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The geochemistry and textures of phosphate minerals can provide insights into the geological histories of parental asteroids, but the processes governing their formation and deformation remain poorly constrained. We assessed phosphorus‐bearing minerals in the three lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron microscope, electron microprobe, cathodoluminescence, and electron backscatter diffraction techniques. The majority of studied phosphate grains appear intergrown with olivine. However, microtextures of phosphates (apatite [Ca5(PO4)3(OH,Cl,F)] and merrillite [Ca9NaMg(PO4)7]) are extremely variable within and between the differently shocked lithologies investigated. We observe continuously strained as well as recrystallized strain‐free merrillite populations. Grains with strain‐free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock‐metamorphic features. Complete melting of portions of the meteorite is recorded by the shock‐melt lithology, which contains a population of phosphorus‐rich olivine grains. The response of phosphorus‐bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. We propose a paragenetic history for P‐bearing phases in Chelyabinsk involving initial phosphate growth via P‐rich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and instead contains P‐enriched olivine. We generalize our findings to propose a new classification scheme for Phosphorus‐Olivine‐Assemblages (Type I–III POAs). We highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of chondritic asteroids. [ABSTRACT FROM AUTHOR]

X-ray diffraction (XRD), Fields Emission Scanning Electron Microscope (FE-SEM) with EDS and Mössbauer Spectroscopy (MS), were applied to investigate a newly fallen solid piece of debris named the Aba Panu meteorite, after a city in south western Nigeria (Lat: N 08° 14′ 25.7″ and Long: E 003° 33′ 47.0″). Matching X-ray diffraction results, together with the FE-SEM analysis confirms the presence of four kinds of iron-bearing minerals, namely olivine, pyroxene, kamacite (Fe-Ni alloys) and troilite (FeS). The Mössbauer spectra recorded at 295 K and 78 K consist of two strong paramagnetic doublets emanating from olivine of quadrupole splitting 2.9 mm/s and pyroxene of quadrupole splitting 2.1 mm/s. These are superimposed on two magnetic sub-spectra attributed to kamacite and troilite phases. From the Mössbauer sub-spectra absorption area, the ratio of the olivine absorption area to the pyroxene absorption area indicates that the meteorite can be classified as an L-ordinary chondrite. The mole fraction of the Fe end-member of olivine (fayalite) and the orthopyroxene (ferrosilite) calculated from the EDS data will be used to identify the petrographic type of the meteorite. [ABSTRACT FROM AUTHOR]

Introduction: Following low temperature aqueous alteration (<200 °C), a number of CM carbonaceous chondrites experienced post-hydration heating (>300 °C) on their parent body [1-3]. However, the timing, duration and mechanism of the dehydration events remain poorly defined. Constraining the thermal history of the carbonaceous chondrites is an important step to understanding the evolution of water-rich C-complex asteroids, and the distribution and transport of volatiles in the early solar system. Iron sulfides are widespread in carbonaceous chondrites and are highly sensitive to both aqueous alteration and thermal metamorphism. Dehydrated CM chondrites contain a generation of recrystallized sulfides with compositions and textural features diagnostic of their metamorphic heating [4-6]. Here, we have systematically characterized Fesulfide grains within the matrices of dehydrated CM chondrites that experienced a range of peak temperatures (~300 - >750 °C) to constrain the timescales of metamorphism and infer the thermal history of the parent body. Samples & Methods: The composition and morphology of coarse (10 -150 µm) Fe-sulfide grains within polished sections of Jbilet Winselwan (CM2, Stage II, ~300 - 500°C), Dhofar (Dho) 1434 (CMan, Stage III, ~500 - 750°C) and Pecora Escarpment (PCA) 02012 (CM2, Stage IV, >750°C) were characterized using a ZEISS EVO 15LS scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). Backscattered electron (BSE) images, element maps