# Submicron-sized anhydrous crystalline silicates and their relation to amorphous silicate in the matrix of Acfer 094

Hugues Leroux, Pierre-Marie Zanetta, Corentin Le Guillou, Maya Marinova

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Hugues Leroux, Pierre-Marie Zanetta, Corentin Le Guillou, Maya Marinova. Submicron-sized anhydrous crystalline silicates and their relation to amorphous silicate in the matrix of Acfer 094. Geochimica et Cosmochimica Acta, 2025, Geochimica et Cosmochimica Acta, 390, pp.160-180. 10.1016/j.gca.2024.11.028. hal-05136688

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Submitted on 30 Jun 2025

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# Submicron-sized anhydrous crystalline silicates and their relation to amorphous silicate in the matrix of Acfer 094

Hugues Leroux<sup>a,*</sup>, Pierre-Marie Zanetta<sup>a,1</sup>, Corentin Le Guillou<sup>a</sup>, Maya Marinova<sup>b</sup>

## ARTICLE INFO

Associate editor: Yangting Lin

**Keywords:**

Chondrites

Matrix

Early solar system

Electron microscopy

## ABSTRACT

The study of pristine chondrites provides insight into nebular processes that occurred prior to the accretion of small-sized parent bodies. The interchondrule matrix of the primitive chondrite Acfer 094 is characterized by the presence of submicron-sized anhydrous crystalline aggregates embedded in a silicate groundmass that is mostly amorphous. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDXS) were employed to investigate the matrix of Acfer 094 and its components.

The amorphous silicate groundmass is homogeneous in composition and exhibits a Mg depletion relative to the solar value. It embeds Fe-Ni nanosulfides, whose typical size is a few tens of nanometers. Nanometer-sized fibrous silicates are rare in the groundmass indicating a low degree of aqueous alteration. Submicron-sized Mg-rich crystalline silicates (olivine and pyroxene) occur either as isolated grains or as aggregates. These submicron-sized crystalline silicates occupy around 25 % of the matrix volume. The isolated grains display a wide range of shapes, from rounded to irregularly angular, and could have originated from the fragmentation of type I chondrules or from nebular condensation. The aggregates exhibit variable morphologies and grain sizes (typically a few tens of nm). They are chemically equilibrated, and likely formed by solid-state thermal annealing of amorphous precursors.

The Acfer 094 matrix contains a range of components that have undergone varying degrees of thermal modification. A significant proportion of a precursor material (i.e. nebular dust) resembling matrix is likely to have undergone one or more brief and intense thermal events, potentially associated with the process of chondrule formation. These events resulted in the formation of magnesium-rich anhydrous silicates (forsterite and enstatite) at high temperatures that were embedded in the matrix of Acfer 094 as isolated grains and crystalline aggregates.

## 1. Introduction

The majority of chondrites are composed of high-temperature coarse-grained mineral assemblages (chondrules, Fe,Ni-metal, refractory inclusions and amoeboid olivine aggregates) embedded in a fine-grained, low-temperature matrix (e.g., Krot et al. 2014). The small size of the grains in the matrix (mostly <1 µm) renders it particularly susceptible to post-accretional modifications due to the high specific surface areas. Consequently, matrices are strongly exposed to transformations on parent bodies, such as aqueous alteration or thermal metamorphism. Chondrites of petrologic types 1 and 2 are defined by the extensive aqueous alteration they underwent on their parent bodies

(e.g., Brearley, 2006). This process results in the formation of abundant phyllosilicates (such as serpentine and saponite), the mobilization of elements in fluids, and the precipitation of carbonates, magnetite, or sulfates. In the case of thermal metamorphism, the matrix recrystallizes into an assemblage of anhydrous silicates (e.g., Huss et al., 2006). Only a few chondrites, classified as type 3.0, have been observed to have avoided significant transformations by thermal metamorphism or aqueous alteration. The matrix of type 3.0 chondrites is therefore of great value as it can shed light on a wide range of processes, potentially including those that occur in the interstellar medium and molecular clouds, as well as those that affected the dust in the early Solar System, prior to the accretion of the parent bodies.

<footnote><sup>a</sup> Univ. Lille, CNRS, INRAE, Centrale Lille, UMR 8207 – UMET – Unité Matériaux et Transformations, F-59000 Lille, France</footnote>

<footnote><sup>b</sup> Univ. Lille, CNRS, INRAE, Centrale Lille, Univ. Artois, FR 2638-IMEC-Institut Michel-Eugène Chevreul, F-59000 Lille, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: Hugues.leroux@univ-lille.fr (H. Leroux).</footnote>

<footnote n="1">¹ New affiliation: UJM, CNRS, ENSL, LGL-TPE UMR5276, F-42023, Saint-Étienne, France.</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Low magnification bright field TEM image of a representative area of the matrix of Acfer 094. The matrix is dominated by an amorphous groundmass and contains abundant anhydrous and crystalline Mg-rich silicates (olivine and pyroxene). The larger ones are labelled (En = enstatite; Fo = forsterite: Sulf = Fe-sulfide) but there are many other smaller ones that are difficult to see on bright field or HAADF images. They are much easier to identify on chemical maps (see Fig. 5).</figcaption></figure>

Given the small size of the grains in chondrite matrices, the study of the mineralogy is carried out by transmission electron microscopy (TEM). A number of studies showed that matrices of the most pristine chondrites are dominated by a Fe-rich amorphous silicate groundmass, which contains nano-sized sulfides, organic matter and Mg-rich anhydrous crystalline silicates (e.g., Brearley, 1993; Greshake, 1997; Abreu and Brearley, 2010; Le Guillou and Brearley, 2014; Leroux et al., 2015; Hopp and Vollmer, 2018; Dobrica and Brearley, 2020; Vollmer et al. 2020; Ohtaki et al., 2021; Zanetta et al., 2021; Zanetta et al., 2022). Amorphous silicates mixed with anhydrous crystalline silicates are also abundant in chondritic porous interplanetary dust particles (CP-IDPs) and in some fine-grained and ultracarbonaceous micrometeorites (e.g., Bradley, 1994a; Keller and Messenger, 2011; Dobrica et al., 2012; Noguchi et al., 2015; Yabuta et al., 2017). This Nonequilibrated assemblage of submicron-sized silicate grains thus appear to be ubiquitous components of the protosolar nebula and could be major elementary building blocks of planetesimals. Although much work has been devoted to understanding the origin of amorphous silicates, crystalline silicates have received comparatively less attention. As amorphous silicates, they formed prior to the incorporation in their parent body. They could have originated from thermal annealing in the solar nebulae or could be high-temperature condensates or fragments of chondrules.

Acfer 094 is a type 3.0 ungrouped carbonaceous chondrite, a breccia that shares mineralogical, petrologic, isotopic affinities to the CM and CO groups (Bischoff and Geiger 1995; Newton et al. 1995). Acfer 094 has a low (S1) shock stage. It is considered to be one of the most pristine objects of the early Solar System available. Its components (chondrules, Fe,Ni-metal, refractory inclusions, amoeboid olivine aggregates and matrix) mostly escaped aqueous alteration and thermal metamorphism. The matrix contains amorphous silicates (Greshake, 1997; Hopp and Vollmer, 2018; Matsumoto et al., 2019; Ohtaki et al., 2021). The minimally transformed nature of Acfer 094 is also evidenced by the presence of abundant presolar grains, including silicates (e.g., Nguyen and Zinner, 2004; Nguyen et al., 2007; Floss et al., 2008; Vollmer et al., 2009). Matsumoto et al. (2019) discovered an ultraporous lithology (UPL) in the fine-grained matrix. This local porous texture was interpreted as being representative of the removal by evaporation and/or melting of

primordial ice previously filling pore spaces, thereby attesting to the preservation of the matrix texture. Altogether, amorphous silicates, presolar grains and porosity textures indicate that the Acfer 094 chondrite has not undergone significant aqueous alteration and thermal metamorphism. Despite these clear indices of primitivity, Wasson and Rubin (2010) showed that the chemical composition of the matrix is more homogeneous than that of other carbonaceous chondrites, possibly due to elemental redistribution during terrestrial weathering in hot desert and/or asteroidal processes. Alexander et al. (2007) observed that the insoluble organic matter is weathered during the early stages of parent body processing and/or terrestrial weathering. Keller and Messenger (2012), Hopp and Vollmer (2018) and Ohtaki et al. (2021) reported a high and homogeneous Fe oxidation state in the amorphous silicate, which they interpreted as the consequence of parent body aqueous alteration which led to hydration and oxidation.

