Chemical Analysis of Iron Meteorites Using a Hand-Held X-Ray Fluorescence Spectrometer

Authors


Abstract

We evaluate the performance of a hand-held XRF (HHXRF) spectrometer for the bulk analysis of iron meteorites. Analytical precision and accuracy were tested on metal alloy certified reference materials and iron meteorites of known chemical composition. With minimal sample preparation (i.e., flat or roughly polished surfaces) HHXRF allowed the precise and accurate determination of most elements heavier than Mg, with concentrations > 0.01% m/m in metal alloy CRMs, and of major elements Fe and Ni and minor elements Co, P and S (generally ranging from 0.1 to 1% m/m) in iron meteorites. In addition, multiple HHXRF spot analyses could be used to determine the bulk chemical composition of iron meteorites, which are often characterised by sulfide and phosphide accessory minerals. In particular, it was possible to estimate the P and S bulk contents, which are of critical importance for the petrogenesis and evolution of Fe-Ni-rich liquids and iron meteorites. This study thus validates HHXRF as a valuable tool for use in meteoritics, allowing the rapid, non-destructive (a) identification of the extraterrestrial origin of metallic objects (i.e., archaeological artefacts); (b) preliminary chemical classification of iron meteorites; (c) identification of mislabelled/unlabelled specimens in museums and private collections and (d) bulk analysis of iron meteorites.

Abstract

Nous évaluons la performance d'un spectromètre portable XRF (HHXRF) pour l'analyse chimique globale de météorites de fer. La précision analytique et l'exactitude ont été testées sur des alliages de métaux de référence certifiés et des météorites de fer de composition chimique connue. Avec une préparation minimale de l'échantillon (c'est à dire avec des surfaces plates ou plus ou moins polies) l'HHXRF a permis la détermination précise et exacte de la plupart des éléments plus lourds que le Mg, et présentant des concentrations supérieures à 0,01% m/m dans les alliages de métaux de référence certifiés, et des éléments majeurs Fe et Ni et mineurs Co, P et S (allant généralement de 0,1 à 1% m/m) dans les météorites de fer. En outre, des analyses ponctuelles multiples HHXRF peuvent être utilisées pour déterminer la composition chimique globale de météorites de fer, qui sont souvent caractérisées par la présence de minéraux accessoires de type sulfures et phosphures. En particulier, il a été possible d'estimer les teneurs en P et S, qui sont d'une importance cruciale pour la compréhension de la pétrogenèse et de l'évolution des liquides riches en Fe-Ni et des météorites de fer. Cette étude valide donc l'HHXRF comme un outil précieux pour l'étude des météorites, permettant une rapides et non-destructive (a) identification rapide de l'origine extraterrestre des objets métalliques (versus les artefacts archéologiques); (b) classification chimique préliminaire des météorites de fer; (c) identification des spécimens mal étiquetés/non identifiés dans les musées et les collections privées et (d) analyse globale de météorites de fer.

Since the first archaeological and geological applications in the 1960s (Shackley 2011), X-ray fluorescence spectrometry (XRF) has become one of the most commonly used analytical techniques for determining the chemical composition of a variety of materials. The recently developed hand-held XRF units (HHXRF) have been used for numerous applications in the field. These applications include the determination of metals in soils (Kalnicky and Singhvi 2001, Radu and Diamond 2009) and sediments (Kirtay et al. 1998, Kenna et al. 2011), analysis of artefacts and artworks (Liritzis and Zacharias 2011, Vázquez et al. 2012), quality tests in the metallurgical industry and engineering, and the identification and classification of hazardous wastes (Vanhoof et al. 2013). The reasons for the significant success of HHXRF (Potts and West 2008) include (a) portability of the instrument, (b) the easy handling of the operating system, (c) minimal sample preparation (d) rapid, non-destructive field analyses with remarkable reproducibility and low detection limits for elements heavier than Mg.

The XRF technique has been widely used for the bulk chemical analysis of meteorites since the late 1960s and early 1970s (i.e., Reed 1972). More recently, HHXRF was used for the first time to identify and classify different groups of stony meteorites, and to quantify their terrestrial elemental contamination (Zurfluh et al. 2011). In this work, we tested a commercial HHXRF instrument for its suitability in the bulk chemical analyses of iron meteorites, encouraged by the fact that HHXRF was designed mainly for the metallurgical and mining industry, especially for the analysis of metal alloys.

