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Chemie der Erde
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-ray diffraction, Mössbauer spectroscopic and electrical resistivity studies onohawat meteorite under high-pressure up to 9 GPa
sha Chandraa,∗, G. Parthasarathyb, N.V. Chandra Shekarc, P.Ch. Sahuc
Department of Physics, University of Rajasthan, Jaipur 302004, IndiaCSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500606, IndiaCondensed Matter Physics Division, Material Science Group, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam 603102, India
r t i c l e i n f o
rticle history:eceived 6 October 2012ccepted 16 January 2013
The physical properties of the extraterrestrial materials, meteorites, though important but are not wellstudied. We present here structural and electrical properties of Lohawat meteorite under high-pressureup to 9 GPa, using in situ high-pressure powder X-ray diffraction, 57Fe Mössbauer spectroscopic andelectrical conductivity techniques. The studied meteorite sample fell at Lohawat (Rajasthan) India isclassified as a Howardite based on geochemical and mineralogical studies. Electron Probe Microanalysis(EPMA) indicates that Lohawat meteorite composed mainly of orthopyroxene (Fs0.65En0.34Wo0.09) andplagioclase (An.946Ab.053). All the three experimental studies independently confirm pressure-inducedstructural changes of Lohawat meteorite at ∼2.8 GPa and at 5.6 GPa representing irreversible amorphiza-tion and reversible crystallization respectively. The observed experimental results are explained based
on the high-pressure behavior of orthopyroxene and anorthite, which are dominant mineral phases ofthe Lohawat meteorite. The observation of structural phase transition at lower pressure for the mete-orite sample compared to their analogous pure end member mineral indicates that the meteorite samplepreserved a residual stress in the sample. The present study, therefore, may help in estimating the resid-ual peak metamorphic shock pressure experienced by the parent body transforming into differentiatedachondrite meteorite.
. Introduction
Meteorites provide the information about the bulk compositionf a large differentiated asteroids or planets, they are originatingrom. As rocky planets grow by accreting surrounding materials,hey suffer increasing violent collisions. During the impacts, somehases in minerals like plagioclase and pyroxenes get thermo-ynamically re-equilibrated at different pressures – the pressureenerated depending on the size of the parent body and the speedf the impactor. The process of crystallization or amorphizationrovides some information of the impact processes on the parentody. High-pressure laboratory experiments, therefore, would beseful in measuring the pressure of transitions. Though the phys-
cal properties are very important in understanding the origin of
mpact processes in meteorites, only few reports on such measure-
ents on the meteorites are available (Parthasarathy and Sharma,004). To the best of our knowledge there were no previous reports
∗ Corresponding author at: High-Pressure Physics Laboratory, Department ofhysics, University of Rajasthan, Jaipur 302004, India. Tel.: +91 141 2707423.
on the high-pressure investigations on meteorite sample. Here wereport a case study on the Lohawat Howardite meteorite which isproduced by impact of howardite–eucrite–diogenite “HED” parentbodies.
