The New Peruvian Meteorite Carancas: Mössbauer Spectroscopy and X-Ray Diffraction Studies

The Carancas meteorite fell on 15 September 2007 approximately 10 km south of Desaguadero, near Lake Titicaca, Peru, producing bright lights, clouds of dust in the sky and intense detonations. The Carancas meteorite is classified as a H4–5 ordinary chondrite with shock stage S3 and a degree of weathering W0. The Carancas meteorite is characterized by well defined chondrules composed either of olivine or pyroxene. The Mössbauer spectra show an overlapping of paramagnetic and magnetic phases. The spectra show two quadrupole doublets associated to olivine and pyroxene; and two magnetic sextets, associated with the primary phases kamacite/taenite and Troilite (Fe²⁺). Metal particles were extracted from the bulk powdered samples exhibit only kamacite and small amounts of the intergrowth tetrataenite/antitaenite. X-Ray diffractogram shows the primary phases olivine, pyroxene, troilite, kamacite, diopside and albite. Iron oxides has not been detected by Mössbauer spectroscopy or XRD as can be expected for a meteorite immediately recovered after its fall.     

MÖssbauer spectroscopy,x-ray diffraction,and electron microprobe analysis of the New Haifa meteorite

Abstract— MÖssbauer spectroscopy, x-ray diffraction (XRD) measurements, and electron microprobe analysis (EMPA) have been carried out for the investigation of a newly fallen Sudanese meteorite named New Haifa. The room temperature MÖssbauer spectrum is fitted with three sextets and two doublets. The sextets are assigned to Fe in troilite, kamacite, and taenite, and the two doublets are assigned to Fe²⁺in olivine and pyroxene (no Fe³⁺was found). The microprobe trace of Ni concentration across a kamacite-taenite-kamacite area shows a high-Ni concentration at the interfaces between kamacite and taenite. From the microprobe analysis, olivine appears to have a constant composition, whereas pyroxene has a varying composition. The mole fractions of the Fe end members of olivine (fayalite) and pyroxene (ferrosilite) are found to be 23.5% and 23.2%, respectively. Accordingly, the New Haifa meteorite is classified as an ordinary L-type chondrite.     

Fe‐bearing phases in a ureilite fragment from the asteroid 2008 TC3 (= Almahata Sitta meteorites): A combined Mössbauer spectroscopy and X‐ray diffraction study

The iron‐bearing phases in a ureilite fragment (AS#051) from the Almahata Sitta meteorite are studied using Mössbauer spectroscopy, X‐ray diffraction (XRD), and electron microprobe analysis (EMPA). AS#051 has a typical ureilite texture of medium‐ to coarse‐grained silicates (olivine, orthopyroxene, and pigeonite) with minor opaques (Fe‐Ni metal, troilite, and graphite). The silicate compositions, determined by EMPA, are homogeneous: olivine (Fo_90.2), orthopyroxene (En_86.3Fs_8.6Wo_5.1), and pigeonite (En_81.6Fs_8.9Wo_9.5), and are similar to those of magnesian ureilites. The modal abundance of mineral phases was determined by Rietveld refinement of the powder XRD data. The Mössbauer spectra at 295 K and 78 K are composed of two sharp well‐defined paramagnetic doublets superimposed on a well‐resolved magnetic sextet and other weak absorption features. The two paramagnetic doublets are assigned to olivine and pyroxene (orthopyroxene and pigeonite), and the ferromagnetic sextet to kamacite (magnetic hyperfine field ≈ 33.2 T), in agreement with the XRD characterization. The Mössbauer results also show the presence of small amounts of troilite (FeS) and cohenite ([Fe,Ni,Co]₃C). Using the Mössbauer data, the relative abundance of each Fe‐bearing phase is determined and compared with the results obtained by XRD.     