and spot analyses were acquired at 20 kV and 1.5 nA. Results: In Jbilet Winselwan, most (~80%) of the analysed grains are pyrrhotite with inclusions of pentlandite (PoPn), which form either ~5 µm sized oriented laths or patches 10 - 50 µm in size. The pentlandite patches themselves appear to contain ~1 µm sized pyrrhotite inclusions. We also observed individual grains of pyrrhotite (Po) and pentlandite (Pn) in Jbilet Winselwan. In Dho 1434, the grains are either PoPn, pentlandite with inclusions of metal (PnM), or pyrrhotite with inclusions of both pentlandite and metal (PoPnM). The majority of Fe-sulfide grains within PCA 02012 are PoPn and PoPnM grains, although we also found one individual Po grain, and one pyrrhotite grain with metal inclusions (PoM). Unlike the oriented laths observed in Jbilet Winselwan, all of the metal and pentlandite inclusions within Dho 1434 and PCA 02012 are irregular in shape, randomly oriented, and <~15 µm in size. Discussion: Kimura et al. [5] used Fe-sulfide textures and compositions to classify the dehydrated CM chondrites into three categories: A) unheated meteorites, with pentlandite commonly occuring along the boundary of pyrrhotite grains; B) mildly heated (~Stage II-III), characterized by blebs or lamellae of pentlandite in all pyrrhotite grains; C) severely heated (~Stage III-IV), with abundant Ni-rich metal inclusions and an absence of pentlandite exsolution textures. The evolving Fe-sulfide assemblages with increasing peak metamorphic temperature reflect exsolution of pentlandite from a monosulfide solid solution ([Fe,Ni]1-xS) at temperatures <610 °C, and thermal decompositon of pentlandite into Ni-rich metal at temperatures >610 °C [5,6]. The PoPn grains within Jbilet Winselwan are consistent with the mildly heated Category B Fe-sulfides. However, the small pyrrhotite inclusions within the larger patches of pentlandite are similar to the "snowflake texture" in CM and CR chondrites described by Singerling and Brearley [7], who interpreted them as primary Fe-sulfides that condensed from the solar nebula prior to accretion. We therefore suggest that Jbilet Winselwan contains distinct lithologies that experienced varying degrees of metamorphism [2], including at least one lithology that was never heated to temperatures >300°C and still retains primary Fe-sulfide grains from the nebula. In Dho 1434 and PCA 02012, the Fe-sulfide textures are consistent with both the mildly heated Category B and strongly heated Category C sulfides. This does not appear to be attributable to brecciation as the Fe-sulfide grains of different categories are not associated with a particular lithology in either of the sections. Instead, the range of Fesulfide textures could either result from different Fe-sulfide compositions prior to heating, or indicate localized variations (on the order of 1000's of µm) in peak metamorphic temperature. The presence of PnM and PoPnM grains, where pentlandite has only partially broken down into metal, suggests the metamorphic event was too rapid to allow for equilibrium to be maintained. Rapid and potentially heterogeneous heating is more consistent with impact heating rather than heating via solar radiation or radiogenic decay. [ABSTRACT FROM AUTHOR]

Introduction: Remote observations of C-type asteroid 162173 Ryugu show a weak absorption band at ~2.72 µm suggesting abundant Mg-rich phyllosilicates and a close affinity to the aqueously altered CI and/or thermally metamorphosed CY carbonaceous chondrites [1]. In December 2020, JAXA's Hayabusa2 mission successfully returned >5 g of material from the surface and sub-surface of Ryugu to Earth. Initial characterization of the pristine samples indicates that they are most similar to the CI chondrites [2]. The morphology, composition, and abundance of Fesulfides are distinctive features of the CI and CY carbonaceous chondrites, reflecting different degrees of aqueous and thermal processing on their parent bodies [3-6]. Here, we compare the coarse (>10 µm) Fe-sulfide grains in a Ryugu particle with those found within the matrices of CI and CY chondrites. Methods: The ten largest Fe-sulfide grains in polished sections of Ryugu particle C0025_01, the CI chondrites Ivuna and Orgueil, and the CY chondrites Yamato (Y)-82162 (Stage III, ~500 - 750°C) and Y-86720 (Stage IV, >750°C) were characterized using a ZEISS EVO 15LS scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). Backscattered electron (BSE) images, element maps, and spot analyses were acquired at 20 kV with a beam current of 1.5 nA. Additional BSE images of Fe-sulfide grains in Ryugu particle C0025_01 were obtained using an FEI Quanta field emission SEM at 9 kV. Results: The ten largest Fe-sulfide grains in Ryugu particle C0025_01, and the CI and CY chondrites, are typically euhedral to subhedral and lath-like in shape, although some irregular grains are also present (Fig. 1). The size range of the largest Fe-sulfide grains in C0025_01 (~30 - 70 µm) is similar to Ivuna (~40 - 80 µm) and Orgueil (50 - 150 µm, where 70 % of the grains are <80 µm). In comparison, the Fe-sulfide grains are larger in Y-82162 (~75 - 130 µm) and Y-86720 (~190 - 330 µm). The abundance of Fe-sulfide grains >10 µm in size in Ryugu (~1 area%) differs to that of both the CI chondrites (~0.13 - 0.3 area%) and the CY chondrites (~1.5 - 2.0 area %). The Fe-sulfide grains in Ryugu particle C0025_01 are pyrrhotite [(Fe,Ni)1-xS] with a composition close to (Fe,Ni)7S8, where the average value of x is 0.13. The pyrrhotite grains have an average Ni content of 1.80 ± 0.86 wt.% and an Fe/S ratio of 0.85 ± 0.01 at.% (no. of analyses = 6). This is consistent with the composition of the Fe-sulfide grains in Ivuna (n = 18) and Orgueil (n = 31), where the average value of x is 0.12, the Ni content is ~1.15 wt.%, and the Fe/S ratio is ~0.86 (similar to the results of [6]). Three of the Fe-sulfide grains in Ryugu particle C0025_01 contain <10 µm sized irregular inclusions of pentlandite ([Fe,Ni]9S8) that were not observed in either Ivuna or Orgueil. However, Bullock et al. [3] reported rare ~10 µm sized pentlandite inclusions in Ivuna (plus the CI chondrites Alais and Tonk). The composition of Fesulfides in CY chondrites is clearly distinct from Ryugu and the CIs; the laths are composed of stoichiometric troilite (FeS), and all grains contain <12 µm sized inclusions of pentlandite. In Y-82162 the pentlandite inclusions are oriented laths, whereas in Y-86720 they are unoriented irregular blebs that often occur adjacent to metal inclusions. Discussion: The largest Fe-sulfide grains in Ryugu particle C0025_01 are compositionally and morphologically very similar to those in the CI chondrites, providing further evidence for a close relationship between these materials. The Fe-sulfides in Ryugu and the CI chondrites most likely formed during near-complete low temperature (100°C) aqueous alteration. However, the presence of pentlandite inclusions in three of the ten largest Fe-sulfide grains suggests that hydration of Ryugu particle C0025_01 was less extensive than Ivuna and Orgueil [3]. The Fe-sulfides in Ryugu differ in size, composition, and morphology from those in the thermally metamorphosed CY chondrites, indicating that they were never heated to temperatures >500°C post-hydration [4,5,7]. [ABSTRACT FROM AUTHOR]

Introduction: Two groups of carbonaceous chondrites, the CV and CK chondrites, have similar compositions and textures. The CV chondrites are characterized by a fine-grained matrix, abundant chondrules (≈45 vol%), and Fe-rich olivine [1-3]. However, the CK chondrites contain Ni-rich olivine and Cr-rich magnetite [4]. Both groups also experience different degrees of metamorphism; while CV chondrites are metamorphosed to types 3.0 - 3.6, CK chondrites are metamorphosed to types 3.7 - 6.0 [5-7]. [8-10] suggest that CV and CK chondrites originate from the same parent body, whereas [11] argue for separate parent bodies due to the two groups' distinct magnetite compositions. The single-parent-body model could be strengthened with evidence of transitional CV-CK material. In its original classification, Northwest Africa (NWA) 10588 (CK3) was described as potentially intermediate between CV and CK chondrites [12]. We investigated the mineralogy and texture of this sample to revisit the classification and determine if it is transitional. Methodology: Elemental mapping, Energy Dispersive Spectroscopy (EDS), and backscattered electron (BSE) imaging was completed at Bowdoin College using a Tescan Vega3 Scanning Electron Microscope (SEM). Mineral