Lohawat meteorite fall which took place about 5 km east ofLohawat village in Jodhpur district (Rajasthan), India (26◦57′56′′N;72◦37′36′′E) in 1994 is classified as a Howardite (Chattopadhyay etal., 1998). The high exposure age (∼107 Ma) of sample and absenceof solar gases generally found in howardite sample categorized itsorigin from a parent body other than asteroid 4 Vesta (Mahajanet al., 2000). Thin section and petrographic studies showed mainmineral constituents of the meteorite to be colorless to grayishpyroxene and subhedral to anhedral plagioclase with some amountof anhedral olivine embedded in fine to medium-grained matrixwithout much of the intergrowth relations (Chattopadhyay et al.,1998; Singh et al., 1998; Sisodia et al., 2001). �-Ray spectrome-try measuring the quantity of cosmogenic radionuclides 26Al and22Na in 47.93 g of meteorite sample indicated 22Na 20% lower thanthe average value implying that no fragmentation has occurred
in interplanetary space. Considering period of the meteorite fallto be prior to the minima of solar activity before the begin-ning of solar cycle 23, it is speculated that the system has beenexposed to highly modulate galactic cosmic ray fluxes for several
SiO2 44.52 Si 8.225Al2O3 35.35 Al 7.697FeO 0.42 Fe(ii) 0.112CaO 19.12 Ca 3.784Na2O 0.60 Na 0.215
Total 100.01 Total 20.034
2.4. Electrical conductivity measurements
The high-pressure electrical conductivity studies were carried out in an opposedBridgman anvil cell system consisting of tungsten carbide anvils with the tip diame-ter of 6 mm. The sample is embedded in pyrophyllite gaskets with MgO as pressure
P2O5 0.03 0.05 0.05
Total 98.51 98.41 98.79
ears (Mahajan et al., 2000; Sisodia et al., 2001). Here we presentressure-induced experimental results on the Lohawat meteoritesing X-ray diffraction, 57Fe Mössbauer spectroscopy and electri-al conductivity techniques up to 9 GPa which might be useful instablishing the importance of high pressure studies as an indicatoro assess the new thermodynamical equilibrium after the impactSharma and Chandra, 2011).
. Experimental
.1. Compositional analyses
The compositional analysis on the whole rock performed using Philip XRF ana-ytical system and Electron Probe Microanalysis (EPMA) system were found to be inood agreement with the reported values (Sisodia et al., 2001; Chattopadhyay et al.,998), within the error limits (Table 1a). The experimental details of the EPMA anal-ses were discussed elsewhere (Parthasarathy et al., 2003). The total iron content iseasured as FeO. The sample used for high pressure studies is a mixture of plagio-
lase feldspar and ortho pyroxene separated from the pulverized matrix. The detailsf FT-infrared spectroscopic measurements using KBr pellet method was discussedlsewhere (Parthasarathy et al., 2002).
.2. X-ray diffraction measurements
In situ high-pressure X-ray diffraction experiment was carried out using a Mao-ell type diamond anvil cell (DAC) in an angle dispersive mode. The powderedample was loaded into a 200 �m diameter hole drilled in pre-indented stainlessteel (SS) gasket along with the pressure calibrant material. To avoid SS gasket peak,n internal collimator of dimension ∼100 �m was used. For pressure measurements,he calibration material was chosen to be soft enough to detect the changes in pres-ure within the limit of our experimental errors and whose diffraction lines do notnterfere with those of the sample. In the present experiment silver was the chosenressure calibrant and a mixture of methanol, ethanol and water in the volume ratio6:3:1 was used as a pressure transmitting medium.
The monochromatized Mo X-ray beam from a ULTRAX (18 kW) rotating anode-ray generator using graphite monochromator (Mo K�1 radiation with wavelength.7107 A) was focused on an image plate based mardtb345 diffractometer. The over-ll resolution of the system is �d/d ∼ 0.001. The Mao-Bell type DAC was fitted to theiffractometer and the sample to detector distance was calibrated using a standardpecimen (LaB6). The patterns were collected with an exposure time of ∼2 h at var-ous pressures up to 6.5 GPa to get a good S/N ratio (Chandra Shekhar et al., 2011;ayana et al., 2010). Integration of the intensities was done from the image usingIT2D software (Hammersley, 1997).
