (see, (see,(see, for forfor instance, instance,instance, [34]). [34]).[34]). The TheThe curvesMineralsroomroom temperature demonstrate 2018temperature, 8, x FOR PEER theMössbauer Mössbauer presence REVIEW spectrum ofspectrum daubré ofelite of the the in polycrystalline polycrystalline the troilite inclusion synthetic synthetic extracted FeCr FeCr2S from2S4 4sample sample the Sikhote-Alin measured measured11 of 12 in in room temperature Mössbauer spectrum of the polycrystalline synthetic FeCr2S4 sample measured in iron[35][35] meteorite. shows shows a asingle single peak peak with with δ δ = =1.2 1.2 mm/s mm/s that that is is twice twice larger larger than than those those obtained obtained for for daubréelite daubréelite [35] shows a single peak with δ = 1.2 mm/s that is twice larger than those obtained for daubréelite foundfoundfound in in in the the the troilite troilite troilite inclusion inclusion inclusion extracted extracted extracted from from from the the the Sikhote-Alin Sikhote-Alin Sikhote-Alin iron iron iron meteorite. meteorite. meteorite. However, However, However, other other other results results results obtainedobtainedobtained earlier earlierearlier in inin [36,37] [36,37][36,37] demonstrate demonstratedemonstrate similar similarsimilar δ δδ values: values:values: ~0.59 ~0.59 ~0.59 mm/s. mm/s.mm/s.

(a) (b)

FigureFigure 11.11. TroiliteTroilite inclusioninclusion extractedextracted fromfrom thethe Sikhote-AlinSikhote-Alin iron meteorite: ( (aa)) zero-field-cooled zero-field-cooled (ZFC)(ZFC) and and field-cooled field-cooled (FC)(FC) magnetizationmagnetization curves; and ( (bb)) Mössbauer Mössbauer spectrum spectrum measured measured at at 295 295 K: K: indicatedindicatedcomponents components areare thethe resultresult ofof the fitfit with a simulation of the the full full static static Hamiltonian Hamiltonian for for the the troilitetroilite component, component, the the differentialdifferential spectrumspectrum isis shownshown below.below. DataData adoptedadopted from [11]. [11].