Characterization of the matrix and fusion crust of the recent meteorite fall Ozerki L6

We studied the interior and the fusion crust of the recently recovered Ozerki L6 meteorite using optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy, X‐ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. The phase composition of the interior and of the fusion crust was determined by means of SEM, XRD, and Mössbauer spectroscopy. The unit cell parameters for silicate crystals were evaluated from the X‐ray diffractograms and were found the same for the interior and the fusion crust. Magnetization measurements revealed a decrease of the saturation magnetic moment in the fusion crust due to a decrease of Fe‐Ni‐Co alloy content. Both XRD and Mössbauer spectroscopy show the presence of magnesioferrite in the fusion crust. The temperatures of cation equilibrium distribution between the M1 and M2 sites in silicates calculated using the data obtained from XRD and Mössbauer spectroscopy appeared to be in a good consistency: 553 and 479 K for olivine and 1213 and 1202 K for orthopyroxene.     

Application of Mössbauer spectroscopy,multidimensional discriminant analysis,and Mahalanobis distance for classification of equilibrated ordinary chondrites

Mössbauer spectra of equilibrated ordinary chondrites consist of two doublets due to paramagnetic iron present in olivines and pyroxenes and two sextets due to magnetically ordered iron present in metallic phases and troilite. The spectral areas of the different mineralogical phases found by Mössbauer spectroscopy in meteorites are proportional to the number of iron atoms in this mineralogical phase. This property of Mössbauer spectra can be the basis for constructing a method for the classification of ordinary chondrites. This idea was first explored at the Mössbauer Laboratory in Kanpur. This group suggested a qualitative method based on 2‐dimensional plots of Mössbauer spectral areas and thus classified properly some meteorites. We constructed a quantitative method using Mössbauer spectral areas, multidimensional discriminant analysis, and Mahalanobis distance (4M method) to determine the probability of a meteorite to be of type H, L, or LL. Based on 59 Mössbauer spectra, we calculated by the 4M method, S _cluster , the level of similarity of the Goronyo meteorite to the clusters. On the plot of ferrosilite versus fayalite, the point representing Goronyo is located on the border between H and L areas. Calculated by the 4M method, the meteorite Goronyo is 32% similar to type H, 75% to type L, and 11% to type LL. Additional mineralogical analyses suggested that the Goronyo meteorite would be classified as type L, although it was originally reported as type H in the Meteoritical Bulletin Database.     

A study of ordinary chondrites by Mössbauer spectroscopy with high‐velocity resolution

Abstract— An improvement in the velocity resolution and quality of Mössbauer spectra has been applied to a group of ordinary chondrites. This improvement permitted us to carry out a more detailed study of the iron bearing phases in these samples than has previously been possible. Mössbauer spectra of 11 ordinary chondrites of L and H chemical groups were measured using 4096 channels and presented for further analysis in 1024 channels. Subspectra of the metal grains of several chondrites demonstrated the presence of at least two magnetic sextets related to the main Fe(Ni, Co) phases. Moreover, Mössbauer study of extracted metal grains from Tsarev L5 revealed three sextets and one singlet spectral components related to various α‐Fe(Ni, Co), α‘‐Fe(Ni, Co), α₂‐Fe(Ni, Co), and γ‐Fe(Ni, Co) phases. Each subspectrum of olivine and pyroxene in Mössbauer spectra of ordinary chondrites was fitted by superposition of two quadrupole doublets related to M1 and M2 sites in minerals for the first time. An analysis of relative areas and Mössbauer hyperfine parameters was performed and some differences for L and H chondrites as well as for M1 and M2 sites were observed. Mössbauer parameters of troilite and oxidized iron were analyzed. In contrast to a previous study with 512‐channel spectra, the presence of oxidized iron was found in all chondrites.     

Modal mineralogy of carbonaceous chondrites by X‐ray diffraction and Mössbauer spectroscopy