.3. 57Fe Mössbauer spectroscopic measurements
For ambient measurements, absorber was prepared by spreading the finelyowdered sample (powdered under acetone environment) uniformly over an areaf 1 cm2 between two thin plastic sheets. The 57Fe Mössbauer set up consists of
10 mCi Co57(Rh) point source (active area of 0.5 mm × 0.5 mm) mounted on aissel transducer operating in a constant acceleration mode, a Si-PIN solid state
etector (Amptek XR-100CR with a resolution 250 eV) and CMCA-550 data acqui-ition module (Wissel Make) for data collection. The velocity scale was calibratedith reference to metallic-iron. To conduct high-pressure measurements, a Merrill-
assett type diamond anvil cell (DAC) was used (Chandra et al., 2005; Chandra,007). The DAC sample assembly was prepared in a tantalum gasket hole of diam-ter ∼200 �m using 4:1 methanol:ethanol mixture as pressure-transmitting fluidnd a few tiny ruby crystals as pressure calibrant. The pressure on the sample wasetermined using the shift in the R1 and R2 lines of ruby. Mössbauer patterns were
An 94.63Ab 5.37Or 0.00
recorded in steps of 1.4 GPa for long exposure time (more than two weeks each)to achieve an appreciable signal to noise ratio. The required Mössbauer parame-ters were determined using Mössbauer analysis program of Jernberg and Sundquist(1990).
Fig. 1. X-ray diffraction patterns of the Lohawat meteorite at (a) room pressure, (b)3.1 GPa, (c) 6.3 GPa and (d) after decompression to room pressure. The intense peaksat 17◦ and 20◦ are corresponding to Ag(1 1 1) and Ag(2 0 0) planes used for pressurecalibration.
U. Chandra et al. / Chemie der Erde 73 (2013) 197– 203 199
F fter dR sing D
tpwe6ur0r
3
s
ig. 2. (a) Mössbauer patterns of Lohawat meteorite at ambient condition and (b) aoom temperature Mössbauer spectra of Lohawat meteorite at various pressures u
ransmitting medium. For pressure calibration, high-purity bismuth exhibiting threeressure induced solid–solid phase transitions at 2.5, 2.7 and 7.4 GPa respectivelyas used (Parthasarathy, 2006). For four-probe set up high purity copper wires as
lectrical probes, Keithley source meter (model No. 263), electrometer (model No.14), digital multimeter (model No. 199) were used (Parthasarathy, 2011). Typicalncertainty involved is 0.2 GPa in the pressure region of 7 GPa. Two independentuns were done on two different sets of samples. Due to non-ohmic contact up to.5 GPa, the electrical conductivity values at high-pressures were normalized withespect to 0.5 GPa.
. Results and discussion
Earlier X-ray diffraction studies on powdered bulk meteoritehowed the peaks of anorthite, clinopyroxenes and chromite
ecompression to ambient condition. The velocity scale is relative to metallic Fe. (c)AC – 1.4, 2.8, 4.2, 5.6, 7.0 and 8.4 GPa. The velocity scale is relative to metallic Fe.
and little amount of olivine with the composition Forsterite(65.47 mol%), Fayalite (34.06 mol%) and Tephroite (0.47 mol%)(Sisodia et al., 2001). The presence of olivine without any otherhydrous mineral indicated the sample to be a fresh one as com-pared to Ararki meteorite (found in Thar Desert) where alteredminerals like serpentines in the olivine were found (Bhandari et al.,2008). Electron Probe Microanalysis (EPMA) on the sample usedfor high pressure studies showed orthopyroxene having formula
En0.34Fs0.65Wo0.09 and the plagioclase feldspar having formulaAn0.94Ab0.05 (Table 1b). The room temperature FTIR spectrum ofthe Lohawat sample supported presence of anorthite by showingmajor absorption lines at 980 cm−1 and peaks around 618, 658,
200 U. Chandra et al. / Chemie der Erde 73 (2013) 197– 203
F to metallic Fe and (b) quadrupole splittings (QS) of Lohawat meteorite showing variationsa pressures pressure. The inset clearly indicates the sign change in isomer shift from +ve to−
7o(cdmcrt
p2toatfTaocp
as
ig. 3. Room temperature Mössbauer parameters (a) isomer shift (IS) with referencet M1 and M2 sites at various pressures. (c) The variation of IS of M1 site at various
ve up to 5.6 GPa again increasing back to +ve value at higher pressures.
