American Mineralogist, Volume 95, pages 870–875, 2010

0003-004X/10/0506–870$05.00/DOI: 10.2138/am.2010.3479 870

57Fe Mössbauer spectroscopy and electrical resistivity studies on naturally occurring native iron under high pressures up to 9.1 GPa

Usha Chandra,1,* Pooja sharma,1 G. Parthasarathy,2 and B. sreedhar3

1Department of Physics, University of Rajasthan, Jaipur 302 055, India2National Geophysical Research Institute (CSIR), Hyderabad 500 606, India3Indian Institute of Chemical Technology (CSIR), Hyderabad 500 606, India

aBstraCt

We report the pressure dependence of the Mössbauer spectra and the electrical resistivity up to 9.1 GPa at room temperature for a native iron sample collected from the Precambrian Chaibasa shales, Singhbhum Craton, Eastern India. The Mössbauer spectroscopy of the sample at ambient conditions yields isomer shifts and magnetic hyperfine field values that confirm the presence of Fe0 oxidation state. Many theories have been put forward to explain the origin of this native iron including a Pre-cambrian meteoritic impact. High-pressure Mössbauer spectroscopic measurements using diamond anvil cell (DAC) showed a constant isomer shift up to 5.6 GPa with a subtle variation of –1.07 × 10–3 mm/s/GPa followed by sharper change –4.3 × 10–3 mm/s/GPa above 6.3 GPa, a pressure much lower than the usual value reported for metallic iron. Further increase of pressure to 9.1 GPa results in the emergence of a tiny peak at ~0 isomer shift indicating the onset of the martensitic phase transition of iron from the body-centered-cubic (bcc) to hexagonal-close-packed (hcp) transition 4 GPa lower than the transition pressure normally observed for pure iron. This phase transition in the native iron is confirmed by high-pressure electrical resistivity study. Lowering of the transition pressure could be due to nucleation of hcp by stacking faults caused by shock metamorphism resulting from the Precambrian impact in the region.

Keywords: Mössbauer spectroscopy, high pressure, diamond anvil cell, native iron, Precambrian impact, electrical resistivity, XPS spectroscopy

IntrodUCtIon

Mössbauer spectroscopic studies of iron and its high-pressure polymorphs, important constituents of meteorites and other ter-restrial and extra-terrestrial objects, provide important informa-tion about their origin and pre-terrestrial transformations. Since the discovery of up to 25 tons heavy boulders of native iron on Disko Island, West Greenland, this occurrence has attracted much attention. The origin of native iron has been the subject of much speculation and various theories have been proposed ranging from meteoroid impact to the chemical reaction between magma and carbonaceous sediments (Carpenter 1935; Cameron 1970; Gibb et al. 1970; Kukkonen et al. 1992; Collinson 1990; Abbott et al. 2006; Colwell et al. 2007; Haggerty and Toft 1985). However, only two preliminary reports are published on the possible occurrence of native iron in India: The first study based on the magnetic properties of the sample, the second report characterized the sample with magnetic susceptibility and scan-ning electron microscopy (SEM) measurements. These findings suggested the occurrence of native iron in the Precambrian Chaibasa shales to be the oldest native iron ever found, formed due to the pre-1.6 Ga impact and preserved for over 1.6 billion years by sudden burial of the molten iron and fast sedimentation in the region. The iron melted during the nearby impact event,

flew through air and was deposited as spherical globules. Stud-ies using SEM, on the other hand, showed the presence of Fe without indicating whether it was present as native iron, hematite, magnetite, or wüstite.

The present sample is from Chaibasa shales of Singhbhum Craton in India where the impact ejecta layer in the pre-1.6 Ga sediments are found (Abbott et al. 2006; Mazumdar 2004). In a preliminary study, Sreedhar and Parthasarathy (2008) used X-ray photoelectron spectroscopic (XPS) and X-ray diffraction (XRD) techniques and reported the existence of native iron with traces of cohenite, suggesting an impact event during the Precambrian. In the present study, Mössbauer spectroscopy is used to confirm the occurrence of native iron in nature at room temperature. Moreover, the sensitivity of Mössbauer parameters (isomer shift, quadrupole splitting, and magnetic hyperfine field) to pressure variations is used to study the sample at high pressure and com-pare it with pure metallic iron. Our data suggest that the sample acquired a new thermodynamic equilibrium during a specific post-genetic event, thus supporting the impact theory.

