Experimental simulation of atmospheric entry of micrometeorites

Abstract— Depending on their velocity, entry angle and mass, micrometeorites suffer different degrees of heating during their deceleration in the Earth's atmosphere, leading, in most cases, to significant textural, mineralogical and chemical modifications. One of these modifications is the formation of a magnetite shell around most micrometeorites, which until now could not be reproduced, neither theoretically nor experimentally. The present study was designed to better understand the entry heating effects on micrometeorites and especially the formation of the magnetite shell. Fragments of the Murchison and Orgueil meteorites were used as analogue material in flash‐heating experiments performed in a high‐temperature furnace; effects of temperature, heating duration, and oxygen fugacity were investigated. These experiments were able to reproduce most of the micrometeorites textures, from the vesicular fine‐grained micrometeorites to the totally melted cosmic spherules. For the first time, the formation of a magnetite shell could be observed on micrometeorite analogues. We suggest that the most plausible mechanism for the formation of this shell is a peripheral partial melting with subsequent magnetite crystallization at the surface of the micrometeorite. Furthermore, with this study, it is possible to estimate the atmospheric entry conditions of micrometeorites, such as the peak temperature and the duration of flash‐heating.     

Chondrules in Antarctic micrometeorites

Abstract— Previous studies of unmelted micrometeorites (>50 μm) recovered from Antarctic ice have concluded that chondrules, which are a major component of chondritic meteorites, are extremely rare among micrometeorites. We report the discovery of eight micrometeorites containing chondritic igneous objects, which strongly suggests that at least a portion of coarse‐grained crystalline micrometeorites represent chondrule fragments. Six of the particles are identified as composite micrometeorites that contain chondritic igneous objects and fine‐grained matrix. These particles suggest that at least some coarse‐grained micrometeorites (cgMMs) may be derived from the same parent bodies as fine‐grained micrometeorites. The new evidence indicates that, contrary to previous suggestions, the parent bodies of micrometeorites broadly resemble the parent asteroids of chondrulebearing carbonaceous chondrites.     

A microchondrule‐bearing micrometeorite and comparison with microchondrules in CM chondrites

We report the discovery of a partially altered microchondrule within a fine‐grained micrometeorite. This object is circular, <10 μm in diameter, and has a cryptocrystalline texture, internal zonation, and a thin S‐bearing rim. These features imply a period of post‐accretion parent body aqueous alteration, in which the former glassy igneous texture was subject to hydration and phyllosilicate formation as well as leaching of fluid‐mobile elements. We compare this microchondrule to three microchondrules found in two CM chondrites: Elephant Moraine (EET) 96029 and Murchison. In all instances, their formation appears closely linked to the late stages of chondrule formation, chondrule recycling, and fine‐grained rim accretion. Likewise, they share cryptocrystalline textures and evidence of mild aqueous alteration and thus similar histories. We also investigate the host micrometeorite's petrology, which includes an unusually Cr‐rich mineralogy, containing both Mn‐chromite spinel and low‐Fe‐Cr‐rich (LICE) anhydrous silicates. Because these two refractory phases cannot form together in a single geochemical reservoir under equilibrium condensation, this micrometeorite's accretionary history requires a complex timeline with formation via nonequilibrium batch crystallization or accumulation of materials from large radial distances. In contrast, the bulk composition of this micrometeorite and its internal textures are consistent with a hydrated carbonaceous chondrite source. This micrometeorite is interpreted as a fragment of fine‐grained rim material that once surrounded a larger parent chondrule and was derived from a primitive carbonaceous parent body; either a CM chondrite or Jupiter family comet.     

Shock fabrics in fine‐grained micrometeorites

The orientations of dehydration cracks and fracture networks in fine‐grained, unmelted micrometeorites were analyzed using rose diagrams and entropy calculations. As cracks exploit pre‐existing anisotropies, analysis of their orientation provides a mechanism with which to study the subtle petrofabrics preserved within fine‐grained and amorphous materials. Both uniaxial and biaxial fabrics are discovered, often with a relatively wide spread in orientations (40°–60°). Brittle deformation cataclasis and rotated olivine grains are reported from a single micrometeorite. This paper provides the first evidence for impact‐induced shock deformation in fine‐grained micrometeorites. The presence of pervasive, low‐grade shock features in CM chondrites and CM‐like dust, anomalously low‐density measurements for C‐type asteroids, and impact experiments which suggest CM chondrites are highly prone to disruption all imply that CM parent bodies are unlikely to have remained intact and instead exist as a collection of loosely aggregated rubble‐pile asteroids, composed of primitive shocked clasts.     