00, 725, and 770 cm−1 (Iishi et al., 1971). Very low concentrationf chromium (0.05 wt%) indicated the sample to be chromite-freeTable 1b). Figs. 1–3 show the X-ray diffraction measurementsarried out at ambient pressure, 3.1 GPa and 6.3 GPa and afterecompression to room pressure (Fig. 1), Mössbauer spectroscopiceasurements recorded up to 8.4 GPa (Fig. 2a–c) and electrical
onductivity measurements (Fig. 4) done on Lohawat Meteoriteespectively. Identical changes in the behavior were observed athe same range of pressures in all the three experiments.
High-pressure X-ray diffraction: The ambient pattern showrominent peaks with the d-values 4.042 A, 3.197 A, 3.005 A,.830 A, 2.573 A and 2.493 A corresponding to mixture of anor-hite and ortho and clinopyroxene (Table 2). At 3.1 GPa, majorityf the peaks vanish except broad peaks with ‘d’ values 3.163 And 2.867 A indicating order–disorder transition. At 6.3 GPa, crys-allization corresponding to crystalline orthorhombic Magnesiumerrosilite (JCPDS card 19-0607) indicated by sharp peaks is seen.hus two phase transformations corresponding to order–disordernd crystallization around 3.1 GPa and 6.3 GPa respectively arebserved. Pattern of the decompressed sample to ambientondition resembles the pattern of 3.1 GPa with intense but broad
eaks at d values 3.13 and 2.86 A.
Earlier studies on the pressure-induced structural behavior ofnorthite (CaAl2Si2O8) having triclinic structure by Angel (1992)howed a pressure-induced first order displacive phase transition
Fig. 4. Room temperature normalized electrical conductivity (with reference toconductivity at 0.5 GPa) of Lohawat meteorite at various pressures. Two samplesmeasured under various pressures are represented by � and �, and show identicalresult.
U. Chandra et al. / Chemie der Erde 73 (2013) 197– 203 201
Table 2X-ray diffraction analysis at various pressures on Lohawat meteorite sample.
‘d’ values at ambient pressure (A)with mineral composition
‘d’ values at pressure 3.1 GPa (A) ‘d’ values at pressure 6.3 GPa (A) ‘d’ values on decompression (A)
4.691 4.596 Fs4.042 An+Opx 4.032 Fs
3.9403.7073.565
3.197 An+Cpx+Opx 3.163 (broad) 3.175 Fs 3.13 (broad)3.005 Cpx2.952 An
n, anorthite; Cpx, clinopyroxene (ICDD 01-089-5683); Opx, orthopyroxene (JCPDS
rom low pressure PI to high pressure II phase between 2.55 and.74 GPa with large discontinuities in the unit cell angles and largeecrease in unit volume. Energy dispersive X-ray diffraction stud-
es on anorthite under static pressures using a diamond anvil cellhowed onset of amorphization between 10 and 14 GPa followedy complete irreversible amorphization about 20 GPa (Redfern,996). Thus the presence of anorthite in our meteorite sampleust be showing irreversible amorphization at pressure ∼3.1 GPa.
edistribution of Fe seems to occur with pressure thereby show-ng the peaks corresponding to Fe-containing crystalline ferrosilitet 6.3 GPa. Stability of orthopyroxene is another interesting fea-ure seen at higher pressures. Zhang et al. (2012) while studyingatural Fe-bearing orthoenstatite using single crystal X-ray diffrac-ion confirmed Pbca structure up to 12.66 GPa, new diffractioneaks appeared only at 14.26 GPa. Wang et al. (2012) observ-
ng an order–disorder transition in C60 fullerene (maintainingheir translational symmetry) at ∼35 GPa implied that a mate-ial can still retain long range order though their fundamentaluilding blocks are disordered. However naturally occurring highressure fullerene C60 in the inter-trappean sediments at the
retaceous–Tertiary boundary of the Deccan Trap showed theesidual pressure of 4.0 GPa (Parthasarathy et al., 2008) which couldnly be estimated using a high resolution XRD pattern from theynchrotron source.