GeoloGICal settInG

Singhbhum Craton, situated in the eastern part of Indian shield, covers an area of about 40 000 km2. The Archean of this craton, considered to be one of the oldest (3.5–3.8 Ga) recogniz-able nuclei of Indian shield (Mahadevan 2002), appears to have been squeezed almost from all sides by mobile belts, thrust zones, * E-mail: ushac_jp1@sancharnet.in

CHANDRA ET AL.: HIGH-PRESSURE STUDIES ON NATURALLy OCCURRING NATIvE IRON 871

and rift valleys. This region has been subjected to widespread mafic volcanic activity during the Proterozoic period resulting in several volcanics suites. These include the Dalma, Ongarbira, Dhanjori, Simlipal, Bonai, and Jagannathapur volcanics. Based on high-temperature magnetic studies on the rocks, the probable existence of native iron has earlier been reported from dyke swarms occurring between latitude 22°–22°45′ N, longitude 85°48′–86°25′ E (Verma and Prasad 1975). The studied sample is from the mica schist in the Chaibasa area of the Singhbhum Craton, Jharkhand, India.

exPerImental methodsThe opaque bright grains of iron picked up from the shales under the

microscope, were analyzed by using X-ray diffraction (XRD), electron probe micro analyzer (EPMA), X-ray fluorescence (XRF), and scanning electron microscopy (SEM).

X-ray photoelectron spectroscopy (XPS) measurements were carried out on thin sections of two different samples (A and B) with a KRATOS-AXIS 165 instrument equipped with dual aluminum-magnesium anodes, using MgKα radiation operat-ing at 15 kv and 5 mA under a chamber pressure of 10–9 Torr. The XPS spectra were fitted using a nonlinear square method with the convolution of Lorentzian and Gaussian functions after polynomial background subtraction from the raw data. The details of the instrumentation and uncertainties in measurements were discussed elsewhere (Parthasarathy et al. 2003). Peak positions were calibrated with respect to the C 1s peak (284.6 ev).

The samples were gently ground on a thin sheet under acetone (to avoid oxi-dation and additional stress on the sample) until very fine flakes were obtained. Room-temperature 57Fe Mössbauer measurement was performed using a 10 mCi 57Co(Rh) point source with an active area of 0.5 × 0.5 mm2 operating in a constant acceleration mode on a Wissel transducer, a Si-PIN solid-state detector (Amptek XR-100CR with a resolution 250 ev) and CMCA-550 data acquisition module (Wissel Make) for data collection. The velocity scale was calibrated with refer-ence to α-iron. For high-pressure Mössbauer measurements, a Merrill-Bassett type diamond anvil cell (DAC) was used (Chandra et al. 2001, 2005; Chandra 2007). The DAC sample assembly was prepared in a tantalum gasket hole of diameter ~ 200 µm using 4:1 methanol:ethanol mixture as pressure-transmitting fluid and a few tiny ruby crystals as pressure calibrant. The shift in the R1 and R2 lines of ruby, linearly varying with pressure, was used to determine pressure on the sample. Mössbauer measurements in steps of ~0.7 GPa were carried out for long exposure times (more than one week each) to achieve an appreciable signal to noise ratio. The patterns were analyzed by fitting least-square analysis to Lorentzian lines us-ing the Mössbauer analysis program of Jernberg and Sundquist (1983) mainly to determine the positions of the peaks without considering the intensity ratio. The significant absence of the usual 3:2:1:1:2:3 intensity ratios for metallic iron suggest the polarized nature of the powdered sample.

Electrical resistivity measurements were carried out in an opposed anvil system using MgO as pressure-transmitting medium. The details of the apparatus and calibration are discussed elsewhere (Parthasarathy 2006, 2007). Typical uncertainty involved in the pressure measurements is 0.2 GPa in the pressure region of 6 GPa. Three independent runs were made on two different samples of the native iron.

resUlts and dIsCUssIonCharacterization

Table 1 represents powder X-ray diffraction (XRD) of opaque pure fine grains with d-spacing, intensity, and phase associated indicating bcc-structured native iron along with traces of cohenite (FeNi)3C with Co as an impurity at a parts per million level. Electron probe micro analysis (EPMA) and X-ray fluorescence (XRF) techniques showed the composition to be Fe = 98.95 ± 0.02 wt% and Ni = 0.92 ± 0.01wt%.

X-ray photoelectron spectroscopy (XPS) used to identify the elements present shows the Fe 2p3/2 spectrum for the Chai-basa sample that could be fitted in two components at binding energies 706.9 and 709.0 ev, representing the native iron and cohenite, respectively (Fig. 1). Similar spectrum of native iron

obtained from lunar sample LANM-81 and meteoritic native iron ASAN76 showed a similar Fe0 peak at 706.7 and 706.8 ev, respectively. On the other hand, the binding energy values for Fe2+ and Fe3+ components were reported at 709.8 and 711.0 ev, respectively (Moulder et al. 1992).