3.4. Magnesioferrite in the Fusion Crust of Chelyabinsk LL5 Fragment

The fusion crust is a glass-like solidified melt resulting from meteorite surface combustion in the Earth’s atmosphere during its fall. Various studies of meteorite fusion crusts showed formation of magnetite Fe3O4, a spinel resulting from iron oxidation during combustion of the Fe-Ni-Co alloy [38,39]. We studied the fusion crust removed from ordinary chondrite Chelyabinsk LL5 fragment No 1a [10]. The XRD pattern of the fusion crust from Chelyabinsk LL5 fragment No 1a is shown in Figure 12a. The fit of this pattern using the Rietveld full profile analysis demonstrated the presence of olivine (~50 wt. %), pyroxene (~27 wt. %) and troilite (~4 wt. %) phases, Fe-Ni-Co alloy (~1 wt. %) and an additional phase related to magnesioferrite (~18 wt. %) instead of magnetite. The two main reflexes [2 2 0] and [3 1 1] at 2Θ ~30° and ~35.5°, respectively, corresponding to magnesioferrite (PDF 01-089-6188), are clearly seen in the X-ray diffractogram, confirming the presence of magnesioferrite. There is an X-ray amorphous halo in 2Θ range ~30–36° that may be a result of the presence of some amount of nanosized magnesioferrite particles in the glass-like fusion crust. The first Mössbauer spectrum of the fusion crust from Chelyabinsk LL5 fragment No 1a, which should only be considered as a preliminary result, is shown in Figure 12b. In this spectrum, a pronounced six-line pattern with larger hyperfine field than that for sextets related to the Fe-Ni-Co alloy and troilite is clearly seen. The best fit of this spectrum revealed five magnetic sextets and five quadrupole doublets, as shown in Figure 12b. Besides two sextets related to Fe-Ni-Co alloy and troilite, respectively, and four quadrupole doublets related to the M1 and M2 sites in both olivine and pyroxene, three additional magnetic sextets with the following parameters were found: (1) δ = 0.271 ± 0.020 mm/s, Heff = 481.0 ± 0.6 kOe, A = ~16(2) %; (2) δ = 0.528 ± 0.029 mm/s, Heff = 479.0 ± 1.3 kOe, A = ~6(1) %; and (3) δ = 0.562 ± 0.020 mm/s, Heff = 444.6 ± 1.8 kOe, A = ~11(1) %. One additional quadrupole doublet with parameters δ = 0.502 ± 0.026 mm/s, ΔEQ = 0.993 ± 0.047 mm/s, and A = ~7(1) % was found. It is well known that magnesioferrite has a spinel structure with Fe3+ and Mg2+ cations in both tetrahedral (A) and octahedral [B] positions in different proportions within the formula: (Mg1–xFex)A[MgyFe2−y]BO4. Mössbauer spectra of the bulk magnesioferrite usually demonstrate two magnetic sextets related to the 57Fe in both (A) and [B] positions, as shown above in Section 3.1 (see [28,29]). Mössbauer hyperfine parameters permit sextets related to two different positions in spinel to be distinguished, because the values of δ are smaller with larger values of Heff for sextets related to (A) positions while δ values are larger with smaller values of Heff for sextets related to [B] positions. However, in the case of nanosized magnesioferrite the Mössbauer spectra were fitted using three magnetic sextets: one sextet was related to (A) positions while two sextets were related to

Minerals 2019, 9, 42 12 of 16

The room temperature Mössbauer spectrum of the troilite inclusion extracted from the Sikhote-Alin iron meteorite is shown in Figure 11b. Originally, in [11], this spectrum was fitted using the full static Hamiltonian for troilite component. However, here we present a simple fit using a simulation of the full static Hamiltonian for troilite component and an additional four sextets for non-stoichiometric troilite Fe1–xS for better observation of an additional spectral component related to daubréelite. Its spectral component has a single-peak shape with the following parameters: Γ = 0.776 ± 0.008 mm/s, δ = 0.584 ± 0.009 mm/s, and A = 2.6(3) %. This single peak disappears in the spectrum measured at 90 K [11] due to the magnetic phase transition and the appearance of a very small magnetic sextet related to daubréelite, extraction of which is very difficult in the spectrum with many overlapped sextets. It is well known that the Mössbauer spectra of daubréelite demonstrate a single peak component only above Curie temperature (see, for instance, [34]). The room temperature Mössbauer spectrum of the polycrystalline synthetic FeCr2S4 sample measured in [35] shows a single peak with δ = 1.2 mm/s that is twice larger than those obtained for daubréelite found in the troilite inclusion extracted from the Sikhote-Alin iron meteorite. However, other results obtained earlier in [36,37] demonstrate similar δ values: ~0.59 mm/s.