The study presented here employs a quantitative approach to investigate the mineralogy of the Acfer 094 matrix. This is achieved through the use of analytical transmission electron microscopy, which allows for the study of the chemical composition, size, and modal abundance of the matrix's main components. The results demonstrate that anhydrous Mg-rich crystalline silicates are frequently found in aggregates, exhibiting distinctive characteristics (shape, grain-size, composition) that suggest diverse origins. This leads to the formulation of a scenario that attempts to link the presence of these Mg-rich crystalline silicates with Fe-rich amorphous groundmass.

## 2. Materials and methods

A polished thick section of Acfer 094 was first observed by scanning electron microscopy (SEM) using a JEOL JSM-7800F (see supplementary material). Six electron-transparent lamellae were prepared by focused ion beam (FIB) from the matrix, taking care to avoid large angular grains that are likely chondrule fragments. The FIB instrument was a FEI Strata 235 dual beam, IEMN – University of Lille. The FIB sections were typically 15 × 8 µm² in size and 100 nm thick. The samples were studied using a TEM FEI Tecnai G2-20 twin (200 kV) to obtain conventional bright field images and to examine the structural state of the matrix components by selected area electron diffraction (SAED). A Thermo-
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Bright field TEM images (a) Area dominated by an amorphous silicate groundmass, which appears to be present as domains whose characteristic size is typically ranges between 100 and 200 nm. These amorphous silicate domains have a smooth homogeneous texture and are often separated from each other by a lumpy material containing nanoporosities (which appear lighter). Nano-Fe-sulfides are common and are mainly located at the boundary of the amorphous domains. (b) Area almost completely amorphous but with abundant lumpy to fine-fibrous material (brighter regions containing nanoporosity) at the boundary of the amorphous domains. A aggregate of crystalline silicate is present (surrounded by a dashed white line). (c) Large forsterite (Fo) grain surrounded by a 100 nm thick continuous amorphous rim.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 2. (continued).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 3. Selected area electron diffraction patterns (a) on the smooth domains. The diffuse rings confirm the amorphous state of the domains (b) Two fine rings at 0.25 and 0.15 nm are present on SAED patterns recorded on the lumpy texture associated with the amorphous domains, superimposed on the diffuse rings. The spots correspond to Fe-sulfides.</figcaption></figure>

Fischer TITAN Themis (300 keV) was employed to obtain high angle annular dark field (HAADF) images using an electron probe size of about 0.1 nm in the scanning TEM mode (STEM). Quantitative chemical data were obtained using energy dispersive X-ray spectroscopy (EDXS) with a Super-X windowless, 4 quadrant SDD detector. Thirty chemical maps were recorded using the VELOX software. The acquisition conditions were as follows: the beam current was maintained within in the range 200–300 pA in order to minimize beam damage, a probe size of approximately 2 nm, a semi-convergence angle of 10 mrad and a dwell time of 2 µs. The VELOX software stores the acquisition data frame by frame, enabling the chemical evolution to be tracked during map acquisition (1–4 h). For quantification, the Bruker® Esprit software (version 1.9) was employed. The k-factors and absorption corrections were applied assuming a homogeneous thickness of 100 nm. k-factors of

the major elements relative to Si were experimentally determined using standard minerals for O, Mg, Al, Ca, S, Fe. Uncertainties associated with EDXS data are typically around 3 % for major elements and 10 % for minor elements. For image analysis, we used the software ImageJ for size and surface measurements of the grains (anhydrous silicates and sulfides) observed in the chemical maps. The SEM and TEM instruments utilized are located within the electron microscopy facility of the University of Lille at the Chevreul Institute.

### 3. Results

The matrix was found to be well homogeneous at the SEM scale, as previously reported by Wasson and Rubin (2010). It contains numerous silicate grains, particularly olivine, which is rich in Mg. They have an
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Fe-Mg-Si ternary diagrams showing the composition field of the matrix. Data extracted from EDS-STEM hyperspectral maps. Each point corresponds to an area of 200 * 200 nm² (volume 4.10⁻³ µm³) (a) Composition field of a probed area FIB sample (blind method), including the anhydrous silicates and sulfides. The total surface area probed is about 12 µm². Branches are drawn on this diagram, indicating the presence of forsterite, enstatite and Fe-sulfide grains. Note that some amorphous domains deviate from the average Mg-poor amorphous composition and tend to have a composition close to the CI composition to a CI composition. The CI composition is represented by a yellow star (data from Lodders 2010). (b) Representative diagram illustrating the extent of the composition field of the amorphous silicate at the sub-micrometer scale in a given FIB section (c) Composition of an amorphous rim around a crystalline silicate (enstatite – En) compared to the composition of the surrounding amorphous (Am) bulk. The composition of the amorphous rim is found to be close to the CI composition compared to the amorphous silicate groundmass around the rimmed grains.</figcaption></figure>

angular shape and a size of several tens of micrometers. These grains, which look like clasts, are probably fragments of chondrules. Such material was not included in the samples used for the study. This section brings together the results obtained by TEM at the nanoscale on the fine-grained matrix. On each topic, additional data are available in the **supplementary material**.

### 3.1. Amorphous silicate groundmass

The matrix is dominated by an amorphous silicate groundmass that contains fine-grained Fe-Ni sulfides, crystalline anhydrous silicates, organic matter and porosity (Fig. 1). The amorphous silicates appear as irregularly shaped domains with a size range of 100–200 nm (Fig. 2). The majority of the amorphous domains are texturally homogeneous and free of inclusions. Nano-grains of sulfides are predominantly located at the periphery of the domains, with a size range of 10–50 nm. Two distinct populations of sulfides can be identified, distinguished by their Ni content. The Ni-poor sulfides exhibit a size smaller than the thickness of the FIB sections (100–150 nm), which precludes accurate analysis due

to their overlap with silicates. It is frequently observed that a fine lumpy texture surrounds the smooth texture of the amorphous domains (Fig. 2a). This lumpy texture is attributed to the presence of nano-sized pores. Indeed, these areas appear brighter on the TEM bright field images (and darker on HAADF images shown in the **supplementary material**). The pores range in size from less than 1 nm up to approximately 20 nm. Marginally, fine-grained fibrous silicates are present, particularly at the boundaries of porosity (Fig. 2b). A smooth-textured amorphous rim surrounds a few crystalline silicates, with a constant thickness of 50 to 100 nm (Fig. 2c). SAED reveals that the smooth amorphous silicate displays diffuse rings (Fig. 3a). The lumpy textured areas are poorly crystallized (Fig. 3b), as evidenced by the presence of two thin rings at 0.26 and 0.15 nm on the diffraction patterns, as previously reported by Matsumoto et al. (2019) in Acfer 094. The two thin rings are superimposed on two diffuse rings characteristic of an amorphous material. Additionally, some thick and deep veins filled with an iron-rich fibrous material were observed. These are oxyhydroxide veins that probably originate from terrestrial alteration, as previously interpreted by Greshake (1997). These veins are often present in contact with large
<caption>**Table 1**</caption>

Element/Si atomic ratio for the main elements present in the amorphous silicate groundmass and average composition of the matrix. The composition of the amorphous silicate was measured using regions of interest of 200 × 200 nm² on the elemental STEM-EDXS maps (N = 541). It is noteworthy that the standard deviation is low, indicating that the composition field of the amorphous silicate is relatively well homogeneous (see also Fig. 4a). The average composition of the matrix is based on 16 EDS maps, with a total surface area of 193 µm². The standard deviation is given at 1σ. The average composition of the FIB section (thus including Fe-sulfides and anhydrous silicates) is compared with the average of the Acfer 094 matrix, as measured by electron microprobe analyses by Wasson and Rubin (2010)¹ and Patzer et al. (2021)². Element/Si ratios are also compared with those of the Acfer 094 bulk composition (Patzer et al., 2021)³ and the solar composition (Lodders, 2010)⁴.