A new variant of saponite‐rich micrometeorites recovered from recent Antarctic snowfall

Abstract– Eight saponite‐rich micrometeorites with very similar mineralogy were found from the recent surface snow in Antarctica. They might have come to Earth as a larger meteoroid and broke up into pieces on Earth, because they were recovered from the same layer and the same location of the snow. Synchrotron X‐ray diffraction (XRD) analysis indicates that saponite, Mg‐Fe carbonate, and pyrrhotite are major phases and serpentine, magnetite, and pentlandite are minor phases. Anhydrous silicates are entirely absent from all micrometeorites, suggesting that their parental object has undergone heavy aqueous alteration. Saponite/serpentine ratios are higher than in the Orgueil CI chondrite and are similar to the Tagish Lake carbonaceous chondrite. Transmission electron microscope (TEM) observation indicates that serpentine occupies core regions of fine‐grained saponite, pyrrhotite has a low‐Ni concentration, and Mg‐Fe carbonate shows unique concentric ring structures and has a mean molar Mg/(Mg + Fe) ratio of 0.7. Comparison of the mineralogy to hydrated chondrites and interplanetary dust particles (IDPs) suggests that the micrometeorites are most similar to the carbonate‐poor lithology of the Tagish Lake carbonaceous chondrite and some hydrous IDPs, but they show a carbonate mineralogy dissimilar to any primitive chondritic materials. Therefore, they are a new variant of saponite‐rich micrometeorite extracted from a primitive hydrous asteroid and recently accreted to Antarctica.     

Heating experiments simulating atmospheric entry heating of micrometeorites: Clues to their parent body sources

Abstract— Depending on their velocity, entry angle and mass, extraterrestrial dust particles suffer certain degrees of heating during entry into Earth's atmosphere, and the mineralogy and chemical composition of these dust particles are significantly changed. In the present study, pulse-heating experiments simulating the atmospheric entry heating of micrometeoroids were carried out in order to understand the mineralogical and chemical changes quantitatively as well as to estimate the peak temperature experienced by the particles during entry heating. Fragments of the CI chondrites Orgueil and Alais as well as pyrrhotites from Orgueil were used as analogue material. The experiments show that the volatile elements S, Zn, Ga, Ge, and Se can be lost from 50 to 100 μm sized CI meteorite fragments at temperatures and heating times applicable to the entry heating of similar sized cosmic dust particles. It is concluded that depletions of these elements relative to CI as observed in micrometeorites are mainly caused by atmospheric entry heating. Besides explaining the element abundances in micrometeorites, the experimentally obtained release patterns can also be used as indicators to estimate the peak heating of dust particles during entry. Using the abundances of Zn and Ge and assuming their original concentrations close to CI, a maximum heating of 1100–1200 °C is obtained for previously analyzed Antarctic micrometeroites. Thermal alteration also strongly influenced the mineralogy of the meteorite fragments. While the unheated samples mainly consisted of phyllosilicates, these phases almost completely transformed into olivine and pyroxene in the fragments heated to ≥800 °C. Therefore, dust particles that still contain hydrous minerals were probably never heated to temperatures ≥800 °C in the atmosphere. During continued heating, the grain size of the newly formed silicates increased and the composition of the olivines equilibrated. Applying these results quantitatively to Antarctic micrometeorites, typical peak temperatures in the range of 1100–1200 °C during atmospheric entry heating are deduced. This temperature range corresponds to the one obtained from the volatile element concentrations measured in these micrometeorites and points to an asteroidal origin of the particles.     