3.4. Magnesioferrite in the Fusion Crust of Chelyabinsk LL5 Fragment The fusion crust is a glass-like solidified melt resulting from meteorite surface combustion in the Earth’s atmosphere during its fall. Various studies of meteorite fusion crusts showed formation of magnetite Fe3O4, a spinel resulting from iron oxidation during combustion of the Fe-Ni-Co alloy [38,39]. We studied the fusion crust removed from ordinary chondrite Chelyabinsk LL5 fragment No 1a [10]. The XRD pattern of the fusion crust from Chelyabinsk LL5 fragment No 1a is shown in Figure 12a. The fit of this pattern using the Rietveld full profile analysis demonstrated the presence of olivine (~50 wt. %), pyroxene (~27 wt. %) and troilite (~4 wt. %) phases, Fe-Ni-Co alloy (~1 wt. %) and an additional phase related to magnesioferrite (~18 wt. %) instead of magnetite. The two main reflexes [2 2 0] and [3 1 1] at 2Θ ~30◦ and ~35.5◦, respectively, corresponding to magnesioferrite (PDF 01-089-6188), are clearly seen in the X-ray diffractogram, confirming the presence of magnesioferrite. There is an X-ray amorphous halo in 2Θ range ~30–36◦ that may be a result of the presence of some amount of nanosized magnesioferrite particles in the glass-like fusion crust. The first Mössbauer spectrum of the fusion crust from Chelyabinsk LL5 fragment No 1a, which should only be considered as a preliminary result, is shown in Figure 12b. In this spectrum, a pronounced six-line pattern with larger hyperfine field than that for sextets related to the Fe-Ni-Co alloy and troilite is clearly seen. The best fit of this spectrum revealed five magnetic sextets and five quadrupole doublets, as shown in Figure 12b. Besides two sextets related to Fe-Ni-Co alloy and troilite, respectively, and four quadrupole doublets related to the M1 and M2 sites in both olivine and pyroxene, three additional magnetic sextets with the following parameters were found: (1) δ = 0.271 ± 0.020 mm/s, Heff = 481.0 ± 0.6 kOe, A = ~16(2) %; (2) δ = 0.528 ± 0.029 mm/s,Heff = 479.0 ± 1.3 kOe, A = ~6(1) %; and (3) δ = 0.562 ± 0.020 mm/s,Heff = 444.6 ± 1.8 kOe, A = ~11(1) %. One additional quadrupole doublet with parameters δ = 0.502 ± 0.026 mm/s, ∆EQ = 0.993 ± 0.047 mm/s, and A = ~7(1) % was found. Minerals 2018, 8, x FOR PEER REVIEW 12 of 12

[B] positions, as well as one quadrupole doublet related to the paramagnetic state of the smallest magnesioferrite particles [40]. Moreover, the values of δ = 0.37 mm/s and ΔEQ = 0.99 mm/s obtained for the paramagnetic quadrupole doublet in [40] appeared to be close to above-mentioned Mössbauer parameters for a paramagnetic doublet revealed in the spectrum of the fusion crust in [10]. Therefore, sextet (1) was related to the 57Fe in (A) positions while sextets (2) and (3) were assigned to the 57Fe in [B] positions. The rest quadrupole doublet can be associated with nanosized magnesioferrite particles which are in the paramagnetic state. Thus, the first study of the fusion crust Minerals 2019, 9, 42 13 of 16 from Chelyabinsk LL5 fragment No 1a using XRD and Mössbauer spectroscopy observed the presence of magnesioferrite instead of magnetite.

36500 MF Py

31500 Ol Py Py Tr MF

26500

Py Py MF MF 21500 Ol Ol Py Py

INTENSITY, counts INTENSITY, Py MF Ol Ol Tr Py Tr Tr 16500 Py Py Ol MF Py Py Py

Tr 11500 Ol 20 30 40 50 60 70 80 90 100 2 THETA, ° (a) (b)

FigureFigure 12.12. TheThe fusion fusion crust crust from from Chelyabinsk Chelyabinsk LL5 LL5fragment fragment 1a: (a 1a:) X-ray (a) diffraction X-ray diffraction pattern, pattern,arrows arrowsindicate indicate reflexes reflexes of the of main themain phases phases such such as Ol as Ol(olivine), (olivine), Py Py(pyroxene), (pyroxene), Tr Tr (troilite), (troilite), and and MF MF (magnesioferrite);(magnesioferrite); and and (b ()b Mössbauer) Mössbauer spectrum spectrum measured measured at 295at 295 K: indicated K: indicated components components are the are result the ofresult the preliminary of the preliminary fit, the differentialfit, the differential spectrum spectrum is shown is shown below. belo Adoptedw. Adopted from [ 10from]. [10].