Iron meteorites are made of Fe-Ni metal alloys of asteroidal origin containing minor amounts of Co, P and S and trace amounts of siderophile (Ga, Ge, Ir, Au, Pt, Pd, Mo, W, Rh, Ru) and chalcophile (Cu, Zn, As, Ag) elements in highly variable concentrations (differing by up to five orders of magnitude). The Ni content varies from ~ 4 to 60% m/m, although it most commonly ranges from 5 to 12% m/m. The chemical classification and petrogenesis of iron meteorites is based on siderophile trace element concentrations (i.e., Ir, Ge, Ga, Au) (e.g., Goldstein et al. 2009). Due to their low abundances (typically of the order of 10−4 to 103 μg g−1), their concentrations are determined by means of sensitive analytical methods such as INAA and radiochemical (RNAA) neutron-activation analysis (Wasson et al. 1989) or ICP-MS (D'Orazio and Folco 2003). In the following sections, we illustrate the analytical precision and accuracy of a NITON XL3t GOLDD+ hand-held spectrometer in the analyses of a representative set of iron meteorites. We also discuss the advantages and limitations of using this rapid, non-destructive and practical analytical method in meteoritics, namely in the identification, classification and geochemical analysis of iron meteorites.

Method and samples

The instrument

The instrument used in this study was a NITON XL3t GOLDD+ XRF spectrometer. The instrument was equipped with a miniaturised tube with an Ag anode (50 kV, 200 μA, 2 W) and fitted with an SDD detector capable of acquiring spectra at high count rates. Accordingly, the instrument was equipped with an X-ray tube capable of operating at higher outputs compared with instruments fitted with a Si(PIN) detector. Different measuring modes were available: ‘Soil’, ‘Mining’ and ‘General Metals’. We exclusively used the ‘General Metals’ mode for this study because this was more suitable for the sample types studied. This procedure allowed the simultaneous detection of over eighteen elements (see Table 1), including those of interest in the analysis of iron meteorites (Fe, Ni, Co, P, S, Cr, Cu, W and Mn). In this mode, the instrument operated in different conditions in order to optimise analysis: ‘main’ (excitation 50 kV, 40 μA), ‘low’ (15 kV, 133 μA) and ‘light’ (8 kV, 200 μA). Limit of detection (LOD) for each analyte was calculated as three times the standard deviation of the concentration measured in samples with none or only a trace amount of the analyte.

Table 1. NITON XL3t GOLDD+ instrument operating conditions
ModeGeneral metals
Main (50 kV, 40 μA) – filter material: AlFe
Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Sn, Sb, W
Low (15 kV, 133 μA) – filter material: Fe
Ti, V, Cr
Light (8 kV, 200 μA) – no filter
Al, Si, P, S
Counting timesMain – 60 s
Low – 60 s
Light – 60 s
Total counting time: 180 s
Spot8 mm

The list of elements measured and operative conditions are shown in Table 1. The counting times for the three different operative modes was 60 s each, making a total acquisition time of 180 s for a single analysis. The on-board software for the XL3t used a ‘Fundamental Parameters’ correction algorithm that involves iterative corrections to the measured X-ray counts on the basis of the approximated compositions, accounting for differences in X-ray emission, absorption, secondary fluorescence and other phenomena. The analyses were performed with the device mounted on a stand with a shielded box protecting the user from radiation. Samples were positioned accurately in the analytical plane of the XRF instrument, and no additional corrections for air gap were required. The beam diameter of this specific instrument was ~ 8 mm, but it could be reduced to 3 mm using a built-in spot collimator. The spectra of the measurements were transferred to a computer using Niton Data Transfer 8.0.1 software (Thermo Scientific, Waltham, MA, USA).

Reference samples

In order to define the optimal analytical conditions and verify the quality of the analytical procedure on metals, we selected a set of steel CRMs, which were analysed in each analytical session. The selected CRMs were iron-nickel alloys with compositions similar to iron meteorites to match matrix effects. They were in the form of thin cylinders with flat basal surfaces that are physically similar to the flat surfaces of the analysed iron meteorites. They included the certified NIST reference steels SRM 1262b and SRM 1158, and the Analytical Reference Materials International (ARMI) steels 35JN and AISI 303 (Table 2).

Table 2. List of analysed iron meteorites and CRMs
SampleChemical classificationStructural classificationReference
  1. Ogg, coarsest octahedrite; Og, coarse octahedrite; Of, fine octahedrite; H, hexahedrite; D, ataxite; D-an, anomalous ataxite; ˜, referred to Seymcham metal fraction.