Chondritic micrometeorites from the Transantarctic Mountains

Abstract– On the basis of morphological and petrographic characteristics, eight “giant” unmelted micrometeorites in the 300–1100 μm size range were selected from the Transantarctic Mountain micrometeorite collection, Victoria Land, Antarctica. Mineralogical and geochemical data obtained by means of scanning electron microscopy, electron probe microanalyses, and synchrotron X‐ray diffraction allow their classification as chondritic micrometeorites. The large size of the micrometeorites increases considerably the amount of mineralogical and geochemical information compared to micrometeorites in smaller size fractions, therefore allowing a better definition of their parent material. A large variety of material is observed: five micrometeorites are related to unequilibrated and equilibrated ordinary chondrite, one to CV chondrite, one to CM chondrite, and one to CI chondrite parent materials. Besides reporting the first occurrence of a CV‐like micrometeorite, our study shows that the abundance of chondritic material supports observations from recent studies on cosmic spherules that a large part of the micrometeorite flux in this size range is of asteroidal origin.     

Fine‐grained precursors dominate the micrometeorite flux

Abstract– 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.     

The origins of I‐type spherules and the atmospheric entry of iron micrometeoroids

The Earth's extraterrestrial dust flux includes a wide variety of dust particles that include FeNi metallic grains. During their atmospheric entry iron micrometeoroids melt and oxidize to form cosmic spherules termed I‐type spherules. These particles are chemically resistant and readily collected by magnetic separation and are thus the most likely micrometeorites to be recovered from modern and ancient sediments. Understanding their behavior during atmospheric entry is crucial in constraining their abundance relative to other particle types and the nature of the zodiacal dust population at 1 AU. This article presents numerical simulations of the atmospheric entry heating of iron meteoroids to investigate the abundance and nature of these materials. The results indicate that iron micrometeoroids experience peak temperatures 300–800 K higher than silicate particles explaining the rarity of unmelted iron particles which can only be present at sizes of <50 μm. The lower evaporation rates of liquid iron oxide leads to greater survival of iron particles compared with silicates, which enhances their abundance among micrometeorites by a factor of 2. The abundance of I‐types is shown to be broadly consistent with the abundance and size of metal in ordinary chondrites and the current day flux of ordinary chondrite‐derived MMs arriving at Earth. Furthermore, carbonaceous asteroids and cometary dust are suggested to make negligible contributions to the I‐type spherule flux. Events involving such objects, therefore, cannot be recognized from I‐type spherule abundances in the geological record.     

Noble gas isotopes and mineral assemblages of Antarctic micrometeorites collected at the meteorite ice field around the Yamato Mountains

Abstract— From November 1998 to January 1999, the 39th Japanese Antarctic Research Expedition (JARE) conducted a large‐scale micrometeorite collection at 3 areas in the meteorite ice field around the Yamato Mountains, Antarctica. The Antarctic micrometeorites (AMMs) collected were ancient cosmic dust particles. This is in contrast with the Dome Fuji AMMs, which were collected previously from fresh snows in 1996 and 1997 and which represent modern micrometeorites. To determine the noble gas concentrations and isotopic compositions of individual AMMs, noble gas analyses were carried out using laser‐gas extraction for 35 unmelted Yamato Mountains AMMs and 3 cosmic spherules. X‐ray diffraction analyses were performed on 13 AMMs before the noble gas measurement and mineral compositions were determined. AMMs are classified into 4 main mineralogical groups, defined from the heating they suffered during atmospheric entry. Heating temperatures of AMMs, inferred from their mineral compositions, are correlated with ⁴He concentrations and reflect the effect of degassing during atmospheric entry. Jarosite, an aqueous alteration product, is detected for 4 AMMs, indicating the aqueous alteration during long‐time storage in Antarctic ice. Jarosite‐bearing AMMs have relatively low concentrations of 4He, which is suggestive of loss during the alteration. High ³He/⁴He ratios are detected for AMMs with high ²⁰Ne/⁴He ratios, showing both cosmogenic ³He and preferential He loss. SEP (solar energetic particles)‐He and Ne, rather than the solar wind (SW), were dominant in AMMs, presumably showing a preferential removal of the more shallowly implanted SW by atmospheric entry heating. The mean ²⁰Ne/²²Ne ratio is 11.27 ± 0.35, which is close to the SEP value of 11.2. Cosmogenic ²¹Ne is not detected in any of the particles, which is probably due to the short cosmic ray exposure ages. Ar isotopic compositions are explained by 3‐component mixing of air, Q, and SEP‐Ar. Ar isotopic compositions can not be explained without significant contributions of Q‐Ar. SEP‐Ne contributed more than 99% of the total Ne. As for ³⁶Ar and ³⁸Ar, the abundance of the Q component is comparable to that of the SEP component. ⁸⁴Kr and ¹³²Xe are dominated by the primordial component, and solar‐derived Xe is almost negligible.     