Mössbauer spectroscopy57Fe Mössbauer spectroscopy is a sensitive technique to

study the oxidation state, chemical environment through its parameters—isomer shift (IS), quadrupole splitting (QS), and magnetic hyperfine field (HF) (Bhide 1973). The sensitivity of these parameters to changes in pressure is as follows:

(1) Isomer shift (IS)—the center of gravity of the spectrum consists of two components: (a) the second-order Doppler shift that is proportional to mean square velocity of the atom, and

Table 1. Powder X-ray diffraction data of the native iron sample from the Chaibasa shale, Singhbhum Craton

d-spacing Normalized peak intensity Miller indices Phase(Å) JCPDS card Present study (hkl) identified2.383 65 10 (112) and (201) cohenite2.112 60 10 (121) cohenite2.070 70 10 (210) cohenite2.029 100 100 (110) + (022) iron + cohenite1.990 100 30 103 cohenite1.970 55 10 (211) cohenite1.857 40 5 (220) cohenite1.433 20 15 (200) iron1.170 30 20 (211) iron1.014 10 8 (220) iron0.906 12 8 (310) ironNote: The data for pure iron (JCPDS card 6-696) and cohenite (23-1113) are listed for comparison.

FIGUre 1. Room-temperature X-ray photoemission spectra of the native iron samples (A and B) taken from the same piece collected from the mica schist of Chaibasa area, Singhbhum Craton. The peak at ~707 ev corresponds to the Fe0 valence state while the second peak indicates Fe2+ state in cohenite present on the surface of the sample.

Binding Energy (eV)

Inte

nsit

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CHANDRA ET AL.: HIGH-PRESSURE STUDIES ON NATURALLy OCCURRING NATIvE IRON872

(b) the isomer shift that depends on s-electron density at the nucleus. The effect of pressure on the second-order Doppler shift is negligible, while the s-electron density at the nucleus is sensitive to pressure variation, resulting in changes in the isomer shift (Pipkorn et al. 1964).

(2) Quadrupole splitting (QS)—quadrupole splitting gives information about the nature of the chemical bonds due to asym-metric distribution of charges either on the ions itself or in the crystal. In any specific compound, two opposite contributions originating from valence electrons and the lattice contribute to-ward QS and both are sensitive to pressure. For ions with filled or half filled configuration (where valence electron contribution is zero) the parameter is indicative of lattice distortion. Uniaxial stress could cause distortion in the lattice hence a quadrupole splitting at high pressure.

(3) Magnetic hyperfine field (HF)—the pressure dependency of the internal field is responsible for the change in spin density arising from core polarization of the 4 s electrons. Because the s-wave function for the inner electrons is large even at the nucleus, a slight difference may produce a large variation in the internal field. The intensity ratio is another quantity that describes the polarization in the system. For a randomly oriented magnetic field direction (generally seen in powdered samples), the ratio is 3:2:1:1:2:3. But if the magnetic field is polarized along and perpendicular to the γ-rays, the intensity ratios are 3:4:1:1:4:3 and 3:0:1:1:0:3, respectively.

Figure 2 shows the Mössbauer patterns and the analyzed parameters (IS, QS, and HF) of native iron sample at ambient condition (outside DAC) and at various pressures of the sample inside DAC. Ambient condition Mössbauer pattern of the pow-dered native iron sample exhibits a six line pattern with peak intensity ratios that are not of 3:2:1:1:2:3 due to the polarization effect inherent to this natural sample (Chandra et al. 1999). The almost-zero isomer shift (–0.020 mm/s with respect to α-iron) and magnitude of hyperfine field (~33 Tesla) for the ambient sample confirms the presence of Fe0 (Table 2). However, at 9.1 GPa, the Mössbauer pattern shows an interesting feature—an extra peak (marked with a line) at almost-zero isomer shift (Fig. 3). To obtain a better resolution of the new peak corresponding to the high-pressure phase, the Mössbauer drive was driven to a lower velocity scale, incorporating only the two inner lines of the sextet of iron spectrum. Figure 3 clearly shows the emer-gence of a new peak at zero velocity, indicating transformation of bcc → hcp phase. Earlier studies on the pressure-dependent Mössbauer spectra of α-Fe by various researchers (Pipkorn et al. 1964; Cort et al. 1982; Taylor et al. 1991; Moyzis and Drickamer 1968) have shown a linear change in isomer shift with pressure. Pipkorn et al. (1964) measured 57Fe in iron metal up to 24 GPa at room temperature and reported an initial linear variation in isomer shift of –8.3 × 10–3 mm/s/GPa, quite close to the value (–7.98 ± 0.31) × 10–3 mm/s/GPa reported by Pound et al. (1961). Moyzis and Drickamer (1968) and Williamson et al. (1972) using the inner two lines of the magnetic sextet on Fe foil and bcc iron complemented the results of Pipkorn et al. (1964) and Pound et al. (1961). The parameter was nearly linear up to 6 GPa though at higher pressures the rate of shift was relatively slower. Southwell et al. (1968) used a tetrahedral anvil press for the range 0 to 8.5 GPa and reported linear decrease in IS with a