chemistry analyses were obtained using a JEOL JXA 8200 electron microprobe at Washington University in St. Louis, where we analyzed olivine, pyroxene, sulfides, and oxides present within chondrules, chondrule rims, veins, and matrix. We obtained 90 olivine analyses in 13 chondrules, as well as 35 pyroxene analyses in 10 chondrules. Microprobe operating conditions included a 15kV potential, 25.0nA beam current, and 2μm beam size. Results: The average fayalite (Fa) content of chondrules ranges between Fa11-Fa33; the average is Fa22 (n = 90). Olivine consistently contains NiO abundances <0.04 wt%, and average Cr2O3 is <0.10wt%. The average ferrosilite (Fs) value for pyroxene is Fs11.83 (n = 35). BSE images show that the matrix is fine-grained and not recrystallized. Shock veins and metal grains are present throughout the sample; multiple oxide veins show evidence of replacement by terrestrial limonite (FeO™nH2O). Minimally-altered magnetite grains in the matrix and chondrule rims contain average TiO2, Cr2O3, and NiO abundances of 0.0wt%, 0.02wt%, and 0.13wt%, respectively. Additionally, 2 troilite grains and one chromite grain were identified. Comparison with CV and CK Chondrites: Characteristics consistent with the CV chondrites include the primitive matrix, NiO content of chondrule olivine, chondrule abundance (65-70 vol%), and abundances of TiO2 and NiO in magnetite. However, the equilibrated nature of chondrule olivine suggests that the sample has experienced metamorphism consistent with Type 3.7 CK chondrites [13]. An anomalous characteristic that is not consistent with CV nor CK chondrites is the low (<0.1wt%) Cr in magnetite. Based on these results, we suggest that NWA 10588 is an anomalous CV chondrite. However, the equilibrated chondrule olivine indicates that it experienced a higher degree of metamorphism than recorded in other CV chondrites. In order to better understand the thermal history of the CV parent body, future work will focus on the search for similarly-equilibrated CV chondrites. [ABSTRACT FROM AUTHOR]

Mineralogic, textural, and compositional studies of black and white matrices in the unequilibrated ordinary chondrite Tieschitz (H/L, 3.6) show, for the first time in an ordinary chondrite, the presence of widespread, randomly distributed geode-like voids and veins. Scanning electron microscope ( SEM) and transmission electron microscope ( TEM) studies show that these voids and veins are partially or completely filled by sodic-calcic amphiboles (winchite and barroisite). The occurrence of amphiboles provides unequivocal evidence of the involvement of fluids in the metamorphic evolution of the parent body of Tieschitz. The presence of amphiboles as the main hydrous phases, rather than phyllosilicates, indicates that aqueous fluids were present at or close to the peak of thermal metamorphism, rather than during the waning stages of the cooling history of the parent body. In addition, ferrous olivine crystals, in association with the amphibole, also establish an important link between thermal metamorphism and hydrous phases formed at high temperatures. Mineralogic and textural evidence suggests that the white matrix and amphibole formed contemporaneously from the same hydrous fluid, prior to the formation of ferrous olivine crystals. Additionally, a dark inclusion identified in the host chondrite has mineralogic, petrologic, and bulk chemical characteristics that are similar to the black matrix of host Tieschitz, suggesting that this dark inclusion was emplaced before or during parent body metamorphism. [ABSTRACT FROM AUTHOR]

Five hundred and eighteen cosmic spherules were identified among the 672 spherules handpicked from deep sea sediments by using Scanning Electron Microscope-Energy Dispersive Spectrometry (SEM-EDS). One of the spherules is found to enclose a spherical chondrule-like object that can be distinguished from the rest of the spherule by its shape, texture and composition and whose petrographic features, size and chemical composition are similar to chondrules from a chondritic meteorite, probably of carbonaceous chondritic nature. The present finding suggests that a small fraction of the particulate extraterrestrial matter enters the earth as fragments of larger meteorites. [ABSTRACT FROM AUTHOR]