able 3oom temperature Mössbauer parameters of the Lohawat (Howardite) meteorite at vario
Pressure (GPa) Isomer shift wrt Fe (mm/s)±0.02
Qua±0.0
0 1.15 2.041.17 2.46
1.4 1.13 2.151.15 2.42
2.8 1.16 2.26−0.04 0.19
4.2 1.16 2.26−0.04 0
5.6 1.14 2.24−0.14 0
7.0 1.13 2.31−0.03 0
8.4 1.08 2.410.05 0
Decompressed to ambient condition 1.11 1.971.25 2.23
−0.08 0
45); Fs, magnesia ferrosilite (JCPDS 19-0607).
High-pressure Mössbauer measurements: Fig. 2a and c shows theMössbauer patterns at ambient pressure and various pressuresinside diamond anvil cell respectively. Mössbauer spectroscopyis a sensitive probe for quantitative analysis of oxidation stateand local environment of the Mössbauer nuclei (Fe57). Mössbauerparameters-isomer shift (IS) and quadrupole splitting (QS), bothare very sensitive to pressure (Fig. 3). Isomer shift is determinedby the s-electron density at the Fe-nuclei primarily influenced byvalence of the Fe cation and the octahedron volume. Different elec-tronic wave functions at the Fe nuclei through isomer shift yieldsinformation about the nature of the chemical bonding. On the otherhand, quadrupole splitting (QS) is strongly related to the non-cubicdistortion in the Fe2+-containing octahedral sites. There are twocontributions toward QS – first, the valence term that comes fromthe spatial extensions of the valence electron clouds and the sec-ond, lattice term influenced by the nearby ions in the crystal lattice.Both the contributions act opposite to each other (Ingalls, 1964;McCammon and Tennant, 1996). The IS of the sample at ambi-ent condition indicate the valence of the Fe cation to be divalent.Hence the ambient Mössbauer pattern was analyzed for Fe2+ ions
with two quadrupole doublets corresponding to two inequivalentsites of Fe2+ of pyroxenes (Tripathi et al., 2000) (Table 3). Anorthiteand olivine do not contribute toward the technique due to negli-gible Fe content. Ortho-pyroxenes and clinopyroxenes have same
us pressures in diamond anvil cell.
drupole splitting (mm/s)2
Line width (mm/s)±0.02
Population area (%)±2%
0.43 77.0 0.44 23.0
0.41 82.3 0.69 17.7
0.54 81.2 0.53 18.8
0.50 86.50.60 13.5
0.58 79.20.60 20.8
0.59 79.80.53 20.2
0.60 81.40.40 18.6
0.37 43.8 0.38 35.3
0.39 20.9
2 ie der E
r(MM1idacarofiadcpitilpstd
proa(msocefe1topfXpKebmnps
4
Lhaass∼oi
02 U. Chandra et al. / Chem
ange of Mössbauer parameters imparting difficulty in distinctionTripathi et al., 2000). 77% of Fe2+ at a more distorted octahedral
2 site and 23% of Fe2+ is found at the almost regular octahedral1 site respectively (Eeckhout et al., 2001; Dowty and Lindsley,
973; Lin et al., 1993). Usual behavior of QS observed in pyroxeness seen for M2 site while the parameter at M1 site show entirelyifferent trend with the increase in pressure (Table 3 and Fig. 3and b). At 2.8 GPa, IS and QS (M1 site) show sudden decrease, IShanging from initial value of +1.15 mm/s to −0.04 mm/s, reaching
minimum of −0.14 mm/s at 5.6 GPa. Further increase in pressureesults into a reversal in the sign of IS (+0.05 mm/s at 8.4 GPa, insetf Fig. 3c). As isomer shift is very sensitive of the electronic con-gurations at the Fe nuclei, both these events could be related tomorphization and crystallization respectively seen earlier. Sud-en change in the isomer shift at 2.8 GPa might be due to suddenhange in environment of Fe nuclei related to the onset of amor-hization, the process seems to continue till 5.6 GPa and on further
ncreasing the pressure, reversal of sign indicates prominent par-icipation of crystallization of pyroxene. Decompression patternndicates the presence of residual amorphous phase – a singlet atower isomer shift of −0.08 mm/s (Fig. 2b and Table 3) along witharameters corresponding to pyroxene. Zhang et al. (2011) whiletudying local structure variations in ortho-enstatite revealed thathe phase transformation strongly depends on the local stress con-itions at the iron sites provided by the different pressure media.