Rare,metal micrometeorites from the Indian Ocean

Metal in various forms is common in almost all meteorites but considerably rare among micrometeorites. We report here the discovery of two metal micrometeorites, i.e., (1) an awaruite grain similar to those found in the metal nodules of CV chondrites and (2) a metal micrometeorite of kamacite composition enclosing inclusions of chromite and merrillite. This micrometeorite appears to be a fragment of H5/L5 chondrite. These metal micrometeorites add to the inventory of solar system materials that are accreted by the Earth in microscopic form. They also strengthen the argument that a large proportion of material accreted by the Earth that survives atmospheric entry is from asteroidal sources.     

Ca‐Al‐Fe‐rich inclusion in the Vigarano CV3 chondrite

Abstract– An anomalous Ca‐Al‐Fe‐rich spherical inclusion (CAFI) was found in the Vigarano CV3 chondrite. The CAFI has an igneous texture and contains large amounts of almost pure and coarse‐grained hercynite grains (approximately 56 vol%) as well as refractory phases such as grossite and perovskite. However, melilite and Mg‐spinel, which are common in ordinary Ca‐Al‐rich inclusions, are very rare (<1 vol%). Another unique characteristic of the CAFI is the presence in its core of dmitryivanovite (CaAl₂O₄), which was formed by shock metamorphism of a low‐pressure form of CaAl₂O₄ that was originally crystallized from a molten droplet. The fine‐grained hercynite and unidentified aluminous phase in the rim of the CAFI may have been produced from grossite during aqueous alteration in the Vigarano parent body.     

Ordinary chondritic micrometeorites from the Indian Ocean

Extraterrestrial particulate materials on the Earth can originate in the form of collisional debris from the asteroid belt, cometary material, or as meteoroid ablation spherules. Signatures that link them to their parent bodies become obliterated if the frictional heating is severe during atmospheric entry. We investigated 481 micrometeorites isolated from ~300 kg of deep sea sediment, out of which 15 spherules appear to have retained signatures of their provenance, based on their textures, bulk chemical compositions, and relict grain compositions. Seven of these 15 spherules contain chromite grains whose compositions help in distinguishing subgroups within the ordinary chondrite sources. There are seven other spherules which comprise either entirely of dusty olivines or contain dusty olivines as relict grains. Two of these spherules appear to be chondrules from an unequilibrated ordinary chondrite. In addition, a porphyritic olivine pyroxene (POP) chondrule‐like spherule is also recovered. The bulk chemical composition of all the spherules, in combination with trace elements, the chromite composition, and presence of dusty olivines suggest an ordinary chondritic source. These micrometeorites have undergone minimal frictional heating during their passage through the atmosphere and have retained these features. These micrometeorites therefore also imply there is a significant contribution from ordinary chondritic sources to the micrometeorite flux on the Earth.     

Fine‐grained material associated with a large sulfide returned from Comet 81P/Wild 2

In a consortium analysis of a large particle captured from the coma of comet 81P/Wild 2 by the Stardust spacecraft, we report the discovery of a field of fine‐grained material (FGM) in contact with a large sulfide particle. The FGM was partially located in an embayment in the sulfide. As a consequence, some of the FGM appears to have been protected from damage during hypervelocity capture in aerogel. Some of the FGM particles are indistinguishable in their characteristics from common components of chondritic‐porous interplanetary dust particles, including glass with embedded metals and sulfides and equilibrated aggregates. The sulfide exhibits surprising Ni‐rich lamellae, which may indicate that this particle experienced a long‐duration heating event after its formation but before incorporation into Wild 2.     