Campo del CieloIAB-Main GroupOgWasson and Kallemeyen (2002)
Canyon DiabloIAB-Main GroupOgWasson and Kallemeyen (2002)
ChingaUngroupedDBuchner et al. (2012)
CoahuilaIIABHWasson et al. (2007)
DroninoUngroupedDRussell et al. (2004)
Gebel KamilUngroupedDD'Orazio et al. (2011)
GibeonIVAOfWasson and Richardson (2001)
HobaIVBDWalker et al. (2008)
MuonionalustaIVAOfWasson and Richardson (2001)
North ChileIIABHWasson et al. (2007)
NWA 6583UngroupedD-anFazio et al. (2013)
Santiago PapasquieroUngroupedHBuchwald (1975)
SeymchanPallasite-Main GroupOg˜van Niekerk et al. (2007)
Sikhote-AlinIIABOggWasson et al. (2007)
TishomingoUngroupedDBirch et al. (2001)
CRMs    
ARMI AISI 303Austenitic stainless steel  www.armi.com
NIST SRM 1158High-Ni steel  www.nist.gov
NIST SRM 1262bSteel  www.nist.gov
ARMI 35JNSteel  www.armi.com

Iron meteorite samples

Iron meteorites are made of Fe-Ni metal phases (mostly kamacite and taenite, secondarily tetrataenite, martensite, awaruite) plus accessory sulfides (i.e., troilite, daubreelite), phosphides (i.e., schreibersite), nitrides (i.e., carlsbergite), carbides (i.e., cohenite), oxides (i.e., chromite) and phosphates (i.e., farringtonite), and sometimes by substantial amounts of silicate inclusions (Mittlefehldt et al. 1998). More than about 99.5% m/m of the metallic portion of iron meteorites consists of Fe, Ni and Co, while the remaining mass is made of siderophile and chalcophile trace elements showing a highly variable relative distribution (up to over a factor of 105). Structurally, iron meteorites are classified in octahedrites, ataxites and hexahedrites. Octahedrites consist of kamacite lamellae oriented along octahedral planes separated by Ni-rich lamellae composed of several phases. This structure, particularly evident on polished and etched surfaces, is known as the Widmanstätten pattern (Figure 1). Octahedrites are further subdivided according to the width of the kamacite lamellae, from coarsest (> 3.3 mm) to finest (< 0.2 mm). Ataxites show only microscopic spindles of kamacite. Hexahedrites consist almost entirely of kamacite, with their name referring to the cleavage of this mineral phase. While the structural subdivision is purely descriptive, a genetically more significant classification is based on the Ni and trace element content of the metal phase, particularly Ga, Ge and Ir. The concentration of Ni, the second most abundant element in iron meteorites after Fe, must be known in order to interpret the structure of iron meteorites based on the subsolidus portion of the Fe-Ni phase diagram (Yang and Goldstein 2005). At present, thirteen chemical groups (IAB, IC, IIAB, IIC, IID, IIE, IIF, IIG, IIIAB, IIIE, IIIF, IVA, IVB) have been distinguished (Wasson et al. 1998 and references therein), with the Roman numerals I to IV indicating decreasing contents of Ga and Ge. Each group is composed of at least five distinct meteorites. Iron meteorites that do not fall in any of these chemical groups (about 16%) are called ‘ungrouped’, whereas irons in which concentrations of only one or two elements fall outside the typical range of a specific group are called ‘anomalous’. The study of the structure, chemistry and isotopic composition of iron meteorites is fundamental for understanding the process of planetary differentiation (including that of the proto-Earth) and the chemical evolution of the Solar System (i.e., Goldstein et al. 2009).

Figure 1.

Stereomicroscopic images of polished and etched surfaces of six of the fifteen iron meteorites analysed by HHXRF in this work. All images were taken at the same magnification to better show relative heterogeneity in terms of texture and mineral composition. (a) Campo del Cielo; coarse octahedrite; (b) Canyon Diablo; coarse octahedrite; (c) Seymchan, metal; coarse octahedrite; (d) Muonionalusta; fine octahedrite; (e) Gebel Kamil; ataxite; accessory mineral crystals (arrowed) consist of schreibersite, troilite and daubreelite; (f) Chinga; ataxite.

Hand-held XRF analyses were conducted on a set of fifteen iron meteorites and the metal fraction of a Main Group Pallasite of well-known chemical composition (Table 2). We selected samples with a good compositional variability in order to be representative of the different chemical and structural classes, that is, from coarsest octahedrites to ataxites with Ni contents ranging from ~ 5 to 32% m/m (see Table 4).