Abstract— Carbonaceous chondrites are among the most analyzed geological materials on Earth. However, despite this attention, and unlike most terrestrial rocks, little is known on the abundance of individual phases within them. Here, we show how a combination of several novel X‐ray diffraction (XRD) techniques (including a high‐brightness X‐ray MicroSource®), and Mössbauer spectroscopy, allows a complete modal mineralogy to be ascertained from even the most highly unequilibrated, fine‐grained chondrites for all minerals of abundance >1 wt%. Knowledge of the modal mineralogy of a sample also allows us to calculate grain density. We analyzed Allende, Murchison, Tagish Lake, and Orgueil. Based on our modal data, the grain density estimates for Allende, Murchison, and Orgueil are close to literature values. In the case of Tagish Lake, there is no published grain density, although a bulk density measurement does exist. Taking our estimate of grain density, and the measured bulk density, we calculate an exceptionally high porosity of 41% for this meteorite, similar to some chondritic IDPs and in line with a porosity calculated from an entry model for the Tagish Lake fireball. Although it is an oxidized CV, magnetite is present in Allende at a level of <0.5 wt% or <0.3 vol%, a result that is substantiated by several other instrumental studies. This may be an oxidized meteorite, but that oxidation is not manifested in abundant magnetite. In addition, we note appreciable fayalitic olivine in Orgueil, detected by both XRD and Mössbauer. We employed MicroSource® XRD to look at heterogeneity in mineral abundance in Orgueil and found substantial variation, with phyllosilicates varying inversely with olivine. The data suggest that Orgueil was initially composed primarily of anhydrous materials, which have been partially, but not completely, altered. Although the data are preliminary, comparison between our XRD modal assessment, bulk chemistry, grain density, and Mössbauer data, suggests that our estimates of mineral abundance are robust. The advent of MicroSource® XRD allows similar modal data to be acquired from samples as small as a few hundred micrograms.     

Glassy magnetic cronstedtite signatures in Mukundpura CM2 chondrite based on magnetic and Mössbauer studies

We have studied the Mukundpura CM2 meteorite for magnetic properties as a function of temperature and magnetic field, as well as its Mössbauer spectrum, at room and low temperatures (up to 5 K). We find that the high temperature paramagnetic phase is followed by two magnetic transitions: a weak transition near 125 K and a strong transition at 8 K. The weak (125 K) magnetic phase can be attributed to complex Fe²⁺–Fe³⁺ constituents present in the meteorite. The absence of the characteristic sextet corresponding to magnetite in Mossbauer spectrum indicates that this magnetic phase is not magnetite, which, if present, must be in insignificant amount. The 8 K magnetic ordering is superimposed with weak ferromagnetic ordering, showing spin‐glass transition. The Mössbauer spectrum taken at 5 K substantiates the observed spin‐glassy nature, as very large hyperfine field ~32 T is recorded, causing localized subordering leading to spin‐glass behavior. The Mössbauer spectra also confirm that iron is mainly present in serpentine‐group minerals, both in ferrous and ferric states. The complete serpentinization of basic silicates indicates aggressive hydrous alteration. These results show that the observed spin‐glass signature is a characteristic feature of the cronstedtite phase in CM meteorites. This feature is unique to carbonaceous CM chondrites and could be used for nondestructive, quick, and independent classification of this rare class of meteorites. Furthermore, the absence of olivine and the presence of cronstedtite in Mossbauer spectra show that the degree of aqueous alteration observed is the most severe in Mukundpura CM2 meteorite, as compared to many other CM2 meteorites. The degree of aqueous alteration in CM2 carbonaceous chondrites increases in the sequence: Paris, Murchison, Murray, Mighei, Nogoya, Cold Bokkeveld, and Mukundpura.     

Systematics of Mössbauer absorption areas in ordinary chondrites and applications to a newly fallen meteorite in Jodhpur,India

Abstract— Mössbauer absorption areas corresponding to ⁵⁷Fe in olivine, pyroxene, troilite, and the metallic phase in ordinary chondrites are shown to exhibit certain systematic behaviors. H chondrites occupy 2 distinct regions on the plot of metallic phase absorption area versus silicate absorption area, while L/LL chondrites fall in a separate region. Similar separation is also observed when pyroxene absorption area is plotted against olivine absorption area. The one‐dimensional plot for the ratio of olivine area to pyroxene area separates L and LL chondrites. Based on these systematics, a newly fallen meteorite at Jodhpur, India is suggested to be an LL chondrite.     