<table><thead><tr><th>El./Si (at.)</th><th>Mg/Si</th><th>Fe/Si</th><th>Ca/Si</th><th>S/Si</th><th>Al/Si</th><th>Ni/Si</th></tr></thead><tbody><tr><td>Amorph. silicates</td><td>0.42 ± 0.09</td><td>0.78 ± 0.09</td><td>0.07 ± 0.01</td><td>0.07 ± 0.04</td><td>0.09 ± 0.02</td><td>0.04 ± 0.01</td></tr><tr><td>FIB section average</td><td>0.70 ± 0.10</td><td>0.75 ± 0.12</td><td>0.07 ± 0.01</td><td>0.18 ± 0.06</td><td>0.08 ± 0.01</td><td>0.04 ± 0.01</td></tr><tr><td>Matrix average¹</td><td>0.78</td><td>0.79</td><td>0.08</td><td>0.17</td><td>0.03</td><td>—</td></tr><tr><td>Matrix average²</td><td>0.77</td><td>1.01</td><td>0.09</td><td>0.21</td><td>0.08</td><td>0.06</td></tr><tr><td>Acfer Bulk³</td><td>0.99</td><td>0.80</td><td>0.08</td><td>0.12</td><td>0.09</td><td>0.05</td></tr><tr><td>CI composition⁴</td><td>1.03</td><td>0.87</td><td>0.06</td><td>0.44</td><td>0.08</td><td>0.05</td></tr></tbody></table>

crystalline silicate grains.

The chemistry of the fine-grained matrix components was studied by STEM-EDXS mapping. The amorphous silicate groundmass shows only minor variations between samples (Fig. 4 and Table 1). It was found to be depleted in Mg compared to the solar composition (Table 1), as previously noted by previous authors. The composition of the amorphous silicate is found to be relatively close to those reported by Gre-shake (1997), Keller and Messenger (2012), and Ohtaki et al. (2021), but significantly differs from those reported by Hopp and Vollmer (2018), who found much lower Mg/Si ratios (Table 1). In rare instances, weakly abundant amorphous silicate domains richer in Mg (and closer to a CI composition) are locally present (Fig. 4). The amorphous rim present around a few crystalline silicates also shows a composition richer in Mg (Fig. 2c and 4c-d). The composition of the amorphous silicates in close proximity to the ferrihydrite veins falls within the range of the average composition of the amorphous phase. The ferrihydrite contains a small quantity of Si but almost no Mg (atomic ratios Si/Fe = 0.29 and Mg/Fe = 0.03).

## 3.2. Crystalline material

The amorphous silicate groundmass contains anhydrous Mg-rich crystalline silicates and Fe-Ni sulfide grains. These crystalline silicates are not readily identifiable on conventional bright-field TEM images unless they are oriented in Bragg condition (Fig. 1). On STEM-HAADF images, the Mg-rich silicates exhibit a slightly lighter contrast than the Fe-rich amorphous silicate groundmass, yet they remain challenging to discern. The crystalline silicates (mostly Mg-rich olivine and pyroxene) are clearly distinguishable on the EDXS chemical maps due to their Mg-rich and Fe-poor composition in contrast to the surrounding amorphous silicate (Fig. 5). Furthermore, the stoichiometry of the crystalline phases (predominantly Mg-rich olivine and pyroxene) can be readily discerned in maps presented in false color (RGB). The imaging techniques employed collectively revealed that the crystalline phases occupy about 30 vol% of the sample volume (≈12 ± 2% olivine, ≈11 ± 2% pyroxene and 8 ± 1% Fe-sulfides, based on EDS maps and ImageJ analysis). The grains display a large diversity of size and shape, and two populations were distinguished: isolated grains (Fig. 6) and aggregates (Fig. 7).

The size of the largest grains is around one micrometer (range between 0.3 and 2 µm). They occur as isolated grains within the silicate

groundmass (Fig. 1, Fig. 6). Some grains have an irregular shape with sharp edges whereas others have a rounded shape with very soft and curved edges (Fig. 6). The majority of the grains are pure enstatite (En99.100) and forsterite (Fo99-100). The minor element (Al, Ca, Ti, Cr, Mn) contents are low (Fig. 8a and 8b and Table 2). Some of the enstatite grains have an elongated shape in the [100] crystallographic direction. They contain a high density of clino-ortho inversion on the (100) plane. They resemble enstatite whiskers encountered in chondritic porous interplanetary dust particles (e.g., Bradley et al. 1983). Whiskers or platelets of forsterite are also present (Fig. 6b). Independently of their shape (rounded or sharp edges), forsterite grains are free of crystal defects. In addition, among the isolated grains, one was identified as a Fe-rich olivine grain (Fo43), one as a large Ca-rich pyroxene (En57Wo41Fs2) that exhibited high twinning on (100), and one as a Al-Ca-Ti-Si-rich grain (Fig. 6c and 6d, Table 2). The latter is strongly chemically zoned, with Al and Ti-rich edges. It is likely an Al-Ti-Ca-rich clinopyroxene (diffraction pattern was not obtained for this grain). Several compositional profiles have been recorded through the interface between the Mg-silicate and the iron-rich amorphous silicate. No Fe-Mg zonation could be observed. The interface is always chemically very sharp.

Numerous aggregates of minerals are also present. The grain size is homogeneous within a given aggregate but varies from a few tens to a few hundreds of nanometers among them. Two categories of aggregates can be distinguished: i) coarse-grained aggregates (grain size > 150 nm) are relatively compact and are composed of an assemblage of olivine (Fo90-100), pyroxene (En90-100) and Fe-sulfides in variable proportion; ii) small-grains aggregates (grain size < 150 nm) have a more irregular shape reminiscent of fluffy aggregates. They are mainly composed of olivine (Fo50-95) and do not contain Low-Ca pyroxenes.

Aggregates with larger grains (grain size ranging from ten to several hundred nanometers) are constituted by pyroxene (Ca-rich and Ca-poor) and olivine with subhedral grain shapes (Fig. 7a and 7b). Fe-sulfides are also present, though no Ni-rich sulfides were observed. The relative phase proportion is highly variable across different aggregates. Some aggregates are composed exclusively of pyroxene, while others are comprised solely of olivine, or a combination of the two. The majority of the silicates in aggregates exhibit low Fe content, falling within the ranges Fo90 to Fo100 for olivine and En90 to En100 for pyroxene. One aggregate comprising solely olivine was observed to exhibit elevated Fe content (Fa40). The composition of pyroxenes differs significantly from that of large, isolated grains, as the majority of them contain a significant quantity of Ca, Al, Cr and Mn (Table 3 and Fig. 8). The aggregates are more or less compact, and their shape is often rounded. In some of them, the grain boundaries are clean and cohesive, exhibiting an equilibrated texture as evidenced by the presence of nearly 120° triple junctions (Fig. 7c-d). Within the aggregates, Fe-sulfides grains are common. They are Ni-poor, in contrast with the small sulfide grains within the silicate groundmass which display two populations (Ni-poor and Ni-rich). A significant number of these coarse-grained aggregates show evidence of alteration at their boundaries. In this instance, the crystalline grains lack cohesion and are separated by low-density, nanoporous, fibrous material of a nature and composition comparable to that which was identified at the boundaries of the amorphous domains (Fig. 7a).

Aggregates with the smallest grain sizes (below 150 nm) are dominated by olivine and contain some high Ca-pyroxenes and Ni-poor iron sulfides but almost no low-Ca pyroxene (Fig. 9). There is a diversity in the size of these aggregates, ranging from a few grains to hundreds of grains. Aggregates show a diversity of shape. They can be rounded or have an irregular shape, which resembles a non-compact assemblage. In the 100 nm thick FIB section, the non-compact crystalline aggregates can extend up to 2 µm. Considering a 3D aggregate, the largest of them must contain several thousand grains. The grain size in the crystalline aggregates shows a Gaussian distribution centered around 50 nm (Fig. 10a). Additionally, Fe-sulfides grains are present in the aggregates.
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** (a-b) Representative STEM-EDXS elemental maps of the Acfer 094 matrix displaying the intensity of X-ray counts of Fe (red), Mg (green) and Si (blue). The amorphous silicate groundmass appears in purple, Mg-rich olivine in green, enstatite in light blue and Fe-sulfides in red. Note the high diversity of size and shape for the olivine and enstatite, most of them are found as aggregates. The outline of one of them, which has a complex shape, is highlighted with a dotted white line.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** TEM bright field images of isolated Mg-rich silicates (a) Large, irregularly shaped enstatite (En) grain. (b) Whisker or platelet of forsterite (Fo) in the center of the image. Another coarse forsterite grain is also present in the upper right. (c) Twinned diopside (Di) grain. (d) Al-Si-Ca-Ti-rich grain (here termed fassaite – Fas). The grain is chemically zoned with edges enriched in Ti and Al.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 6. (continued).</figcaption></figure>

They are significantly larger (100–200 nm) than the sulfides nano-grains associated with the amorphous silicate groundmass. For this category of aggregates, the grain boundaries were found to be systematically altered and filled with a Fe-rich lumpy to fibrous material. The chemical composition of the crystalline silicates is not readily quantifiable due to the fact that the grains are frequently smaller in size than the thickness of the FIB sample. In order to minimize superposition with the iron-rich fibrous material present in the grain boundaries, we measured the grains’ compositions in aggregates with grain sizes typically of the order of 100 nm or higher. In general, the grain size and composition of a given aggregate are relatively homogeneous, with a close Mg and Fe concentration between grains (Fig. 10b). This indicates that the crystals are chemically equilibrated. The olivine aggregates composition ranges

from Fo$_{95}$ to Fo$_{40}$, depending on the aggregate. Pyroxene is present as a minor phase and is mostly Ca-rich (Wo$_{30-45}$). Only a few low-Ca pyroxene grains were identified among the two hundred analyses conducted on these small grains.