- We optically classified 5682 micrometeorites (MMs) from the 2000 South Pole collection into textural classes, imaged 2458 of these MMs with a scanning electron microscope, and made 200 elemental and eight isotopic measurements on those with unusual textures or relict phases. As textures provide information on both degree of heating and composition of MMs, we developed textural sequences that illustrate how fine-grained, coarse-grained, and single mineral MMs change with increased heating. We used this information to determine the percentage of matrix dominated to mineral dominated precursor materials (precursors) that produced the MMs. We find that at least 75% of the MMs in the collection derived from fine-grained precursors with compositions similar to CI and CM meteorites and consistent with dynamical models that indicate 85% of the mass influx of small particles to Earth comes from Jupiter family comets. A lower limit for ordinary chondrites is estimated at 2-8% based on MMs that contain Na-bearing plagioclase relicts. Less than 1% of the MMs have achondritic compositions, CAI components, or recognizable chondrules. Single mineral MMs often have magnetite zones around their peripheries. We measured their isotopic compositions to determine if the magnetite zones demarcate the volume affected by atmospheric exchange during entry heating. Because we see little gradient in isotopic composition in the olivines, we conclude that the magnetites are a visual marker that allows us to select and analyze areas not affected by atmospheric exchange. Similar magnetite zones are seen in some olivine and pyroxene relict grains contained within MMs. [ABSTRACT FROM AUTHOR]

- Among the several hundred, mostly small meteorite fragments, recovered within the Almahata Sitta strewn field, one fragment (MS-CH), weighing 5.68 g, was detected that represents a new type of chondritic meteorite. The detection of short-lived cosmogenic radionuclides clearly indicates that this chondrite fragment results from a fresh meteorite fall consistent with the Almahata Sitta event in October 2008. The fundamental mineralogical characteristics of the Almahata Sitta fragment MS-CH can be summarized as follows: (1) the almost equilibrated olivine has high Fa contents of about 36 mole%. The fragment is of petrologic type 3.8 ± 0.1; (2) the metal abundance of the rock is on the order of 2.5 vol%; (3) the mean chondrule size has been determined to be roughly 450 μm; (4) point-counting and imaging indicate that the matrix abundance is approximately 45 vol%; (5) Cr-spinels have much lower TiO2 concentrations than typical spinels within R chondrites; (6) calcium-aluminum-rich inclusions are spinel-rich and severely altered having abundant Na- and/or Cl-rich alteration products. Spinel also contains significant concentrations of Fe and Zn; (7) magnetites and platinum-group element-rich phases (sulfides, tellurides, and arsenides) characteristic of both R and CK chondrites were not found in fragment MS-CH; and (8) the mean oxygen isotope composition of three small fragments of Almahata Sitta MS-CH is δ17O = +4.35‰, δ18O = +4.94‰, and Δ17O = +1.76‰. The oxygen isotopes relate MS-CH to R chondrites. No established chondrite group having all these characteristics exists. [ABSTRACT FROM AUTHOR]

Abstract: We have performed an experimental study of the aqueous alteration of the Allende CV3 carbonaceous chondrite under highly oxidizing conditions, in order to examine the alteration behavior of Allende’s anhydrous mineralogy. The experiments were carried out at temperatures of 100, 150, and 200°C, for time periods between 7 and 180 days, with water/rock ratios ranging from 1:1 to 9:1. Uncrushed cubes of Allende were used so that the spatial relationships between reactant and product phases could be examined in detail. Scanning electron microscope studies show that in all the experiments, even those of short duration (7 days), soluble salts of Ca and Mg (CaSO4, CaCO3, and MgSO4) precipitated on the sample surface, indicating that these elements are rapidly mobilized during alteration. In addition, iron oxides and hydroxides formed on the sample surfaces. The sulfates, carbonates, and the majority of the iron-bearing secondary minerals are randomly distributed over the surface