New insights into the mineralogy and weathering of the Meridiani Planum meteorite,Mars

Meridiani Planum is the first officially recognized meteorite find on the surface of Mars. It was discovered at and named after the landing site of the Mars Exploration Rover Opportunity. Based on its composition, it was classified as a IAB complex iron meteorite. Mössbauer spectra obtained by Opportunity are dominated by kamacite (α‐Fe‐Ni) and exhibit a small contribution of ferric oxide. Several small features in the spectra have been neglected to date. To shed more light on these features, five iron meteorite specimens were investigated as analogs to Meridiani Planum with a laboratory Mössbauer setup. Measurements were performed on (1) their metallic bulk, (2) troilite (FeS) inclusions, (3) cohenite ((Fe,Ni,Co)₃C) and schreibersite ((Fe,Ni)₃P), and (4) corroded rims. In addition to these room‐temperature measurements, a specimen from the Mundrabilla IAB‐ungrouped meteorite was measured at Mars‐equivalent temperatures. Based on these measurements, the features in Meridiani Planum spectra can be explained with the presence of small amounts of schreibersite and/or cohenite and iron oxides. The iron oxides can be attributed to a previously reported coating on Meridiani Planum. Their presence indicates weathering through the interaction of the meteorite with small amounts of water.     

Mineralogy and 57Fe Mössbauer spectroscopy of opaque phases in the Neuschwanstein EL6 chondrite

Abstract— This study presents compositional data and ⁵⁷Fe Mössbauer spectra, taken at 295 K and 85 K, of two fragments of the enstatite (EL6) chondrite Neuschwanstein that fell near the famous Neuschwanstein castle (Bavaria, southern Germany) on April 6, 2002. Main silicate minerals are enstatite (Fs 2) and plagioclase (An 20), the main opaque minerals are kamacite and troilite. Small amounts of oldhamite, daubreelite, and schreibersite have been found. The presented Mössbauer data are the first data gathered for an EL6 chondrite. The dominant parts of each Mössbauer spectrum consist of two six‐line patterns due to the presence of ferromagnetic phases kamacite and troilite. In contrast to other chondrites, peaks of other iron species in the central parts of the spectra are missing due to an extremely low content of Fe‐bearing paramagnetic components. The hyperfine interaction parameters for kamacite are internal magnetic hyperfine field H_hf = 333.2 kOe, isomer shift (relative to a metallic Fe foil) IS = 0.01 mm/s, quadrupole splitting QS = 0 mm/s, line width W = 0.41 mm/s. The data for troilite are H_hf = 305.5 kOe, IS = 0.75 mm/s, QS = ?0.85 mm/s, W = 0.34 mm/s.     

Mössbauer study of Meridiani Planum,the first iron‐nickel meteorite found on the surface of Mars by the MER Opportunity

Abstract— –Meridiani Planum is the first iron meteorite found on Mars. It was discovered in 2005 by the Mars Exploration Rover Opportunity (MER‐B). Mössbauer spectra (MS) of the unbrushed and brushed meteorite species were acquired in 10 degrees temperature windows in the range of 210–260 K. Earlier examinations of these MS have led to the conclusion that the meteorite, which contains ～～7 wt% Ni, belongs to the IAB meteorite group. Here, making use of a recently developed calibration/folding procedure for MER MS, we report the results of the MS analyses for the single temperature windows m5 (210–220 K), m6 (220–230 K), m7 (230–240 K), and m89 (240–260 K). All spectra consist of a sextet and a ferric doublet. The hyperfine field of the sextet, extrapolated to room temperature, is ～～34.5 T, which is, based on Mössbauer studies of meteorites found on Earth, indeed consistent with the presence of kamacite. The fractional spectral area of the sextet is ～～0.96 of the total spectrum. The ferric doublet has an average quadrupole splitting of 0.70 mm/s and is not diagnostic of any specific Fe mineral.     