High-pressure electrical conductivity measurements: Theressure-induced variation of normalized conductivity (witheference to conductivity at 0.5 GPa) on the sample supportsur other results revealing a change in the slope of the curvet ∼1.4 GPa and a sudden drop in the conductivity at ∼5.6 GPaFig. 4). Considering above results, the change in slope at ∼1.4 GPa
ight represent the onset of amorphization. Similar change inlope has also been observed in vacancy-doped (La,Sr) perovskitexides at low pressure (Kumar et al., 2006). A sudden drop inonductivity at 5.6 GPa in Lohawat meteorite (a mixture of differ-ntiated pyroxenes) might be due to realignment of atoms to formerrosilite phase. Xu and Shankland (1999) observed for pyrox-nes a decrease in conductivity of about 0.7 log units at pressure3 GPa and temperature between 1000 and 1400 ◦C indicating theransformation from ortho to clinopyroxene. In our earlier studiesn native iron from an impact site showed that the transitionressure required for the bcc–hcp transition decreased by ∼4 GPaor the impacted sample (Chandra et al., 2010) though ambient-ray diffraction characteristics did not show any changes. Theresence of high-pressure minerals like stishovite (Gilmour andoeberl, 2000), high-pressure phase of fullerene (Parthasarathyt al., 2008), diamond (Gilmour and Koeberl, 2000) have alsoeen reported in the impact metamorphic terrain. The Lohawateteorite being the achondrite, the inherent impact metamorphic
ature of the sample is responsible for the residual stress. Our highressure experiments provide a unique opportunity to estimateuch residual stress in small sample like meteorite.
. Conclusion
Here we present the comprehensive report on the howarditeohawat meteorite composed of pyroxene and plagioclase underigh pressure using X-ray diffraction, Mössbauer spectroscopynd electrical conductivity techniques. While X-ray diffractionnd electrical resistivity probe bulk properties and Mössbauerpectroscopy is a local technique, all the three measurements
how identical behavior. Two phase transitions at ∼2 GPa and5.6 GPa are observed in all the three experiments. The presencef anorthite in the meteorite seems to be mainly responsible forrreversible amorphization, pyroxene retaining the orthorhombic
rde 73 (2013) 197– 203
structure. Our studies demonstrate that high-pressure is a usefulmethodology in estimating the peak shock metamorphic pressureexperienced by impact ejection of meteorite from the parent body.Future studies on Eucrites (mainly consisting of plagioclase) mightimprove our understanding of the behavior. Such quantitative stud-ies can be useful in relation to shock grade classification.
Acknowledgements
We thank the anonymous reviewers for their critical commentsand useful suggestions. We are grateful to Council of Scientificand Industrial Research (CSIR), PLANEX program of Indian SpaceResearch Organization, Department of Space (Government of India)for funding, Prof. R.P. Tripathi, Jai Narayan Vyas University, Jodh-pur and Professor N. Bhandari of Physical Research Laboratory,Ahmedabad for providing the Lohawat meteorite sample. UCacknowledges CSIR for providing the Emeritus scientist Fellowship.We specially thank Prof. N. Bhandari for his motivation in this fieldof meteoritic science and acknowledge his valuable suggestions andcritical analysis.
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