### 3.3. Aqueous alteration phases

Lumpy to fine fibrous material containing nanopores is frequently associated with the amorphous silicates, at the edge of the domains (Fig. 2b). In crystalline aggregates, number of grain boundaries are thick and filled by this lumpy material, which tends to be clearly fibrous in some of them (Fig. 7a, 9a, 9b). In some regions of the FIB sections, there are larger, highly transformed areas of homogeneous lumpy material
<figure><img src="image_12.png" /><figcaption>**Fig. 7.** TEM bright field images of crystalline aggregates (a) Aggregate composed of olivine, pyroxene, and Fe-sulfide grains. The grains are not cohesive and are separated by an Fe-rich fibrous material. A carbon nanoglobule with a central hole is indicated by an arrow. (b) Aggregate dominated by enstatite with a high density of clino-ortho inversion along (100) planes. Most of the grain boundaries are clean but some areas are filled with fibrous material. (c) Large crystalline aggregate (only a part is shown here) consisting of olivine (Fo₅₀). The grain boundaries are clean, and exhibit equilibrated triple junctions. The grains contain Al and Cr-rich inclusions (arrowed). Their small size (20 – 50 nm) precludes the determination of their composition but they probably correspond to Al-Cr spinels. The aggregate is in contact with a large, rounded enstatite grain. (d) Small, rounded aggregate consisting of three olivine grains (Fo₅₀) with clean boundaries and equilibrated triple junction.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 7. (continued).</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 8.** Composition diagram of the crystalline silicates, distribution of minor elements, in at.% (a) Composition diagrams for minor elements (Ca vs Al) in low Ca Px in the isolated grains (in green) and in the coarse-grained aggregates (in blue). Their different composition fields suggest that they were formed by different processes. (b) Compositional diagram for Mn and Cr in olivine in the isolated grains and in the coarse-grained aggregates.</figcaption></figure>

containing nano-voids (Fig. 11a). Its composition and texture are comparable to those present between amorphous domains and in the grain boundaries of the crystalline aggregates. Some enstatite grains exhibit local replacement at their edges by a homogeneous Fe-rich fine fibrous material (Fig. 11b). There is no evidence for alteration of olivine. Apart from these indices of alteration, no typical secondary phases that are common in altered carbonaceous chondrites were detected, including well-developed phyllosilicates, carbonates, and Fe-bearing secondary phases such as magnetite, ferrihydrite, or cronstedtite. It is noteworthy that the preferential alteration of enstatite likely results to an underestimation of the amount of crystal grains in the matrix, as it should be at the time of the parent body accretion. It is also likely to affect the estimation of the initial ratio olivine/pyroxene. This can explain partly why the aggregates with small grain size contain only olivine but do not contain low-Ca pyroxene.

# 4. Discussion

## 4.1. A high abundance of Mg-rich crystalline anhydrous silicates

The observations made by SEM and electron microprobe revealed the presence of crystalline phases in the Acfer 094 matrix. These phases were most often observed in the form of large lithic fragments (>4 µm) originating from chondrule fragmentation. This finding is consistent

with previous studies by Wasson and Rubin (2010), Ebel et al. (2016), Zanetta et al. (2021), and Patzer et al. (2021). In Acfer 094, Vaccaro et al. (2023) estimated the average size of these crystalline silicates to be 175 nm, using SEM-EDXS high-resolution elemental maps at low accelerating voltage. Our study, conducted using by TEM, explored the finest grain size range of the matrix (< 3 µm), with an estimated crystalline fraction of approximately 30 % (equivalent to 12 ± 2 vol% olivine, 11 ± 2 vol% pyroxene, and 8 ± 1 vol% sulfides). These estimates are lower than those of Greshake (1997), who reported in the matrix approximately 30 vol% olivine, 20 vol% pyroxene and 5 vol% sulfides. Given that Acfer 094 shows no evidence of thermal metamorphism (e.g., Newton et al., 1995; Grossman and Brearley, 2005), it can be concluded that these small crystalline silicates did not form through secondary processes by the interaction with the amorphous silicates in which they are embedded. Indeed, no evidence of interdiffusion was detected in the forsterite and enstatite grains in contact with the iron-rich amorphous silicate. It is therefore proposed that this population of small (< 3 µm) anhydrous silicates were formed prior to the accretion of the parent body, either by chondrule fragmentation or other nebular processes, and incorporated into the matrix.

Our study has shown that the fine fraction of crystalline silicates in the Acfer 094 matrix can be classified into two distinct categories: isolated grains of forsterite and enstatite and polycrystalline aggregates. We will discuss these occurrences in the next two paragraphs and attempt to interpret how they were formed. We will consider the following possibilities: condensation, crystallization from a liquid, and subsolidus crystallization.

The isolated grains of forsterite and enstatite are several hundred nanometers in size and contain very low Fe and minor element contents (Figs. 1, 6, 8a). Their origin has been discussed by Greshake (1997) who concluded, based on the rounded shape of many grains, that they were formed by condensation and would not be the result of a chondrule fragmentation process. Additionally, we have also identified round-shaped grains, a morphology that is not commensurate with that of fragments. We have also found enstatite whiskers comparable to those present in chondritic porous interplanetary dust particles (CP-IDPs) (e. g., Bradley et al., 1983; 1994b), in ultracarbonaceous Antarctic micro-meteorites (UCAMMs) (Dobrica et al., 2012; Noguchi et al., 2015), or in primitive chondrite matrices (Leroux et al., 1995). We have also identified a whisker/platelet of forsterite. These very peculiar shapes (whiskers and rounded grains) cannot have been produced by chondrule fragmentation, thus a formation by direct condensation at high temperature, under reducing conditions, in the protoplanetary nebula is a more consistent explanation. However, our study shows that many of the isolated crystalline silicates have a complex, irregular shape, with flat surfaces and sharp edges. This angular-shaped population is more compatible with a fine-grained population derived from chondrule fragmentation. In order to attribute these populations to a specific process (condensation or chondrule formation), we attempted to compare the minor element content (Al, Ca, Cr, Mn) of the isolated forsterite and enstatite of various shapes using published data of Acfer 094 type I chondrules (data from Jones Ushikubo et al., 2012) and ameboid olivine aggregates (AOAs) (data from Krot et al., 2004a; 2004b) in Acfer 094, as well as data from type I chondrules in other primitive chondrites (data from Jones, 1994; Berlin, 2009; Ushikubo and Kimura, 2021). However, the composition of isolated shards and rounded grains is similar, and is comparable to that of AOAs and Type 1 chondrules. Therefore, we propose that the matrix of Acfer 094 contains two populations of isolated crystalline silicates (condensation and chondrule formation), although we can only use their shape as an indicator of their origin. Consequently, the determination of the relative proportions of these two populations resulting from these two formation mechanisms is limited.

Crystalline silicates are also present in the form of aggregates, displaying a diversity of mineralogy and grain size (Figs. 7-9). For those whose grain size is greater than 150 nm, they consist of olivine,
<caption>**Table 2**</caption>

Representative compositions of large, isolated forsterite (#1–3) and enstatite (#4–6) grains recorded by EDS-STEM (at. %). Analysis #7 shows the composition of an unusual Fe-rich olivine. Analyses #8 and #9 give respectively the composition of the large twinned augite (Fig. 6c) and the Ti-Al-rich diopside (Fig. 6d). nd = not detected.