Characterization of iron meteorites by scanning electron microscopy,x-ray diffraction,magnetization measurements,and Mössbauer spectroscopy: Mundrabilla IAB-ung

A fragment of Mundrabilla IAB-ung iron meteorite was analyzed using optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. The polished section of meteorite fragment characterization by optical microscopy and SEM shows the presence of the γ-Fe(Ni, Co) phase lamellae, plessite structures and schreibersite inclusions in the α-Fe(Ni, Co) phase. EDS indicates variations in the Ni concentrations in the following ranges: (i) ∼6.3–6.5 atom% in the α-Fe(Ni, Co) phase and (ii) ∼22 to ∼45 atom% in the γ-Fe(Ni, Co) phase lamellae including the range of ∼29–33 atom% of Ni leading to the paramagnetic state of the γ-Fe(Ni, Co) phase. Schreibersite inclusions contain ∼23 atom% of P, ∼33 atom% of Fe, ∼43 atom% of Ni, and ∼0.7 atom% of Co. Plessite structure contains the average Ni concentration of ∼17 atom% while detailed EDS analysis shows: (i) the lowest Ni concentrations of ∼5 to ∼8 atom%, (ii) the intermediate Ni concentrations of ∼9 to ∼19 atom%, and (iii) the highest Ni concentration up ∼38 atom% (some individual micro-grains demonstrate up to ∼47 and ∼59 atom% of Ni) that may indicate the presence of the (i) α-Fe(Ni, Co), (ii) α₂-Fe(Ni, Co), and (iii) γ-Fe(Ni, Co) phases. These may be a result of the γ-phase decomposition with mechanism γ → α + α₂ + γ that indicates a slow cooling rate for Mundrabilla IAB-ung iron meteorite. The presence of ∼98.6 wt% of the α-Fe(Ni, Co) phase and ∼1.4 wt% of the γ-Fe(Ni, Co) phase is found by XRD while schreibersite is not detected. Magnetization measurements show the saturation magnetization moment of Mundrabilla IAB-ung of 188(2) emu g⁻¹ indicating a low average Ni concentration in Fe-Ni-Co alloy. Mössbauer spectrum of the bulk Mundrabilla powder demonstrates five magnetic sextets related to the ferromagnetic α₂-Fe(Ni, Co), α-Fe(Ni, Co), and γ-Fe(Ni, Co) phases and one singlet associated with the paramagnetic γ-Fe(Ni, Co) phase, however, there are no spectral components corresponding to schreibersite. Basing on relatively larger and smaller values of the magnetic hyperfine field, two magnetic sextets associated with γ-Fe(Ni, Co) phase can be related to the disordered and more ordered γ-phases. The iron fractions in the detected phases can be roughly estimated as follows: (i) ∼17.6% in the α₂-Fe(Ni, Co) phase, (ii) ∼68.5% in the α-Fe(Ni, Co) phase, (iii) ∼11.5% in the disordered γ-Fe(Ni, Co) phase, (iv) ∼2.0% in the more ordered γ-Fe(Ni, Co) phase, and (v) ∼0.4% in the paramagnetic γ-Fe(Ni, Co) phase.     

A high spatial resolution synchrotron Mössbauer study of the Tazewell IIICD and Esquel pallasite meteorites

Metallic phases in the Tazewell IIICD iron and Esquel pallasite meteorites were examined using ⁵⁷Fe synchrotron Mössbauer spectroscopy. Spatial resolution of ~10–20 μm was achieved, together with high throughput, enabling individual spectra to be recorded in less than 1 h. Spectra were recorded every 5–10 μm, allowing phase fractions and hyperfine parameters to be traced along transects of key microstructural features. The main focus of the study was the transitional region between kamacite and plessite, known as the “cloudy zone.” Results confirm the presence of tetrataenite and antitaenite in the cloudy zone as its only components. However, both phases were also found in plessite, indicating that antitaenite is not restricted exclusively to the cloudy zone, as previously thought. The confirmation of paramagnetic antitaenite as the matrix phase of the cloudy zone contrasts with recent observations of a ferromagnetic matrix phase using X‐ray photoemission electron spectroscopy. Possible explanations for the different results seen using these techniques are proposed.     