Chondritic ingredients: I. Usual suspects and some oddballs in the Leoville CV3 meteorite

Abstract– Reduced CV3 chondrites are relatively pristine rocks and prime candidates for studies exploring processes that predated planet formation. We closely examined the petrographic features and trace elemental composition of different CV3 constituents in the accretionary breccia Leoville. The petrographic results are presented here. Our sample (2.2 cm²) is not brecciated. The main ingredient—about 65 area%—is fine‐ to coarse‐grained ferromagnesian type I chondrules. Minor constituents (in order of 2‐D abundance) include refractory inclusions, Al‐rich chondrules, and very fine‐crystalline clasts of moderately volatile composition. Type II chondrules and metal nuggets occur sporadically. The chondrule–matrix ratio is approximately 3:1. Medium‐ and coarse‐grained chondrules exhibit porphyritic textures, probably caused by incomplete melting, and frequent, partial or continuous, recrystallized dust rims. The fine‐grained population most likely represents randomly sectioned dust rims. The rim material and some of the medium‐grained objects are relatively troilite‐rich. Iron‐nickel metal is rare. In addition, almost all constituents show strikingly ragged or convoluted outlines. Only a few, rim‐less components exhibit smooth contours. Evidence for incomplete melting and the formation of recrystallized or igneous rims in carbonaceous chondrites is well established, suggesting that both processes were widespread events. The observed features in Leoville support this conclusion. In addition, our findings indicate that surface abrasion in a turbulent dust‐filled regime may have taken place after the consolidation of dust rims. Alternatively, the irregular, convoluted nature of at least the rimmed chondrules may have been inherent to the dust accretion event and was not erased by subsequent heating.     

Melted micrometeorites from Antarctic ice with evidence for the separation of immiscible Fe-Ni-S liquids during entry heating

Abstract— We report the discovery of four large (>50 μm) cosmic spherules (CSs) and a single scoriaceous micrometeorite (SMM) that contain evidence for the separation of immiscible Fe-Ni-S liquids during atmospheric entry heating. The particles contain segregated Fe-rich regions dominated by either Ni-S-bearing Fe-oxides or iron sulphides and have textural relations that suggest these separated from the silicate portions of the particles as metallic liquids. The oxides, which may be hydrous, are thought to result from alteration of metal and sulphide. The compositions of the silicate portions of the CSs are equivalent to spherules without Fe-rich regions, implying that metallic liquids are exsolved during the heating of most spherules, but completely separate. The single SMM has a very different composition from other scoriaceous particles, and the occurrence of an exsolved metallic liquid probably indicates extreme reduction during entry heating. The pyrolysis of carbonaceous materials is the most likely explanation for reduction and suggests that the precursor material of this particle was unusually C-rich. This SMM might be, therefore, an appropriate candidate for a large melted anhydrous or smectite interplanetary dust particle (IDP). The exsolution of immiscible Fe-Ni-S liquids during entry heating will result in systematic changes in the compositions of the remaining silicate melt.     

The entry heating and abundances of basaltic micrometeorites

Basaltic micrometeorites (MMs) derived from HED‐like parent bodies have been found among particles collected from the Antarctic and from Arctic glaciers and are to date the only achondritic particles reported among cosmic dust. The majority of Antarctic basaltic particles are completely melted cosmic spherules with only one unmelted particle recognized from the region. This paper investigates the entry heating of basaltic MMs in order to predict the relative abundances of unmelted to melted basaltic particles and to evaluate how mineralogical differences in precursor materials influence the final products of atmospheric entry collected on the Earth's surface. Thermodynamic modeling is used to simulate the melting behavior of particles with compositions corresponding to eucrites, diogenites, and ordinary chondrites in order to evaluate degree of partial melting and to make a comparison between the behavior of chondritic particles that dominate the terrestrial dust flux and basaltic micrometeroids. The results of 120,000 simulations were compiled to predict relative abundances and indicate that the phase relations of precursor materials are crucial in determining the relative abundances of particle types. Diogenite and ordinary chondrite materials exhibit similar behavior, although diogenite precursors are more likely to form cosmic spherules under similar entry parameters. Eucrite particles, however, are much more likely to melt due to their lower liquidus temperatures and small temperature interval of partial melting. Eucrite MMs, therefore, usually form completely molten cosmic spherules except at particle diameters <100 μm. The low abundance of unmelted basaltic MMs compared with spherules, if statistically valid, is also shown to be inconsistent with a low velocity population (12 km s^?1) and is more compatible with higher velocities which may suggest a near‐Earth asteroid source dominates the current dust production of basaltic MMs.     