<table><thead><tr><th>Element (at%)</th><th>#1</th><th>#2</th><th>#3</th><th>#4</th><th>#5</th><th>#6</th><th>#7</th><th>#8</th><th>#9</th></tr></thead><tbody><tr><td>O</td><td>57.2</td><td>57.4</td><td>57.5</td><td>60.0</td><td>60.1</td><td>59.9</td><td>57.0</td><td>60.1</td><td>60.1</td></tr><tr><td>Al</td><td>nd</td><td>0.06</td><td>nd</td><td>0.04</td><td>0.16</td><td>nd</td><td>nd</td><td>1.62</td><td>9.61</td></tr><tr><td>Mg</td><td>27.8</td><td>27.5</td><td>27.2</td><td>19.9</td><td>19.3</td><td>20.1</td><td>12.2</td><td>10.7</td><td>4.72</td></tr><tr><td>Si</td><td>14.3</td><td>14.7</td><td>14.9</td><td>19.9</td><td>20.0</td><td>19.8</td><td>14.0</td><td>19.0</td><td>13.3</td></tr><tr><td>Ca</td><td>0.26</td><td>0.08</td><td>0.07</td><td>0.03</td><td>0.12</td><td>0.08</td><td>0.51</td><td>7.56</td><td>9.40</td></tr><tr><td>Ti</td><td>nd</td><td>nd</td><td>0.03</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>0.24</td><td>1.47</td></tr><tr><td>Cr</td><td>0.05</td><td>0.31</td><td>nd</td><td>0.13</td><td>0.21</td><td>0.04</td><td>nd</td><td>0.36</td><td>1.12</td></tr><tr><td>Mn</td><td>0.16</td><td>0.12</td><td>0.06</td><td>0.04</td><td>nd</td><td>nd</td><td>0.17</td><td>0.04</td><td>0.10</td></tr><tr><td>Fe</td><td>0.22</td><td>0.10</td><td>0.18</td><td>0.05</td><td>0.23</td><td>0.07</td><td>16.1</td><td>0.31</td><td>0.07</td></tr><tr><td colspan="10">Cations per formula unit for olivine (#1–3 and #7) and for pyroxene (#4–6, 8–9)</td></tr><tr><td>Al</td><td>nd</td><td>0.004</td><td>nd</td><td>0.004</td><td>0.016</td><td>nd</td><td>nd</td><td>0.162</td><td>0.961</td></tr><tr><td>Mg</td><td>1.95</td><td>1.92</td><td>1.91</td><td>1.98</td><td>1.93</td><td>2.01</td><td>0.85</td><td>1.07</td><td>0.472</td></tr><tr><td>Si</td><td>1.00</td><td>1.03</td><td>1.04</td><td>1.98</td><td>2.00</td><td>1.98</td><td>0.98</td><td>1.91</td><td>1.33</td></tr><tr><td>Ca</td><td>0.02</td><td>0.006</td><td>0.005</td><td>0.003</td><td>0.011</td><td>0.008</td><td>0.036</td><td>0.759</td><td>0.940</td></tr><tr><td>Ti</td><td>nd</td><td>nd</td><td>0.002</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td><td>0.024</td><td>0.147</td></tr><tr><td>Cr</td><td>0.003</td><td>0.021</td><td>nd</td><td>0.013</td><td>0.021</td><td>0.004</td><td>nd</td><td>0.036</td><td>0.112</td></tr><tr><td>Mn</td><td>0.011</td><td>0.008</td><td>0.004</td><td>0.004</td><td>nd</td><td>nd</td><td>0.012</td><td>0.004</td><td>0.010</td></tr><tr><td>Fe</td><td>0.015</td><td>0.007</td><td>0.012</td><td>0.005</td><td>0.023</td><td>0.007</td><td>1.13</td><td>0.031</td><td>0.007</td></tr></tbody></table>

pyroxenes, and Fe-(Ni-poor)-sulfides in very variable proportions. Their composition differs greatly from that of isolated grains, showing a higher iron and minor elements content. This suggests that they are not inherited from chondrule fragmentation or condensation processes (Fig. 8a). Therefore, they likely result from a process affecting the dust of the nebula prior to accretion into the matrix. The silicates that fed the protoplanetary disk were mostly amorphous and had their origin in the interstellar medium (e.g., Kemper, 2004). Among the various processes that could have affected the dust in the nebula, our favored scenario is solid-state crystallization of amorphous silicates due to a high thermal event in the protoplanetary disk. This interpretation is supported by the microstructure's similarities between the Acfer 094 aggregates, and the products of annealing experiments conducted beyond the melting temperature of silicates having compositions close to that of the protoplanetary disk (e.g., Murata et al., 2007; Whattam et al., 2008; Roskosz et al., 2009; Gillot et al., 2011; Matsuno et al., 2012; Morgano et al., 2023). Additionally, they exhibit notable similarities in terms of shape, grain size, and equilibrated compositions, with the characteristics of the so-called equilibrated aggregates (EAs) found in CP-IPDs (Bradley, 1994b; Keller and Messenger, 2005; Messenger et al., 2015; Ishii, 2019;

Bradley et al., 2022) and UCAMMs (Dobrică et al., 2012; Noguchi et al., 2015; Noguchi et al., 2017; Yabuta et al., 2017). In Acfer 094, they were recently highlighted by Matsumoto et al. (2019). The crystalline EAs are interpreted as originating from thermal annealing process in the nebula, leading to subsolidus crystallization of amorphous precursors (Keller and Messenger, 2005) or crystallization from a melt droplet (Bradley, 1994b). The diverse textures of the studied crystalline aggregates likely reflect a wide range of temperatures to which the dust was subjected in the protoplanetary disk. This may also include sintering and re-equilibration of some initially crystalline assemblages, such as AOAs or a crystallization product from a previous thermal event. The local temperature was certainly higher for aggregates which possess cohesive and clean grain boundaries (Fig. 7c). It is also for these aggregates that the grain size is the largest. For those with non-cohesive grain boundaries, the temperature should be lower and did not allow for the sintering of the grains or the evacuation of the residual interstitial phase. In CP-IDPs and UCAMMs, the interspace between grains of aggregates is filled by a SiO₂-rich glassy material (Keller and Messenger, 2005; Dobrică et al., 2012). In Acfer 094, this interspace displays clear evidence of transformation into a Fe-rich fine-fibrous material due to

<caption>**Table 3**</caption>

Representative compositions of low-ca pyroxene (#1,2,3), high ca-pyroxenes (#4,5,6) and olivine (#7,8,9) in coarse grained aggregates (edxs-stem). nd = not detected.

<table><thead><tr><th>Element (at%)</th><th>#1</th><th>#2</th><th>#3</th><th>#4</th><th>#5</th><th>#6</th><th>#7</th><th>#8</th><th>#9</th></tr></thead><tbody><tr><td>O</td><td>60.0</td><td>60.1</td><td>60.1</td><td>59.6</td><td>59.6</td><td>60.7</td><td>57.4</td><td>57.2</td><td>57.7</td></tr><tr><td>Al</td><td>1.47</td><td>1.61</td><td>0.63</td><td>0.35</td><td>2.29</td><td>0.20</td><td>nd</td><td>nd</td><td>0.05</td></tr><tr><td>Mg</td><td>17.7</td><td>17.5</td><td>17.2</td><td>12.4</td><td>8.19</td><td>10.7</td><td>26.94</td><td>27.9</td><td>23.4</td></tr><tr><td>Si</td><td>19.1</td><td>19.3</td><td>19.6</td><td>18.5</td><td>17.3</td><td>20.8</td><td>14.8</td><td>14.3</td><td>15.3</td></tr><tr><td>Ca</td><td>0.72</td><td>0.62</td><td>0.63</td><td>7.67</td><td>8.86</td><td>6.13</td><td>0.24</td><td>0.05</td><td>0.15</td></tr><tr><td>Ti</td><td>0.05</td><td>0.04</td><td>0.07</td><td>0.08</td><td>0.40</td><td>0.06</td><td>0.06</td><td>nd</td><td>0.03</td></tr><tr><td>Cr</td><td>0.26</td><td>0.27</td><td>0.40</td><td>0.76</td><td>0.02</td><td>0.80</td><td>0.10</td><td>0.10</td><td>0.15</td></tr><tr><td>Mn</td><td>0.35</td><td>0.34</td><td>0.61</td><td>0.08</td><td>nd</td><td>0.18</td><td>0.26</td><td>0.40</td><td>0.35</td></tr><tr><td>Fe</td><td>0.28</td><td>0.23</td><td>0.69</td><td>0.61</td><td>0.14</td><td>0.37</td><td>0.15</td><td>0.03</td><td>2.76</td></tr><tr><td colspan="10">Cations per formula unit for pyroxene (#1–6) and for olivine (#7–9)</td></tr><tr><td>Al</td><td>0.147</td><td>0.161</td><td>0.063</td><td>0.035</td><td>0.246</td><td>0.020</td><td>nd</td><td>nd</td><td>0.003</td></tr><tr><td>Mg</td><td>1.77</td><td>1.75</td><td>1.72</td><td>1.23</td><td>0.880</td><td>1.09</td><td>1.89</td><td>1.96</td><td>1.63</td></tr><tr><td>Si</td><td>1.91</td><td>1.93</td><td>1.96</td><td>1.83</td><td>1.86</td><td>2.12</td><td>1.04</td><td>1.00</td><td>1.09</td></tr><tr><td>Ca</td><td>0.721</td><td>0.062</td><td>0.063</td><td>0.758</td><td>0.952</td><td>0.625</td><td>0.017</td><td>0.003</td><td>0.011</td></tr><tr><td>Ti</td><td>0.005</td><td>0.004</td><td>0.007</td><td>0.008</td><td>0.004</td><td>0.006</td><td>0.004</td><td>nd</td><td>0.002</td></tr><tr><td>Cr</td><td>0.026</td><td>0.027</td><td>0.040</td><td>0.075</td><td>0.002</td><td>0.081</td><td>0.007</td><td>0.007</td><td>0.011</td></tr><tr><td>Mn</td><td>0.035</td><td>0.034</td><td>0.061</td><td>0.008</td><td>nd</td><td>0.018</td><td>0.002</td><td>0.028</td><td>0.025</td></tr><tr><td>Fe</td><td>0.028</td><td>0.023</td><td>0.069</td><td>0.060</td><td>0.015</td><td>0.038</td><td>0.011</td><td>0.002</td><td>0.196</td></tr></tbody></table>
<figure><img src="image_15.png" /><figcaption>**Fig. 9.** TEM bright field images of fine-grained crystalline aggregates. (a) Aggregate of olivine and sulfide grains Some Fe-rich fibrous material is present in the grain boundaries. (b) Irregularly shaped aggregate containing olivine and sulfide grains. On the left side, an enstatite whisker is visible. It contains a high density of planar defects along (100) planes which correspond to clino-ortho pyroxene alternation.</figcaption></figure>