Thermal and shock history of Almahata Sitta meteorite inferred from structure refinement of pyroxene and Mössbauer spectroscopy of Fe-Ni metal

The crystal structures of orthopyroxene (En_86.3Fs_8.6Wo_5.1, space group Pbca) and pigeonite (En_81.7Fs_8.8Wo_9.5, space group P2₁/c) from the Almahata Sitta ureilite (fragment#051) have been refined to R1 indices of 3.10% and 2.53%, respectively, using single-crystal X-ray diffraction data. The unit formulas were calculated from electron microprobe analysis, and the occupancies at the M1 and M2 sites were refined for both pyroxenes from the single-crystal diffraction data. The results indicate a rather disordered intracrystalline Fe²⁺-Mg cation distribution over the M1 and M2 sites, with a closure temperature of 726(±55)°C for orthopyroxene and 704(±110)°C for pigeonite, suggesting fast cooling of these pyroxenes. The Mössbauer spectrum of the Fe-Ni metal particles of Almahata Sitta ureilite (fragment#051) is dominated by two overlapping magnetic sextets that are assigned to Fe atoms in Si-bearing kamacite, and arise from two different nearest-neighbor configurations of Fe* (=Fe+Ni) and Si atoms in the bcc structure of kamacite; (8F*, 0Si) and (7Fe*, 1Si). In addition, the spectrum shows weak absorption peaks that are attributed to the presence of small amounts of cohenite [(Fe,Ni)₃C], schreibersite [(Fe,Ni)₃P], and an Fe-oxide/hydroxide phase. The fast cooling of pyroxene to the closure temperature (after equilibration at ~1200°C) and the incorporation of Si in kamacite can be interpreted as due to a shock event that took place on the meteorite parent body, consistent with the proposed formation history of ureilites parent body where a fast cooling has occurred at a later stage of its formation.     

Weathering of ordinary chondrites from the Atacama Desert,Chile, by Mössbauer spectroscopy and synchrotron radiation X‐ray diffraction

Some terrestrial areas have climatic and geomorphologic features that favor the preservation, and therefore, accumulation of meteorites. The Atacama Desert in Chile is among the most important of such areas, known as DCA. This desert is the driest on Earth, one of the most arid, uninhabitable localities with semiarid, arid, and hyper‐arid conditions. The meteorites studied here were collected from within the DCA of San Juan and Pampa de Mejillones, located, respectively, in the Central Depression and the Coastal Range of the Atacama Desert. ⁵⁷Fe Mössbauer spectroscopy was used for quantitative analysis of the degree of weathering of the meteorites, through the determination of the proportions of the various Fe‐bearing phases and in particular the amount of oxidized iron in terrestrial alteration products. The abundance of ferric ions in weathered chondrites can be related to specific precursor compositions and to the level of terrestrial weathering. The aim of the study was the identification, quantification, and differentiation of the weathering products in the ordinary chondrites found in the San Juan and the Pampa de Mejillones areas of the Atacama Desert. The ⁵⁷Fe Mössbauer spectroscopy study was complemented by synchrotron radiation X‐ray diffraction and magnetic susceptibility measurements. The results allow a clear differentiation of the rate of weathering in meteorite samples collected from the San Juan versus the Pampa de Mejillones areas of the Atacama Desert.     

Characterization of iron meteorites by scanning electron microscopy,X-ray diffraction,magnetization measurements,and Mössbauer spectroscopy: Gibeon IVA

Gibeon IVA iron meteorite fragment was characterized using optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. Optical microscopy and SEM made on the polished section of the meteorite, show the presence of α-Fe(Ni, Co) and γ-Fe(Ni, Co) phases and plessite structures. There are no troilite inclusions observed in the studied section. EDS studies indicate some variations in the Ni concentrations: (i) within the α-Fe(Ni, Co) phase in the range ~5.0 ± 0.1 – ~7.5 ± 0.1 at% and (ii) within the γ-Fe(Ni, Co) phase in the range ~26.0 ± 0.2 – ~36.1 ± 0.2 at%. The latter Ni concentration range indicates the presence of small amount of the paramagnetic γ-phase in addition to the ferromagnetic γ-phase. EDS also shows that Ni content in two plessite structures is varying in the range ~16–37 at%, which can indicate the presence of only the α₂-Fe(Ni, Co) and γ-Fe(Ni, Co) phases in the duplex plessite structure. This may be a result of the γ-phase decomposition with the incomplete martensitic transformation: γ → α₂ + γ due to a faster cooling rate. XRD indicates the presence of ~1.3 wt% of the γ-Fe(Ni, Co) phase in Gibeon VIA. The saturation magnetization moment of 185(2) emu g⁻¹ obtained also confirms the presence of phases with low and high Ni concentrations. The most appropriate fit of the Gibeon IVA Mössbauer spectrum demonstrates the presence of five magnetic sextets and one paramagnetic singlet which are assigned to the ferromagnetic α₂-Fe(Ni, Co), α-Fe(Ni, Co), γ-Fe(Ni, Co), and paramagnetic γ-Fe(Ni, Co) phases. The relative average Fe contents in these phases are: 13.4% in the α₂-Fe(Ni, Co) phase, 78.3% in the α-Fe(Ni, Co) phase, and 8.3% in the ferromagnetic and paramagnetic γ-Fe(Ni, Co) phases.     