3+ 2+ 4. ConclusionsIt is well known that magnesioferrite has a spinel structure with Fe and Mg cations in both tetrahedral (A) and octahedral [B] positions in different proportions within the formula: (Mg 1–xFex)A[MgyFe2−y]BO4. Mössbauer spectra of the bulk magnesioferrite usually demonstrate two magnetic sextets related to the 57Fe in both (A) and [B] positions, as shown above in Section 3.1 (see [28,29]). Mössbauer hyperfine parameters permit sextets related to two different positions in spinel to be distinguished, because the values of δ are smaller with larger values of Heff for sextets related to (A) positions while δ values are larger with smaller values of Heff for sextets related to [B] positions. However, in the case of nanosized magnesioferrite the Mössbauer spectra were fitted using three magnetic sextets: one sextet was related to (A) positions while two sextets were related to [B] positions, as well as one quadrupole doublet related to the paramagnetic state of the smallest magnesioferrite particles [40]. Moreover, the values of δ = 0.37 mm/s and ∆EQ = 0.99 mm/s obtained for the paramagnetic quadrupole doublet in [40] appeared to be close to above-mentioned Mössbauer parameters for a paramagnetic doublet revealed in the spectrum of the fusion crust in [10]. Therefore, sextet (1) was related to the 57Fe in (A) positions while sextets (2) and (3) were assigned to the 57Fe in [B] positions. The rest quadrupole doublet can be associated with nanosized magnesioferrite particles which are in the paramagnetic state. Thus, the first study of the fusion crust from Chelyabinsk LL5 fragment No 1a using XRD and Mössbauer spectroscopy observed the presence of magnesioferrite instead of magnetite.

4. Conclusions Different meteorites (stony, stony-iron and iron) contain the minor spinel phases as accessories to the main minerals. Therefore, observation of the iron-bearing spinels in the Mössbauer spectra of bulk meteorite materials is not easy. Nevertheless, application of the high velocity resolution Mössbauer spectroscopy permits us to observe the minor spectral components in the complex meteorite spectra related to spinel phases. The complex study, using optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, X-ray diffraction and magnetization measurements, in addition to Mössbauer spectroscopy, permits us to identify and prove the presence of iron-bearing spinels in meteorites. Thus, it is possible to observe various spinels such as chromite, hercynite, magnesiochromite, daubréelite and magnesioferrite in different meteorites using Mössbauer spectroscopy. Minerals 2019, 9, 42 14 of 16

Author Contributions: Conceptualization, M.I.O.; reviewed measurements were carried out mainly by: A.A.M. (optical microscopy), A.A.M., M.I.O. (SEM with EDS), A.V.C. (XRD, full profile Rietveld analysis), I.F. (magnetization measurements and analysis), A.A.M., M.I.O. (Mössbauer measurements and spectra fits); writing—original draft preparation, M.I.O.; writing—review and editing, A.A.M., I.F., A.V.C. Acknowledgments: The authors wish to thank all their colleagues in various studies, reviewed in the present work, for fruitful cooperation: Dr. V.I. Grokhovsky, Dr. V.A. Semionkin, Dr. E.V. Petrova, Dr. M.Yu Larionov, Dr. M.S. Karabanalov, M.V. Goryunov and G.A. Yakovlev (Ural Federal University, Ekaterinburg); Prof. E. Kuzmann, Prof. Z. Homonnay and Z. Bend˝o(Eötvös Loránd University, Budapest); Dr. Z. Klencsár (Centre for Energy Research, Hungarian Academy of Sciences, Budapest); and Dr. T. Kohout (University of Helsinki, Helsinki). This work was supported by the Ministry of Science and Higher Education of the Russian Federation (the Project No. 3.1959.2017/4.6) and by Act 211 of the Government of the Russian Federation, contract No. 02.A03.21.0006. Conflicts of Interest: The authors declare no conflict of interest.

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