aqueous alteration on the parent body. This configuration is very close to that of chondrule's mesostases which are known to be highly sensitive to aqueous alteration.

A noteworthy outcome of our investigation is the identification of aggregates comprising small grains (≤150 nm) that are almost exclusively olivine. Their morphology is intricate, and there is no evidence of sintering or development of clean interfaces between grains (Fig. 5). These aggregates are interpreted as having formed from initially predominantly amorphous grains, which were then subjected to a moderate annealing process at a temperature between 900 and 1000 K. At this annealing temperature, which is below or close to the glass transition temperature of the amorphous component, the mobility of the cations is reduced. There are several orders of magnitude between the mobility of Mg-type cations (network modifiers) and that of network-forming Si atoms (e.g., Roskosz et al., 2009; 2011). In this context, several studies have demonstrated that the crystallization of olivine (which is composed

of isolated SiO₄ tetrahedra) is strongly favored while pyroxenes (in which SiO₄ tetrahedra are linked together in a single chain) require a higher temperature (> 1000 K) to form (Roskosz et al., 2009; 2011; Gillot et al., 2011, Matsuno et al., 2012). The complex shape of these small grain aggregates also testifies that they formed at moderate temperature, since a slower diffusion rate can explain the limited reduction of the free surface and the limited rounding of the aggregates. If olivine is formed, then an interstitial SiO₂-rich glassy material should also be present, as it was observed in crystalline aggregates of CP-IDPs and UCAMMs (Keller and Messenger, 2005; Dobrica et al., 2012). However, this phenomenon was not observed in Acfer 094. It is probable that the interstitial phase was unable to resist aqueous alteration and was transformed into a Fe-rich fine-fibrous material.

In summary, the Acfer 094 matrix collected a mixture of crystalline material that had undergone various annealing temperatures in the protoplanetary disk. The most reactive regions (interstitial residual
<figure><img src="image_16.png" /><figcaption>**Fig. 10.** Fine-grained aggregates (a) Grain size distribution in a representative fine-grained aggregate dominated by olivine. (b) Frequency histogram showing the distribution of the fayalite content in olivine in three fine-grained aggregates from which accurate composition can be measured although we cannot exclude a small contribution of the Fe-rich thick grain boundaries.</figcaption></figure>

glassy material) were later minimally transformed on the parent body. These high-temperature components are cemented by the amorphous silicate groundmass (low-temperature component).

## 4.2. A Mg-depleted amorphous silicate groundmass

If the crystalline fraction of Acfer 094 exhibits similarities with that of CP-IDPs or UCAMMs, this is not the case for the amorphous fraction. Indeed, the amorphous silicate is richer in iron than the amorphous silicate of IDPs or UCAMMs and contains only a few nano-sulfides and no metal.

Numerous studies have shown that matrices of primitive type 3 chondrites (or close to this type), that have escaped extensive aqueous alteration and thermal metamorphism, contain abundant amorphous silicates and sub-micrometer anhydrous Mg-rich crystalline silicates (e. g., Brearley, 1993; Greshake, 1997; Chizmadia and Brearley, 2008; Abreu and Brearley, 2010; Le Guillou and Brearley, 2014; Le Guillou et al., 2015; Leroux et al., 2015; Hopp and Vollmer, 2018; Matsumoto et al., 2019; Dobrică and Brearley, 2020; Ohtaki et al., 2021; Zanetta et al., 2022). A common feature of the amorphous silicate of these primitive chondrites is their low Mg/Si atomic ratio compared to the solar value. For Acfer 094, this Mg/Si ratio is approximately half that of the solar value (Table 4). The Fe/Si ratio is more variable (ranging from ≈ 0.5 to 1.5) according to the chondrites, and it is also possibly heterogeneous at the matrix scale. Additionally, the Fe/Si ratio of the silicate containing nano-Fe-sulfide inclusions is challenging to measure. On average, the Fe/Si ratio of the amorphous silicate in Acfer 094 is closer to the solar value.

There is a pronounced discrepancy in the Fe content of the amorphous silicate in primitive chondrites, varying depending on the

chondrite and also within a given chondrite as in the case of Acfer 094 (Table 4). In Acfer 094, the degree of aqueous alteration is relatively low to moderate, as attested by the absence of developed phyllosilicates, carbonates and/or magnetite, ferrihydrite or cronstedtite. Most of the amorphous silicate is relatively well preserved and the porosity is not filled by alteration products as it is the case for the more altered chondrites. For instance, the ultraporous pockets are preserved, which would not be expected if water had circulated extensively (Matsumoto et al., 2019). However, the indices of aqueous alteration are indisputable in the Acfer 094 matrix. In CR chondrites, Le Guillou et al. (2015) and in Acfer 094, and Hopp et Vollmer (2018) and Ohtaki et al. (2021) measured the valency of iron in the amorphous silicate. They found a high proportion of Fe³⁺, which they interpreted as resulting from aqueous alteration which led to hydration and oxidation of the amorphous silicate groundmass. Our study also showed that the amorphous silicate is locally replaced by lumpy to fine-fibrous material at the boundaries of amorphous domains. Some enstatite grains are partially altered as well as number of grain boundaries of crystalline assemblages. The local replacement of enstatite by iron-rich fibrous material illustrates the extent of the mobilization of iron in the matrix under the effect of the aqueous alteration, which can reach 100 nm in the most altered Fe-rich rim. The iron originates from the dissolution of Fe-sulfides and metal (e.g., Hanowski and Brearley, 2000; Abreu and Brearley, 2010; Hopp and Vollmer, 2018, Vollmer et al., 2020). In CR chondrites, Vollmer et al. (2020) noted that increasing alteration degree was correlated with the decrease of the amorphous silicate content and its increasing Fe/Si ratio. The Fe content in the amorphous silicate groundmass of Acfer 094 is likely to have been modified by parent body aqueous alteration, even if this was localised and of a low intensity.

The Mg/Si ratio of the amorphous silicate of Acfer 094 differs significantly from that of the CI composition (~ 0.42 vs. 1.03). These two elements are only present in silicates and such a discrepancy from the solar composition raises questions about the primitive nature of amorphous silicates. The solar nebula is fed by dust from the molecular cloud from which it originates. At this stage, the silicates are predominantly amorphous (e.g., Kemper et al., 2002; Min et al., 2007), and thus their composition should be chondritic, with a Mg/Si ratio close to 1. The composition of the amorphous silicates of Acfer 094 therefore bears a signature of processes that took place in the protoplanetary disk, before their accretion on the parent body. The presence of a high abundance of Mg-rich crystalline silicates in the matrix is likely related to this, and thus these two components are probably chemically linked to each other.