Bulk chemical composition analyses were carried out on interior ground surfaces (600 mesh) of meteorite slabs (Figure 1) or end cuts. This minimal specimen preparation, which is the customary approach used by researchers or dealers to start characterising new iron meteorites, is enough for quantitative X-ray analyses to minimise inconsistencies caused by small variations in the surface-to-instrument distance and random unaccounted absorption due to the roughness of the surface.

To avoid surface contamination, all samples were washed in an ultrasonic bath with acetone and then allowed to dry prior to analyses. Care was taken to analyse surfaces devoid of accessory minerals visible to the naked eye, to obtain the actual metal phase composition, which is the composition used for the chemical classification of iron meteorites. The number of spot analyses on each iron meteorite increased with increasing mineralogical heterogeneity of the specimen in order to better approximate the representativeness of the analyses. For instance, the number of spot analyses was typically < 10 for homogeneous samples such as some ataxites (i.e., Figure 1f), and 10–20 for samples showing some heterogeneity of the metal phase at the scale of the analysed surfaces, such as the coarse octahedrites (Figure 1b, c). Gridded spot analyses of Gebel Kamil were conducted to assess the capability of HHXRF in determining the bulk meteorite composition (i.e., metal plus accessory minerals) of heterogeneous irons characterised by scattered mm-sized sulfide and phosphide crystals (Figure 1e). Lastly, we performed HHXRF analyses on the external surface of the latter meteorite to show how this method can be used for the rough identification of an iron meteorite in cases where internal, flat or polished surface and not available, as may happen in the field during discovery of the material.

Results

Hand-held XRF compositional data of CRMs are presented in Table 3. In Figure 2 they are plotted against reference values. HHXRF data showed a nearly one-to-one relationship across a broad range of elemental compositions. Relatively larger deviations were observed only for those elements present in very low concentrations (< 0.1% m/m). % RSD varied from ~ 10 to 20 for P, S, V, Sn and Sb. Furthermore, analyses of CRMs were performed over a 7-month period using the same analytical procedure and setting to check the long-term precision of the instrument. Results indicated very good stability over time for several elements, with% RSD ranging between 1 and 5 (Figure 3).

Table 3. HHXRF analyses on metal alloy CRMs
 UnitNIST SRM 1262bARMI 35JNARMI AISI303NIST SRM 1158
Ref.aEstimated uncertaintyHHXRF (n = 11) s % RSDRef.bEstimated uncertaintyHHXRF (n = 11) s % RSDRef.bEstimated uncertaintyHHXRF (n = 11) s % RSDRef.aEstimated uncertaintyHHXRF (n = 11) s % RSD
  1. Ref., Reference values from aNIST certificate, bARMI certificate.

Alμg g−181020< LOD29010< LOD
Sim/m0.40.010.4120.047110.60.010.4150.042100.220.010.0970.023290.1940.003< LOD
Pm/m0.0440.0010.0480.005100.0060.001< LOD0.0250.001< LOD0.0030.001< LOD
Sm/m0.0370.0010.0360.007200.0250.002< LOD0.340.010.4030.01370.0050.0020.0210.00733
Tiμg g−1100040710304201< LOD
Vμg g−1410101602016404< LOD10002097012012
Crm/m0.30.010.30.0621.180.021.2110.007118.240.0318.070.0420.20.0630.0080.1230.0076
Mnm/m1.050.011.0410.00910.550.010.5510.00811.980.022.0060.03220.470.0070.470.0061
Fem/m95.395.80.0500.0597.197.20.10.168.769.10.10.263.263.80.040.1
Com/m0.30.010.1890.051270.2080.0020.2420.0080.0020.0580.058100
Nim/m0.590.010.5670.01320.0860.0020.0980.011129.50.039.3170.0280.436.10.02935.40.10.3
Cuμg g−151001005790801870208304045100100512012024002074011015
Zrμg g−122001001850302
Nbμg g−1300010031203012010< LOD
Moμg g−1700106601024500100459030113002013203011102011032
Snμg g−1160102202085010701013
Sbμg g−112010150108205< LOD
Wμg g−12000100281040230< LOD
Figure 2.

HHXRF elemental concentrations of CRMs plotted versus reference values. The line shows the 1:1 linear correlation.

Figure 3.

Temporal variations of the Ni, Fe, P, Si, Mn and S concentrations in CRMs by HHXRF over a period of 6 months to exemplify the long-term instrumental precision. All concentrations are % m/m. Top and bottom continuous lines on each diagram represent positive and negative 2s variation range, respectively; top and bottom dotted lines on each diagram represent positive and negative 1s variation range, respectively.