slope (–7.46 ± 0.21) × 10–3 mm/s/GPa, increase in QS as ∆EQ/∆P = (0.96 ± 0.23) × 10–3 mm/s/GPa and decrease in the magnetic field as ∆H/H0 = (–1.34 ± 0.25) × 10–1P – (0.60 ± 0.39) × 10–6P2 with P in kbar.

Cort et al. (1982) observed a variation in IS as –6.5 × 10–3

mm/s/GPa on a foil of natural iron (doped with 57Co) with a linear decrease in the internal magnetic field and line width

∆lnH/∆P = (–3.15 ± 0.09) × 10–3 GPa–1

which agrees with Pipkorn’s estimation (Moyzis and Drickamer 1968)

∆H/H0∆P = (–1.69 ± 0.05) × 10–3 GPa–1

Ambient

1.4 GPa

2.8 GPa

5.6 GPa

7.7 GPa

8.3 GPa

100

60100

60

100

60

100

60

100

60

20100

60

20

-9.70 -4.85 0 4.85 9.70Velocity (mm/sec)

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FIGUre 2. Room-temperature Mössbauer pattern of the native iron sample from the mica schist of Chaibasa area, Singhbhum Craton, at various pressures—ambient (outside DAC), 1.4, 2.8, 5.6, 7.7, and 8.3 GPa (inside DAC). The velocity scale is relative to α-iron. The absence of usual intensity ratio related to random crystallites indicates polarized nature of the sample.

Table 2. Room-temperature Mössbauer parameters (isomer shift, quadrupole splitting, and magnetic hyperfine field) of the native iron sample from the Chaibasa shale, Singhbhum Craton, at various pressures up to 10 GPa

Experimental Mössbauer parametersPressure (GPa) Isomer shift Quadrupole splitting Hyperfine field±0.3 GPa (mm/s) (±0.01) (mm/s) (±0.1) (Tesla) (±0.1) Ambient –0.02 0.04 33.0 1.4 –0.02 0.05 33.0 2.8 –0.03 0.05 33.0 5.6 –0.03 0.05 32.9 6.3 –0.04 0.05 32.8 7.7 –0.04 0.03 32.8 8.3 –0.05 0.03 32.7 9.1 –0.06 0.03 32.7Note: IS is measured relative to α-Fe.

CHANDRA ET AL.: HIGH-PRESSURE STUDIES ON NATURALLy OCCURRING NATIvE IRON 873

However, isomer shift and quadrupole splitting of our native iron sample are found pressure invariant up to 5.6 GPa (Fig. 4). Above 6.3 GPa, a linear variation in the IS is observed and QS showed a sudden drop in the value from 0.049 to 0.020 mm/s suggesting a decrease in lattice distortion in contrast to the observation by Nicol and Jura (1963). Reichlin (1983) through discontinuity in the relative electrical resistance and Zou et al. (1981) through X-ray diffraction study of iron in DAC observed bcc-hcp transition at about 15 GPa in contrast to Pipkorn’s observation of ~13 GPa. Taylor et al. (1991) found the transfor-mation to be sluggish with its onset varying between 8.6 to 15 GPa supporting the electrical resistivity measurements by Wong et al. (1968) for a possible transformation at 8 GPa, far below the much-confirmed 13 GPa transition of pure iron. Wang and Ingalls (1998) have studied the X-ray absorption fine structure (EXAFS) of iron under high-pressure using boron carbide anvils with epoxy as pressure medium, and suggested that bcc-hcp transition mechanism involving an intermediate fcc phase during the shear induced bcc-hcp phase transition. However, a detailed molecular dynamics simulations study of the hcp/bcc nucleation and growth by adiabatic compression along the [001], [011], and [111] directions showed that the bcc-hcp phase transition occurs at 14 GPa and it is invariant at different crystal orientations (Wang et al. 2009). Our high-pressure electrical resistivity measurements with quasi-hydrostatic MgO medium on the native iron sample also showed a phase transition between