## 4.3. Proposed scenario

The protoplanetary disk originates from a cold and dense molecular cloud, composed of gas and dust of interstellar origin. This interstellar silicate dust is amorphous (e.g., Kemper et al., 2004) and should have a bulk composition close to the solar value for the condensable elements (e.g., Ueda et al., 2005; Jones and Ysrad, 2019). Several studies have suggested that a portion of Fe in amorphous silicates may be present as nanometric inclusions, metal, sulfides, or oxides (e.g., Köhler et al., 2014; Altobelli et al., 2016; Jones and Ysrad, 2019). The interstellar silicates could be close to the GEMS grains in primitive CP-IDPs, which consist of an amorphous silicate containing numerous nano-inclusions of Fe-metal and Fe-sulfides (e.g., Bradley, 1994b; Keller and Messenger, 2011; Bradley et al., 2022). In the cold molecular cloud, the dust is more or less agglomerated and is composed largely of amorphous silicates mixed together with Fe-sulfides and Fe-metal, presolar grains, carbonaceous grains and ices. This molecular cloud collapsed to form the proto-sun, surrounded by the protoplanetary disk.

If we assume that the precursor of the matrix of Acfer 094 is a mineral assemblage similar to the dust inherited from the molecular cloud with a solar composition, we can estimate the distribution of major elements (Mg, Si, S and Fe) in the major phases can be roughly
<figure><img src="image_17.png" /><figcaption>**Fig. 11.** TEM bright field images (a) Area dominated by amorphous silicates which are partly transformed into lumpy material with numerous nano-voids. (b) Alteration rim around a large enstatite grain.</figcaption></figure>
<caption>**Table 4**</caption>

Atomic ratio Mg/Si and Fe/Si of the amorphous silicates groundmass from this study compared to previous studies and the solar abundance.

<table><thead><tr><th>Amorphous silicate, Meteorite/Authors</th><th>Mg/Si</th><th>Fe/Si</th></tr></thead><tbody><tr><td>Acfer 094 (this study)</td><td>0.42 ± 0.09</td><td>0.78 ± 0.09</td></tr><tr><td>Acfer 094 (Greshake, 1997)</td><td>0.32</td><td>0.47</td></tr><tr><td>Acfer 094 (Keller and Messenger, 2012)</td><td>0.48</td><td>0.98</td></tr><tr><td>Acfer 094 (Ohtaki et al., 2021)</td><td>0.32</td><td>0.69</td></tr><tr><td>Acfer 094 (Matsumoto et al., 2019)</td><td>0.47</td><td>1.42</td></tr><tr><td>Acfer 094 (Hopp and Vollmer, 2018)</td><td>0.17–0.22</td><td>0.6–1.6</td></tr><tr><td>Paris (Leroux et al., 2015)</td><td>0.71</td><td>0.68</td></tr><tr><td>Paris (Zanetta et al., 2021)</td><td>0.54</td><td>0.61</td></tr><tr><td>Paris (Ohtaki et al., 2021)</td><td>0.42</td><td>0.52</td></tr><tr><td>MET 00426 (Le Guillou and Brearley, 2014)</td><td>0.64</td><td>0.64</td></tr><tr><td>QUE 99177 (Zanetta et al., 2022)</td><td>0.48</td><td>0.86</td></tr><tr><td>DOM 08006 (Zanetta et al., 2022)</td><td>0.44</td><td>0.80</td></tr><tr><td>Semarkona (Zanetta et al., 2021)</td><td>0.33</td><td>0.56</td></tr><tr><td>Semarkona (Dobrica and Brearley, 2020)</td><td>0.36</td><td>0.36</td></tr><tr><td>UCAMMs (Dobrica et al., 2012)</td><td>0.81</td><td>0.53</td></tr><tr><td>GEMS in IDPs (Ohtaki et al., 2021)</td><td>0.81</td><td>0.15</td></tr><tr><td>Solar abuncance (Lodders, 2010)</td><td>1.03</td><td>0.87</td></tr></tbody></table>

estimated. Nearly all of Si and Mg are contained in the amorphous silicates, resulting in a Mg/Si atomic ratio close to 1. The case of Fe is more complicated. Indeed, Fe can be partitioned into three major phases (silicates, sulfides and metal). Sulfides (troilite or pyrrhotite) have a S/Fe atomic ratio close to 1 whereas the solar value is close to 1/2. Since all the S is carried by the sulfides, about half of the iron is thus carried by sulfides and the other half is carried by silicates. It should be noted that metal grains are neglected since their abundance is low in carbonaceous chondrites. If we distribute the elements according to the solar abundances in the major mineral phases, the composition of the amorphous silicates, including possible embedded Fe-metal nano-inclusions, should be as follows: Si = 1, Mg = 1.03 and Fe = 0.44 (i.e., Point 1 in Fig. 12a, calculation based on the solar abundances of Lodders, 2010).

The presence of chondrules and crystalline silicates in the matrix of primitive chondrites indicates that one or several brief transient episodes of high temperature occurred in the dusty proto-planetary disk. The dust is heated to various temperatures locally, as suggested by the diversity of crystalline aggregates in the Acfer 094 (Fig. 13). In the local hottest regions of the disk, the precursor dust has certainly been fully vaporized. The subsequent cooling of the gas led to the condensation of solid phases. In high temperature regimes, refractory oxides and silicates are formed according to a condensation sequence, similar that observed in refractory minerals produced close to the sun where the temperature was high enough to entirely volatilize the solar dust (e.g., Grossman, 1972). This crystal population is largely dominated by forsterite and enstatite. This condensation process can explain the presence of the rounded isolated crystals and of the whiskers and platelets observed in the matrix of Acfer 094. The condensation of Mg-silicates must have produced a residual gas depleted in Mg. At lower temperatures and if cooling rates were fast enough, condensation would not occur at thermodynamic equilibrium and the residue of the gas would be condensed as amorphous silicates depleted in Mg (Fig. 12a). The abundant presence of Fe in the amorphous silicate suggests oxidizing conditions, above the Fe-FeO oxygen buffer, which is compatible with moderate temperatures. The formation of amorphous silicates by non-equilibrium condensation following a transient heating event in the protoplanetary nebula is also the process described by numerous previous studies (e.g., Brearley, 1993; Abreu and Brearley, 2010; Hopp and Vollmer, 2018).

In regions that have experienced less heating, the dust was not evaporated, but the temperature was sufficient to either melt or anneal the dust. In the densest regions, in which the dust has probably coagulated into grains aggregates, the transient episodes of high temperature favored the formation of large silicates crystals/aggregates of crystals (and possibly chondrules). These large crystals are Mg-rich, which reflects a low oxygen fugacity, below the iron-wüstite buffer, likely due to the high C/O ratio of the dust precursor material. Consequently, the

formation of these crystals is accompanied by a preferential evaporation and loss of iron-rich species, as has been shown by experimental and modeling studies (e.g., Ogawa and Nagahara, 2001; Imae and Isobe, 2017; Costa et al., 2017). For forsterite-rich objects (including type I chondrules), the gas should also be enriched in Si, although to a lesser extent (e.g., Tachibana et al., 2002; Tissandier et al., 2002; Libourel et al., 2006; Imae and Isobe, 2017). It is similarly probable that the gas enriched in Fe and Si (depleted in Mg) may have interacted with dust that was not significantly transformed by local high-temperature episodes, resulting in a reduction in their Mg/Si ratio.

In regions of intermediate temperatures (typically within the range 1000–1300 K), below the melting point of the components of the dust, pre-existing amorphous silicates crystallized by sub-solidus transformations. The observed textures and morphologies of the crystalline aggregates suggest a wide range of temperature variability. The aggregates having the largest grains also have cohesive joints, which testify for intense sintering. These aggregates were brought to a temperature close to the melting point. The others, those which have a complex shape and smaller grains, experienced a lower temperature, yet sufficient to recrystallize the amorphous silicates toward a mineralogy dominated by olivine (typically around 900–1000 K). Solid state crystallization did not result in significant bulk chemical fractionation of aggregates.

Finally, a portion of the dust completely escaped these transient thermal events. In this case, the interstellar precursors were minimally or not at all modified. The survival of this fraction of precursors is attested by the presence of presolar grains and poorly evolved carbonaceous material in the Acfer 094 matrix (Quirico et al., 2014; Hoppe et al., 2015 and references therein).