The HHXRF bulk metal composition of fifteen iron meteorites obtained from the analyses of cut surfaces is listed in Table 4, along with standard deviation, % RSD for each sample and reference values from the literature. The match is good and the % RSD varied from < 1 to 5 for the most abundant elements, that is, Fe, Ni and Co. Figure 4 shows HHXRF measurements plotted against reference data. The best results were obtained for Fe, Ni and Co, which are the most abundant elements in iron meteorites. The relatively large deviations for some elements such as Cr and Cu are possibly due to the very low concentrations of these elements close to the limit of detection (i.e., 450 μg g−1 for Cu and 80 μg g−1 for Cr (Table 5). Poor correlations of P and S in the Campo del Cielo, Canyon Diablo and North Chile meteorites are related to weak reference values, which were derived through modal estimation models (Buchwald 1975).

Table 4. HHXRF analyses of bulk metal of the studied meteorite samples
 Campo del CieloCanyon Diablo
Ref.Average (n = 15) s % RSDRef.Average (n = 21) s % RSD
Si< LOD< LOD
P0.250.1120.024220.260.110.0113
S0.40.0130.0021910.040.0115
Ti< LOD< LOD
Cr379707607940010013
Fe92.892.70.20.289.8–92.492.80.20.2
Co0.42–0.470.320.0280.280.310.014
Ni6.5–7.136.60.237.16.70.22
Cu50< LOD< LOD
W1510509< LOD
 ChingaCoahuila
Ref.Average (n = 15) s % RSDRef.Average (n = 3) s % RSD
Si< LOD< LOD
P0.050.0180.002110.1280.0065
S0.1260.221173< LOD
Ti< LOD< LOD
Cr810122041033370970505
Fe82.7–83.283.30.20.293.9–94.193.90.0040.004
Co0.54–0.570.460.0360.41–0.440.420.0051
Ni16.2–16.616.20.215.49–5.595.40.0020.04
Cu< LOD120–170420307
W< LOD< LOD
 DroninoGebel Kamil
Ref.Average (n = 20) s % RSDRef.Average (n = 22) s % RSD
Si< LOD< LOD
P0.0170.002130.0410.0127
S1.21.21010.0230.0729
Ti< LOD< LOD
Cr405904006760030050
Fe89.688.71.4278.678.50.20.3
Co0.550.410.04100.760.690.057
Ni9.89.60.5520.620.60.211
Cu30320702170010014
 GibeonHoba
Ref.Average (n = 11) s % RSDRef.Average (n = 3) s % RSD
Si< LOD< LOD
P0.0160.003220.0550.0270.00310
S0.0950.031320.020.0380.038100
Ti28015055< LOD
Cr130–37026016060< LOD
Fe91.2–92.391.80.10.282.4–82.882.90.10.2
Co0.37–0.390.250.0150.74–0.790.70.0030.4
Ni7.25–8.277.80.10.716.4–16.816.30.10.6
Cu140–200< LOD< LOD
W< LOD< LOD
 MuonionalustaNorth Chile
Ref.Average (n = 15) s % RSDRef.Average (n = 5) s % RSD
Si< LOD< LOD
P0.040.0390.30.210.084
S< LOD0.10.0220.0063
Ti< LOD< LOD
Cr10040030061502101708
Fe90.7–91.491.30.040.0593.4–93.793.70.20.03
Co0.39–0.410.240.0150.210.3810.0130.3
Ni8.2–8.98.30.0515.6–5.75.60.20.3
Cu110< LOD130450400.8
W< LOD< LOD
 NWA 6583Santiago Papasquiero
Ref.Average (n = 6) s % RSDRef.Average (n = 4) s % RSD
Si0.130.190.1156< LOD
P0.30.1190.073620.01< LOD
S0.040.0370.016420.0220.0300.00311
Ti30047014029
Cr0.060.0344
Fe81.881.70.20.292.0891.70.1060.115
Co0.390.330.03100.380.3950.0092.3
Ni17.717.50.10.427.517.710.0310.407
Cu1400141021015
W< LOD
 Seymchan metalSikhote-Alin
Ref.Average (n = 15) s % RSDRef.Average (n = 10) s % RSD
Si< LOD< LOD
P0.0670.025370.1840.02111
S0.0330.019570.0310.0133
Ti< LOD< LOD
Cr3041035084< LOD
Fe90.189.80.30.493.6–93.893.20.51
Co0.530.40.0240.47–0.510.370.0616
Ni9.39.70.345.7–5.875.80.34
Cu< LOD130–1901902011
W< LOD140107
 Tishomingo
Ref.Average (n = 3) s % RSD
  1. All elements expressed as % m/m except Ti, Cr, Cu, W which are in μg g−1.