6 to 7 GPa (Fig. 5). The sluggishness in the pressure-induced transformation might arise due to shear conditions caused by the shocked nature of the studied sample as well as due to the nature of the solid pressure-transmitting medium. In the pres-ent Mössbauer studies, methanol-ethanol mixture was used as pressure-transmitting medium in DAC where the pressure can be assumed to be hydrostatic up to about 12 GPa (Downs et al. 1996). Despite the different pressure medium used, the transition pressure of the native iron is found to be same as in the electrical resistivity measurements, i.e., around 7 GPa (Fig. 5). Mao et al. (1967) used the rigid-sphere model to derive hcp structure from bcc structure by displacing and repositioning the atoms. Such relationship could occur only if the sample is an intimate mixture of bcc and hcp and the domains of the hcp in bcc sample might be little more than a stacking fault (Bassett and Huang 1987). Caspersen et al. (2004) studied the importance of shear on mar-tensitic transformation in iron from a theoretical viewpoint and showed through sequential lamination that the transformation pressure ranges from 11 to 25 GPa for shear 0.2–0.6. This may also include the different shear states observed through diffrac-tion studies in DAC (Rueff et al. 1999). They also observed that shear stress initiates the bcc-hcp transformation and stabilizes the hcp phase in iron. With increased shear stress, the transformation occurs at larger volumes and lower applied pressures via the formation of a complex microstructure comprising mixed phases. This departure from linear behavior of Mössbauer spectroscopic

FIGUre 3. (a) Room-temperature Mössbauer patterns of the native iron sample from the mica schist of Chaibasa area, Singhbhum Craton, at 9.1 GPa. Lower figure (b) shows the raw spectrum at the same pressure taken at velocity range (–2.0 to +2.0 mm/s) where only the inner two lines of the sextet corresponding to α-iron is visible. The velocity scale is relative to α-iron. The marker indicates the onset of bcc-hcp transformation.

FIGUre 4. The variation in the isomer shift (IS), quadrupole splitting (QS), and magnetic hyperfine field (HF) with pressure of the native iron sample from the mica schist of Chaibasa area, Singhbhum Craton. Isomer shift is measured with reference to α-Fe. The figure also shows error bars along x and y directions.

Channnels

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CHANDRA ET AL.: HIGH-PRESSURE STUDIES ON NATURALLy OCCURRING NATIvE IRON874

parameters, like isomer shift is quite distinct in the native iron sample studied here. Pressure invariant isomer shift indicates that the s-electron density remains unaffected by the applied pressure. In other words, it is plausible that during the shock event, the sample had attained a thermodynamic equilibrium of hcp stacks within the bcc structure. This state remained frozen in after the impact. Upon application of pressure, the hcp stacks within the bcc structure, served as nuclei for the bcc → hcp phase transition, thus initiating the pressure driven phase transition at much lower pressure than observed in unshocked material. Similar arguments could be applied to interpret the high-pressure behavior of QS and the magnetic field.

In summary, the present study on the Mössbauer measurement on the native iron sample from Chaibasa Mica Schist confirms the existence of Fe0 oxidation state of iron in nature. Pressure invari-ance of the Mössbauer parameters up to 5 GPa and observation of bcc-hcp phase transition at a lower pressure than previously reported are the result outcome of the high-pressure Mössbauer spectroscopic studies conducted on the sample. It is proposed that the stacking faults comprising of hcp stacks on bcc structure might have been introduced by shock loading and contained in nature for over 1.6 billion years. The decrease in the transition pressure might be because of the presence of inherent deviatoric stress caused by the hcp stacking faults.

aCknowledGmentsWe are grateful to Bill Bassett, M.D. Dyar, Martin Kunz, and anonymous

reviewers for their painstaking corrections and very useful as well as construc-tive suggestions, in improving the manuscript. We thank Christian Koeberl, T.M. Mahadevan, D. Mukhopadhyay, B.P. Radhakrishna, and W.U. Reimold for many useful discussions and suggestions. We thank PLANEX program of ISRO, De-partment of Space, Government of India, CSIR and DST Government of India, New Delhi for funding.

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Manuscript received deceMber 16, 2009Manuscript accepted January 28, 2010Manuscript handled by M. darby dyar