New measurement technique for characterizing small extraterrestrial materials by X‐ray diffraction using the Gandolfi attachment

Identification and characterization of small extraterrestrial samples, such as small Antarctic meteorites <~1 cm, require the development of convenient laboratory‐based nondestructive analytical techniques using X‐ray diffraction (XRD). We explore the characterization criteria using an X‐ray diffractometer with a Gandolfi attachment using sub‐mm small fragments and powder aggregates for various kinds of stony meteorites and develop a new analytical technique. We primarily focus on olivine and pyroxene because they are the most abundant and important minerals for stony meteorite classification. A new calibration is performed to estimate the FeO content of the olivine in unequilibrated ordinary chondrites, which is useful for determining the meteorite chemical group irrespective of powder aggregate diameter but dependent on fragment grain diameter. This is because X‐ray intensity absorption is more effective for grains than for powders. Clinoenstatite (Cen) and orthoenstatite (Oen) were distinguished using the presence or absence of the isolated Oen 511 index peak. The method is also applied to other stony meteorites including carbonaceous chondrites and achondrites. The XRD results are consistent with studies based on polished sections involving textural observations by scanning microscope and chemical compositions of the constituent minerals. The new measurement technique presented here is convenient because of its use in air by the laboratory‐based X‐ray diffractometer, which makes it useful for the initial analyses of restricted extraterrestrial sample characterization.     

A Mössbauer spectroscopy and X‐ray diffraction study of ordinary chondrites: Quantification of modal mineralogy and implications for redox conditions during metamorphism

Abstract— We present a method that combines Mössbauer spectroscopy and X‐ray diffraction to quantify the modal mineralogy of unequilibrated ordinary chondrites (UOCs). Despite being a fundamental tool in the interpretation of geological systems, there are no modal mineralogical data available for these meteorites. This is due to their fine‐grained nature, highly heterogeneous silicate mineralogy, and the presence of poorly characterized phases. Consequently, it has not been possible to obtain accurate modal mineralogy by conventional techniques such as point counting. Here we use Mössbauer spectroscopy as a preliminary identification technique and X‐ray diffraction provides the quantification for a suite of recent UOC falls. We find the most primitive UOCs to contain a significant amount of phyllosilicate material that was converted during metamorphism to form ferromagnesian silicates. A complete suite of Antarctic samples is analyzed by each method to observe mineralogical trends and these are compared with trends shown by recent falls. The fact that mineralogical relationships shown by finds and falls are in agreement allows us to be confident that we are observing the products of pre‐terrestrial alteration. Mössbauer spectroscopy reveals evidence of steadily increasing reduction with metamorphism in the UOCs. Because this technique allows comparisons to be made between UOCs and EOCs, our reduction sequence can be combined with other evidence showing progressive oxidation in the EOCs. This yields an integrated model of changing redox conditions on equilibrating ordinary chondrite parent bodies.     

THE HAVERÖ UREILITE*

The Haverö ureilite fell on August 2, 1971 on the Island of Haverö, Finland, lat 22° 03‘ 43“ E., long 60° 14’ 44” N. The meteorite contains curved open fractures partly filled with kamacite foils or drops, clusters of olivine mosaic with preferred orientation, very fine-lamellar polysynthetic twinned clino-***bronzite and carbonaceous matter as laths up to 4 mm in length. The carbon laths are in preferred orientation and contain in addition to graphite, kamacite, chromite and diamonds. The petrology, textural features and origin are discussed.