  2. Reference values (min–max) mainly from: Buchwald (1975, and references therein), Scott and Wasson (1976), Wlotzka and Jarosewich (1977), Jochum et al. (1980), Wasson and Ouyang (1990), Choi et al. (1995), Wasson et al. (1998), Benedix et al. (2000), Birch et al. (2001), Wasson and Richardson (2001), Wasson and Kallemeyen (2002), Petaev and Jacobsen (2004), Russell et al. (2004), van Niekerk et al. (2007), Wasson et al. (2007), Walker et al. (2008), D'Orazio et al. (2011), Buchner et al. (2012), Fazio et al. (2013).

Si< LOD
P< LOD
S0.0340.01545
Ti< LOD
Cr10094011011
Fe66.767.10.30.4
Co1.261.130.011
Ni32.131.30.31
Cu< LOD
W< LOD
Table 5. HHXRF average limits of detection (μg g−1) for the elements determined in the studied iron meteorites. Limits of detection for Fe, Ni and Co are not reported here as the concentrations of these elements are orders of magnitude higher
ElementMean LOD
  1. Limit of detection was calculated as three times the standard deviation of the concentration measured in samples with none or only a trace amount of the analyte.

W140
Cu340
Cr60
Ti40
S210
P200
Si500
Figure 4.

HHXRF elemental concentrations of iron meteorites plotted vs. reference values from the literature (see Table 4 for data sources). The grey line shows the 1:1 linear correlation. For some elements in concentrations < 0.1% m/m (dotted line) such as Cu and Cr, there is a weak correlation between HHXRF analysis and reference data.

One of the major problems in the determination of the bulk composition of iron meteorites is their heterogeneity, determined by the size and spatial distribution of the constituent phases (e.g., the kamacite-taenite intergrowths, accessory minerals Figure 1), relative to the size of the X-ray beam. We thus focused on the systematic analysis of a highly heterogeneous meteorite at the specimen scale in order to assess how many HHXRF spot analyses are required to obtain a representative bulk chemical composition that takes into account the occurrence of mm-sized crystals (or larger). For this purpose, the recently classified iron meteorite Gebel Kamil (D'Orazio et al. 2011) from Egypt was selected, which has millimetre-sized troilite (FeS), schreibersite ([Fe,Ni]3P) and daubreelite ([Fe,Cr]2S4) crystals in a cm-scale spacing arrangement (Figure 1). We performed 166 HHXRF spot analyses on numerous meteorite slabs adopting an 8-mm spot and a grid spacing of 1 cm for a total of 83.4 cm2 of analysed surface. The dynamic average of the concentrations of Fe, Ni, S, P and Co (i.e., the variations of the average values of the concentrations of these elements with increasing number of analyses) is plotted in Figure 5. The plot reveals significant offsets and systematic divergences associated with the occasional analyses of mm-sized sulfide and phosphide crystals. Note in fact that each positive spike of P and S coincides with a negative spike of Ni and Fe. Overall, these divergences reflect the different P, S and Fe, Ni ratios of the mm-sized phosphide and sulfide crystals and host metal. As expected, after an initial scattering, data tended to stabilise around constant values, and ~ 3× differences in the P and S bulk contents were observed relative to the metal composition (Table 4). Furthermore, examining HHXRF bulk meteorite analysis of the Gebel Kamil specimen (Table S1), it was possible to count the same number of visible phosphide and sulfide crystals and then to estimate a ~ 1:1 ratio between phosphide and sulfides that is different from that estimated by D'Orazio et al. 2011.

Figure 5.

Dynamic average profiles (see text for explanation) of Fe, Ni, S, P and Co concentrations from 166 HHXRF spot analyses of the Gebel Kamil iron meteorite. Final average values are reported in each diagram.

The HHXRF analysis of the external surface of the Gebel Kamil meteorite is given in Table 6. The analysis revealed a lower Fe/Ni (3.2) ratio relative to bulk metal and bulk meteorite compositions (3.8) from interior surfaces, and the occurrence of considerable Si, Al, S up to 9.1, 3.8 and 1.8% m/m, respectively.