The precise configuration of these high-temperature stages is difficult to assess because it depends on several parameters (intensity and duration of heating, local composition and density of the dust, more or less strong aggregation of this dust at the time of heating). The diversity of microstructures observed in the matrix of Acfer 094, as well as in those of other primitive chondrites, supports the complexity of the parameterization that transformed the dust component of the protoplanetary disk before the accretion of the parent bodies. What emerges from this analysis is that the most heated material allowed the formation of Mg-rich crystals (high-temperature condensates or chondrules), whereas the fine-grained cementing silicate fraction is on the contrary depleted in Mg. The chemical evolution of the solid silicate fraction is summarized in Fig. 12a, which shows the initial composition, the formation of magnesian crystals (enstatite and forsterite) and the formation or evolution of amorphous silicates depleted in Mg. This chemical trend has also been noted by previous authors. For example, in a recent study including the case of Acfer 094, Patzer et al. (2021) concluded that the matrix is not CI-like and its composition reveals the extraction of a fraction (10–15 wt%) of forsterite from an initial CI-like material. When the compositions (bulk, matrix, isolated silicates and chondrules) obtained by electron microprobe published by Patzer et al. (2021) are plotted in a Mg-Si-Fe ternary diagram, it is interesting to note that they align along a line roughly connecting the Mg-silicates (forsterite + enstatite) and the CI compositions (Fig. 12b). The compositions obtained in this study were obtained by EDXS-STEM. The analytical spatial resolution is well adapted to the sub-micron compounds of the matrix. These compositions are also aligned on the same line, defining the end member compounds of this line.

After the thermal event(s). The disk is then composed of objects of different sizes, also including residual gas enriched in Fe and Si relative to Mg. If this high-temperature stage was not immediately followed by the accretion of the parent bodies, the fine dust certainly did not remain coupled to the larger objects (such as the chondrules), which could have been a factor of further chemical fractionation. The final stage is the accretion of dust and chondrules on the parent body, after a mixing of the different components. Our study showed that secondary modifications on the parent body were minimal, allowing Acfer 094 to retain much of the primitive signature of the accreted materials. However, the
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

**Fig. 12.** Chemical evolution of the silicate components in the Fe-Mg-Si ternary diagram. (a) Evolution of the composition of the amorphous silicates. It is assumed that Fe is partitioned between the amorphous silicates and Fe-sulfides. Consequently, the composition of the amorphous silicate lies on the mixing line connecting the Fe apex and the solar composition (dashed line). The initial composition of the amorphous silicates, before the chemical differentiation within the protoplanetary disk, is positioned in (1). Transient thermal events led to the crystallization of Mg-rich crystalline phases crystallized (blue arrows pointing to enstatite and forsterite). The composition of the remaining amorphous silicate shifts accordingly from point 1 to point 2. This point 2 is the measured composition of the amorphous silicates in the matrix of Aćfer 094. Purple dot: average matrix composition from Wasson and Rubin (2010). Red dot: average composition of the matrix samples in this study. The CI composition is represented by a yellow star (data from Lodders 2010). (b) Composition field of the amorphous silicates compared with the compositions obtained by electron microprobe of the bulk, matrix, chondrules and isolated silicates (in blue) by Patzer et al. 2021. The compositions obtained in this study (in grey) align well along a line connecting the Mg-silicates and the amorphous silicates with their nano-sulfides inclusions.
<figure><img src="image_20.png" /><figcaption>**Fig. 13.** Schematic representation of the proposed scenario. In a dust- and gas-dense region of the protoplanetary disk, brief and more or less intense thermal events take place. Initially, the precursor material is the dust of the disk that is inherited directly from the molecular cloud. The dust has an average CI composition, contains abundant interstellar amorphous silicates and is more or less aggregated (1) In the hottest regions (> 2000 K) the dust is vaporized. During cooling, Mg-rich crystalline silicates condense, however, rapid cooling inhibits the crystallization of minerals at intermediate temperatures (~1500–1000 K). The condensation process is therefore out of equilibrium and result in the formation of amorphous, Mg-depleted components. The temperature and dust/gas ratio conditions can cause condensation of infrequent refractory minerals similar to those found in CAIs or AOAs. (2) When heated in the temperature range 1600–2000 K, the dust melts and forms chondrules. The surrounding gas becomes enriched in Fe and Si. Upon cooling, the gas surrounding the molten objects condenses out of equilibrium to form amorphous silicates depleted in Mg. (3) In sub-regions heated in the 900–1600 K range, the dust is annealed. The initially amorphous precursors crystallize in the solid state. Texture, mineralogy and grain size are directly related to the annealing temperature (see text). (4) Some regions have escaped local heating, so the precursor material, i.e., the dust inherited from the molecular cloud is not significantly modified during the protoplanetary transit. Presolar grains, organic matter and amorphous silicates of interstellar origin are preserved. It is important to note that this figure presents several thermal events of varying intensities. However, for a region subject to a given thermal event, there can also be a significant thermal gradient, with a high temperature in the center and a more moderate temperature at the periphery. After these more or less intense transient thermal events, the material formed in regions from 1 to 4 has mixed and accreted to form the Acfer 094 parent body.</figcaption></figure>

matrix underwent a moderate aqueous alteration activity. During this
episode, the mobility of Fe in the amorphous silicate favored local ho-
mogenization of the amorphous silicate chemistry.

**5. Conclusion**

Our study shows that the amorphous silicate in Acfer 094 display a
magnesium depletion compared to the CI composition, while the Fe/Si
ratio remains closer to the CI ratio. This amorphous silicate embeds
numerous submicron-sized Mg-rich olivine and pyroxene grains. We
suggest that these two populations (amorphous silicates and anhydrous
crystalline silicates) are related. The Mg-rich crystalline silicates formed
at high temperatures and their microtextural variations depend on the
mechanisms involved during their formation (condensation, crystalli-
zation from a liquid or subsolidus crystallization). The amorphous sili-
cate was probably formed by non-equilibrium condensation at lower
temperatures. Both populations formed after a transient heating event(s)
possibly related to the formation of chondrules.

The Acfer 094 matrix has preserved sensitive phases such as primi-
tive organic matter and presolar grains, it is possible that some inter-
stellar silicates (mostly amorphous) have also been preserved. However,
these interstellar silicates have no reason to have a composition far from
the solar composition, in particular for the Mg/Si ratio, since these two
elements are exclusively partitioned in silicates. These amorphous sili-
cates of solar composition are rare in the matrix, suggesting that most of
the initial characteristics of the interstellar dust have been obliterated
within the protoplanetary disk.

It is noteworthy that in the matrix of Acfer 094 the submicron ag-
gregates of crystalline silicates are well preserved, with characteristics
comparable to those found in primitive IPDs and UCAMMs. Their
abundance in this matrix provides a unique opportunity to study in
detail the matter transformations associated with the thermal events
that affected the protoplanetary disk prior to the accretion episode.

CRediT authorship contribution statement

**Hugues Leroux:** Writing – original draft, Visualization, Methodology, Investigation, Funding acquisition, Conceptualization. **Pierre-Marie Zanetta:** Writing – review & editing, Conceptualization. **Corințin Le Guillou:** Writing – review & editing, Conceptualization. **Maya Marinova:** Writing – review & editing, Investigation.

Declaration of competing interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Acknowledgments

This work was supported by the Programme National de Planetologie (PNP) of CNRS/INSU/CNES, the ISITE ULNE and the “Métropole Européenne de Lille” through the “TEM-Aster” project. This work was carried out on the electron microscopy facility of the Advanced Characterization
Platform of the Chevreul Institute, funded through the CHEMACT project supported by the "Ministère de l'Enseignement Supérieur de la Recherche et de l'Innovation", the region "Hauts-de-France" and the "Métropole Européenne de Lille". We thank the Muséum National d'Histoire Naturelle (Paris) for providing the section of the Acfer 094 chondrite. We thank David Troadec for the high-quality FIB sections, partly supported by the French RENATECH network. We would like to thank the three anonymous reviewers for their valuable comments and suggestions, which helped us to improve the quality of the manuscript.

## Appendix A. Supplementary material

Readers will find additional data in the supplementary material. These include SEM images of the matrix (not shown in the main document), bright-field TEM images of the amorphous and crystalline silicates, STEM-HAADF images of the amorphous and crystalline assemblages (not shown in the main section), additional EDXS chemical maps, additional ternary diagrams to allow comparison of the 6 FIB samples studied. Finally, the document contains all the chemical compositions of the crystalline and amorphous silicates collected during this study, tabulated and related to the figures presented in the main document. Supplementary material to this article can be found online at https://doi.org/10.1016/j.gca.2024.11.028.

## Data availability

Data are available through Mendeley Data at https://doi.org/10.17632/9bsywddwzy.1.

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