of samples. In some instances the iron oxides and hydroxides are constrained to the boundaries of altering mineral grains. Transmission electron microscope studies show that the FeO-rich olivine in the interior of the samples has altered to form interlayered serpentine/saponite and Fe-oxyhydroxides. The degree of alteration increases significantly with increasing water/rock ratio, and to a lesser extent with increasing duration of heating. The serpentine/saponite forms both by direct replacement of the olivine in crystallographically oriented intergrowths, and by recrystallization of an amorphous Si-rich phase that precipitates in pore space between the olivine grains. The alteration assemblage bears many similarities to those found in altered carbonaceous chondrites, although in detail there are important differences, which we attribute to (a) the relatively high temperatures of our experiments and (b) comparatively short reaction times compared with the natural examples. In terms of mineral assemblage, our experiments most closely resemble alteration in the CI chondrites, although the degree of alteration of our experiments is much lower. CI chondrites contain serpentine/saponite intergrowths and veins of Ca-sulfate and Ca-carbonate as well as the Fe-oxyhydroxide, ferrihydrite. However, the phyllosilicate phases formed in our experiments are somewhat coarser-grained than the finest phyllosilicate fraction present in CI chondrites, suggesting that alteration of the CI chondrites occurred at lower temperatures. In terms of mineral assemblage, our experiments also appear to come close to matching CR chondrites, although we infer that CR alteration probably occurred at temperatures <100°C, based on the very fine-grained size of phyllosilicates in CR matrices. [Copyright &y& Elsevier]

A petrographic and scanning electron microscopic study of the four CO3 chondrites Kainsaz, Ornans, Lance´, and Warrenton reveals for the first time that dark inclusions (DIs) occur in all the meteorites. DIs are mostly smaller in size than those reported from CV3 chondrites. They show evidence suggesting that they were formed by aqueous alteration and subsequent dehydration of a chondritic precursor and so probably have a formation history similar to that of DIs in CV3 chondrites. DIs in the CO3 chondrites consist mostly of fine-grained, Fe-rich olivine and can be divided into two types on the basis of texture. Type I DIs contain rounded, porous aggregates of fine grains in a fine-grained matrix and have textures suggesting that they are fragments of chondrule pseudomorphs. Veins filled with Fe-rich olivine are common in type I DIs, providing evidence that they experienced aqueous alteration on the parent body. Type II DIs lack rounded porous aggregates and have a matrix-like, featureless texture. Bulk chemical compositions of DIs and mineralogical characteristics of olivine grains in DIs suggest that these two types of DIs have a close genetic relationship.The DIs are probably clasts that have undergone aqueous alteration and subsequent dehydration at a location different from the present location in the meteorites. The major element compositions, the mineralogy of metallic phases, and the widely dispersed nature of the DIs suggest that their precursor was CO chondrite material. The CO parent body has been commonly regarded to have been dry, homogeneous, and unprocessed. However, the DIs suggest that the CO parent body was a heterogeneous conglomerate consisting of water-bearing regions and water-free regions and that during asteroidal heating, the water-bearing regions were aqueously altered and subsequently dehydrated. Brecciation may also have been active in the parent body.The DIs and the matrices are similarly affected by thermal metamorphism in their own host CO3 chondrites (petrologic subtypes 3.1 to 3.6), but the degree of the secondary processing (aqueous alteration and subsequent dehydration) of the DIs has no apparent correlation with the petrologic grades of the host chondrites. These observations suggest that the DIs had been incorporated into the host chondrites before the thermal metamorphism took place and that the secondary processes that affected the DIs largely occurred before the thermal metamorphism. [Copyright &y& Elsevier]