Table 6. HHXRF bulk metal and bulk meteorite (i.e., metal phase plus accessory minerals) compositions from flat, roughly polished interior surfaces of the Gebel Kamil specimen and of its external surface
ElementBulk metalBulk meteoriteExternal surface
  1. All values in % m/m.

Fe78.578.166.7–67.9
Ni20.620.619.2–23.3
Co0.690.700.87–1.05
P0.040.190.06–0.09
S0.020.211.35–1.85
Cr0.060.090.03–0.05
Cu0.070.060.07–0.10
Al1.85–3.80
Si3.48–9.10

Discussion

Results suggest that the HHXRF employed in this study yields precise and accurate analyses of metal alloys for most elements heavier than Mg with minimum concentrations of 0.01% m/m, as documented by analyses of CRMs (Table 3). In addition, the instrument showed very good stability, as revealed by analyses of CRMs over a seven-month period (Figure 3).

Hand-held XRF is very effective in the quantification of elements in iron meteorites, especially major elements such as Fe, Ni and minor elements such as Co, P and S, which generally range from 0.1 to 1% m/m. This is documented by the good agreement between HHXRF data from cut (and roughly polished) surfaces of the analysed iron meteorites and reference data from the literature (Figure 4). As a result, the HHXRF analyses undertaken allowed discrimination of different iron meteorites.

Hand-held XRF analyses of cut surfaces could also be used to constrain the classification of iron meteorites. Figure 6 shows the Ni vs. Co diagram of the iron meteorites analysed in this work by HHXRF relative to the major iron meteorite groups from the literature. The analysed meteorites plot in the compositional fields of their respective chemical groups (Table 2). When coupled with petrographic and textural analysis, this information can be used to assign unknown iron meteorites to a limited number of chemical (and structural) classes.

Figure 6.

Ni vs. Co classification diagram for iron meteorites. Compositional fields show the ranges of the major iron meteorite classes from the literature. The bulk metal compositions of the fourteen iron meteorites analysed by HHXRF are shown. GK, Gebel Kamil; H, Hoba; CH, Chinga; NWA, NWA6583; CO, Coahuila; NC, North Chile; SP, Santiago Papasqueiro; SA, Sikhote-Alin; DRO, Dronino; SEY, Seymchan, metal; CDC, Campo del Cielo; CD, Canyon Diablo; GI, Gibeon; MU, Muonionalusta. The ungrouped meteorite Tishomingo (Ni = 31.3% m/m, Co = 1.3% m/m) is omitted here.

The bulk P and S concentrations determined by HHXRF were adequate, in terms of precision and accuracy, to study the chemical evolution and petrogenesis of iron meteorites. Note that the concentration of non-metal elements such as P, S and C determines the solidification behaviour and the distribution of major, minor and trace elements in iron meteorites (Goldstein et al. 2009). Furthermore, the amount of P present in the metal greatly influences the nucleation temperature, the reaction process and the diffusion rate of Ni as the Widmanstätten pattern develops. The identified importance of P in the nucleation and growth of the Widmanstätten pattern has allowed to the development of new and more sophisticated models for the determination of cooling rates of iron meteorites (Goldstein et al. 2009). Since P and S are preferentially contained in accessory phases such as sulfides and phosphides, their size and distribution in the meteorite must be carefully assessed in order to select an appropriate analytical method for determining its bulk composition. According to the analytical protocol for INAA and ICP-MS analyses of iron meteorites, the metal sample must not contain visible inclusions and sulfide and phosphide crystals. The true meteorite bulk composition could be obtained by either dissolving a sample large enough to be representative, namely hundred grams of meteorite (or much more), or by integrating INAA or ICP-MS data with the geochemical contribution of the mm-sized accessory minerals obtained by modal analyses plus mineral chemistry (Buchwald 1975, Wasson et al. 2007). The first approach usually requires the destruction of large amounts of precious material and is often avoided; the second approach is often favoured, but can be inaccurate. In the case of meteorites containing large accessory minerals relative to the spot analysis, HHXRF is a suitable tool for determining bulk meteorite composition, including P and S. A comparison between HHXRF bulk metal composition and bulk composition of the Gebel Kamil meteorite is given in Table 6. The 3× differences in P and S contents highlight the geochemical contribution of the mm-sized sulfide and phosphide crystals to the bulk meteorite composition and the usefulness of the method.