Density,porosity, mineralogy,and internal structure of cosmic dust and alteration of its properties during high‐velocity atmospheric entry

X‐ray microtomography (XMT), X‐ray diffraction (XRD), and magnetic hysteresis measurements were used to determine micrometeorite internal structure, mineralogy, crystallography, and physical properties at μm resolution. The study samples include unmelted, partially melted (scoriaceous), and completely melted (cosmic spherules) micrometeorites. This variety not only allows comparison of the mineralogy and porosity of these three micrometeorite types but also reveals changes in meteoroid properties during atmospheric entry at various velocities. At low entry velocities, meteoroids do not melt and their physical properties do not change. The porosity of unmelted micrometeorites varies considerably (0–12%) with one friable example having porosity around 50%. At higher velocities, the range of meteoroid porosity narrows, but average porosity increases (to 16–27%) due to volatile evaporation and partial melting (scoriaceous phase). Metal distribution seems to be mostly unaffected at this stage. At even higher entry velocities, complete melting follows the scoriaceous phase. Complete melting is accompanied by metal oxidation and redistribution, loss of porosity (1 ± 1%), and narrowing of the bulk (3.2 ± 0.5 g cm^?3) and grain (3.3 ± 0.5 g cm^?3) density range. Melted cosmic spherules with a barred olivine structure show an oriented crystallographic structure, whereas other subtypes do not.     

A TEM study of four particles extracted from the Stardust track 80

Abstract— Four particles extracted from track 80 at different penetration depths have been studied by analytical transmission electron microscopy (ATEM). Regardless of their positions within the track, the samples present a comparable microstructure made of a silica rich glassy matrix embedding a large number of small Fe‐Ni‐S inclusions and vesicles. This microstructure is typical of strongly thermally modified particles that were heated and melted during the hypervelocity impact into the aerogel. X‐ray intensity maps show that the particles were made of Mg‐rich silicates (typically 200 nm in diameter) cemented by a fine‐grained matrix enriched in iron sulfide. Bulk compositions of the four particles suggest that the captured dust particle was an aggregate of grains with various iron sulfide fraction and that no extending chemical mixing in the bulb occurred during the deceleration. The bulk S/Fe ratios of the four samples are close to CI and far from the chondritic meteorites from the asteroidal belt, suggesting that the studied particles are compatible with chondritic‐porous interplanetary dust particles or with material coming from a large heliocentric distance for escaping the S depletion.     

Yamato 86029: Aqueously altered and thermally metamorphosed CI‐like chondrite with unusual textures

Abstract— We describe the petrologic and trace element characteristics of the Yamato 86029 (Y‐86029) meteorite. Y‐86029 is a breccia consisting of a variety of clasts, and abundant secondary minerals including coarse‐ and fine‐grained phyllosilicates, Fe‐Ni sulfides, carbonates, and magnetite. There are no chondrules, but a few anhydrous olivine‐rich grains are present within a very fine‐grained phyllosilicate‐rich matrix. Analyses of 14 thermally mobile trace elements suggest that Y‐86029 experienced moderate, open‐system thermal metamorphism. Comparison with data for other heated carbonaceous chondrites suggests metamorphic temperatures of 500–600°C for Y‐86029. This is apparent petrographically, in partial dehydration of phyllosilicates to incompletely re‐crystallized olivine. This transformation appears to proceed through ‘intermediate’ highly‐disordered ‘poorly crystalline’ phases consisting of newly formed olivine and residual desiccated phyllosilicate and their mixtures. Periclase is also present as a possible heating product of Mg‐rich carbonate precursors. Y‐86029 shows unusual textures rarely encountered in carbonaceous chondrites. The periclase occurs as unusually large Fe‐rich clasts (300–500 μm). Fine‐grained carbonates with uniform texture are also present as small (10–15 μm in diameter), rounded to sub‐rounded ‘shells’ of ankerite/siderite enclosing magnetite. These carbonates appear to have formed by low temperature aqueous alteration at specific thermal decomposition temperatures consistent with thermodynamic models of carbonate formation. The fine and uniform texture suggests crystallization from a fluid circulating in interconnected spaces throughout entire growth. One isolated aggregate in Y‐86029 also consists of a mosaic of polycrystalline olivine aggregates and sulfide blebs typical of shock‐induced melt re‐crystallization. Except for these unusual textures, the isotopic, petrologic and chemical characteristics of Y‐86029 are quite similar to those of Y‐82162, the only other heated CI‐like chondrite known. They were probably derived from similar asteroids rather than one asteroid, and hence may not necessarily be paired.