The comparison of the bulk compositions obtained by the HHXRF analyses of the interior and external surfaces of the Gebel Kamil specimen (Table 6) shows that HHXRF not only enables detection of the extraterrestrial signature of iron meteorites, namely the combination of major elements Fe, Ni and Co, but also the detection of their alteration in the terrestrial environment due to ablative flight, weathering and contamination. For instance, in the specific case of the Gebel Kamil shrapnel (i.e., a meteorite fragment devoid of fusion crust that formed upon hypervelocity impact), the lower Fe/Ni ratio is probably due to oxidation during weathering. The high concentrations of S, Si and Al are due contamination from the Sahara desert where it was found (Folco et al. 2010), most likely desert varnish (i.e., Lee and Bland 2003, Giorgetti and Baroni 2007).

Furthermore, since Fe, Ni and Co, along with P and S, are the most abundant diagnostic elements in iron meteorites, HHXRF can be used as a first analytical approach to distinguish extraterrestrial iron from iron artefacts. This is relevant as many valuable archaeological artefacts are made of meteoritic iron, as recently documented (e.g., Buchner et al. 2012, Johnson et al. 2013). Likewise, HHXRF can be used to identify paired specimens in meteorite collections, that is, from dense meteorite collection areas, or mislabelled specimens in museum meteorite collections. The advantage of rapid, non-destructive methods in the curation of meteorites has already been demonstrated by Rochette et al. (2003, 2008) and Folco et al. (2006) in the case of magnetic susceptibility measurements.

Conclusions

Analyses of CRMs and iron meteorites of known composition showed that a commercial hand-held XRF spectrometer (NITON XL3t GOLDD+) allowed precise and accurate determination of the major elements Fe and Ni, and the minor elements Co, P and S (generally ranging from 0.1 to 1% m/m) in iron meteorite metal. The % RSD values varied from less than 1 to 5 for the most abundant elements such as Fe, Ni and Co.

The procedure required minimal sample preparation, that is, flat, ground (≤ 600 Mesh) representative surfaces larger that the mm-sized X-ray spot size (3 or 8 mm in diameter in the XRF spectrometer used in this study). Analyses were rapid (180 s) and non-destructive. Analyses of irregular external surfaces provided qualitative information about the extraterrestrial geochemical signature of iron meteorites, namely the detection of diagnostic major and minor elements Fe, Ni, Co, P and S. They also provided information about their surface alteration in terrestrial environments due to ablative flight, weathering and contamination.

Hand-held XRF thus proves to be a valuable and practical tool in meteoritics for curatorial purposes. It can be used to: (a) confirm/verify the extraterrestrial origin of metallic objects; (b) complete the preliminary chemical classification of new iron meteorites; (c) identify mislabelled/unlabelled specimens in museums and private collections.

Multiple HHXRF spot analyses can be used to determine the bulk chemical composition of iron meteorites characterised by up to cm-sized crystals of accessory minerals with a mm- to cm-scale spacing (most commonly sulfides and phosphides). A test conducted on the heterogeneous Gebel Kamil iron meteorite, which is characterised by mm-sized and cm-spaced sulfide and phosphide crystals, required about 160 spot analyses (total analysed surface: 83 cm2; total analysis time: ~ 8 hr) to obtain a representative bulk meteorite composition for Fe, Ni, Co, P and S. Note that only a few spot analyses were required for homogeneous meteorites such as Hoba, Chinga, North Chile and Coahuila. Bulk P and S contents are of crucial petrological importance in modelling parent liquid evolution and subsolidus cooling rates. Their determination in a heterogeneous iron meteorite such as Gebel Kamil by means of other customary methods such as INAA or ICP-MS would require the destruction (digestion) of hundreds of grams of precious material.

Due to its principal characteristics and capabilities (portability, and rapid, non-destructive, accurate analyses), HHXRF has great potential applications in archaeometry, namely on-site identification and the examination and study of iron artefacts. It can be useful not only during archaeological excavations, but also when museums do not allow sampling of precious artefacts (as required for INAA or ICP-MS analysis) or even their temporary transfer to the laboratory.

Acknowledgements

This work was supported by the Italian Programma Nazionale delle Ricerche in Antartide, PNRA (project ID#: PEA2009, A2.08, Meteoriti Antartiche) and by the Italian Ministero degli Affari Esteri, Progetti di Grande Rilevanza 2013–2015, Protocollo Italia-Egitto (project ID#: PGR 00187, Geologia, geofisica e geocronologia del Kamil Crater, Egitto). M. Gemelli is also supported by the Italian Ministero dell'Istruzione, dell'Università e della Ricerca, MIUR ‘Futuro in Ricerca Programme 2013’ (project ID#: RBFR13FIVO). We thank two anonymous referees for constructive review and the Thomas Meisel for editorial handling.

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