# Mid-infrared detection and characterization of refractory inclusions in CM and CO chondrites: A non-destructive approach for returned space samples

Jean Charlier, Alice Aléon-toppani, Rosario Brunetto, Jérôme Aléon, Ferenc Borondics

► To cite this version:

Jean Charlier, Alice Aléon-toppani, Rosario Brunetto, Jérôme Aléon, Ferenc Borondics. Mid-infrared detection and characterization of refractory inclusions in CM and CO chondrites: A non-destructive approach for returned space samples. Meteoritics and Planetary Science, 2025, 60 (3), pp.544 - 569. 10.1111/maps.14314. hal-05346074

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# Mid-infrared detection and characterization of refractory inclusions in CM and CO chondrites: A non-destructive approach for returned space samples

Jean CHARLIER ID<fnref n="1" />,<fnref n="2" />*, Alice ALÉON-TOPPANI<fnref n="1" />, Rosario BRUNETTO<fnref n="1" />, Jérôme ALÉON<fnref n="2" />, and
Ferenc BORONDICS<fnref n="3" />

*Correspondence

Jean Charlier, Institut d'Astrophysique Spatiale, CNRS, Université Paris-Saclay, Orsay 91405, France.

Email: jean.charlier@mnhn.fr

(Received 19 July 2024; revision accepted 09 January 2025)

**Abstract**—Refractory inclusions (RIs) in chondrites are widely used as tracers of early solar system formation conditions. In the context of sample-return missions, a non-destructive and non-invasive analytical tool that can rapidly detect and characterize RIs in space samples during their early phase of study is highly needed. Here, we performed mid-infrared (MIR) fine-scale hyperspectral imaging over large fields of view to detect RIs in CM and CO chondrites. A database of MIR spectra of typical RIs minerals was built (1) to support future remote sensing observations in astronomical environments and (2) to develop a detection method based on machine-learning algorithms and spectral distance between sample and reference minerals. With this method, up to 96.5% of the RI content is detected in a meteorite section. Further comparison between scanning electron microscopy and spot analysis acquired in reflectance in the full MIR range shows that RIs can be classified following their mineralogy based on infrared (IR) properties. Finally, we show that the relative OH content of several RIs in CM chondrites determined from IR spectroscopy can be used to infer the extent of modification caused by aqueous alteration on the asteroidal parent body.

## INTRODUCTION

Carbonaceous chondrites are primitive chondrites from primordial asteroids that contain objects formed at the beginning of the solar system. Refractory inclusions (RIs) are the first objects to have formed in the early solar system by direct gas to solid condensation of a gas of solar composition at high temperature (Grossman, 1972) approximately 4.567 Gyr ago (Amelin et al., 2010; Connelly et al., 2012), and are therefore essential tracers of early solar system processes. RIs from primitive chondrites include objects belonging to two different populations: inclusions rich in refractory elements, the so-called calcium-aluminum-rich inclusions (CAIs) on one hand and amoeboid olivine aggregates (AOAs), which contain

CAI-like nodules in an olivine groundmass, on the other hand. Carbonaceous chondrites are the main carriers of RIs.

Comets and primordial asteroids may have accreted RIs during their formation, as they represent bodies formed early in the solar system. To date, four space missions have collected samples from comets (the Stardust mission) or asteroids (Hayabusa, Hayabusa 2, and OSIRIS-REx). So far, one RI-like object has been reported in a grain from asteroid Ryugu (Kawasaki et al., 2022) and a few CAIs from fragments of the comet 81P/Wild 2 (Joswiak et al., 2017; Matzel et al., 2010; Simon et al., 2008). The presence and characterization of RIs in samples returned from primitive bodies should bring multiple outcomes in terms of understanding solar

<footnote n="1">¹Institut d'Astrophysique Spatiale, CNRS, Université Paris-Saclay, Orsay, France</footnote>

<footnote n="2">²Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Sorbonne Université, Museum National d'Histoire Naturelle, CNRS UMR 7590, IRD, Paris, France</footnote>

<footnote n="3">³SOLEIL Synchrotron, L'Orme des Merisiers, RD 128, Saint Aubin, France</footnote>
system history. Given that few of these objects are
available for study, that they contain only scarce and
small refractory objects, and that future sample return
missions such as Martian Moon eXplorer may
return carbonaceous chondrite material, it is essential to
be able to efficiently identify RIs in returned space
samples as a prerequisite to detailed mineralogical and
geochemical studies.

RIs are commonly detected in meteorites using false
color magnesium–calcium–aluminum (MgCaAl) X-ray
maps acquired by scanning electron microscopy (SEM)
combined with energy dispersive spectroscopy (EDS).
However, SEM is an invasive technique that often requires
gold or carbon coating, which inevitably contaminates the
surface of precious samples such as returned space
samples. In addition, SEM can also slightly modify the
surface of the sample (e.g., irradiation and volatilization of
light elements). RIs can be centimeter-sized in CV
chondrites (Macdougall & Goswami, 1981) and thus easily
detectable optically, but can be much smaller, down to
tens of micrometers in CM chondrites (Rubin, 2007),
which is more challenging to detect without MgCaAl
X-ray maps. Therefore, another analytical tool to detect
RIs in returned space samples that does not require any
coating and allows the detection of small RIs is highly
needed.

Whereas transmittance infrared (IR) spectroscopy
requires invasive/destructive preparation such as
extraction and/or grinding to analyze a sample, reflectance
IR spectroscopy is an analytical tool that (1) is neither
invasive nor destructive, (2) can probe both the mineral
and organic composition of a sample, (3) can be
performed under N₂ environment or vacuum (e.g.,
Pilorget et al., 2022), and (4) can be performed in imaging
mode. Therefore, IR reflectance hyperspectral imaging is a
particularly well-suited approach to analyze priceless
returned space samples. Considering that rock-forming
minerals exhibit their main characteristic features in the
mid-infrared (MIR) range (Beck et al., 2018), this spectral
region is promising for the detection of RIs. The term
MIR used here refers to the astronomy division scheme
(MIR: 4000 to 400 cm⁻¹).

In the context of sample-return missions, MIR
hyperspectral imaging is therefore ideal (1) to develop a
method for rapid detection and characterization of RIs in
returned space samples during their early stage of study,
and (2) for non-destructive and non-invasive analyses of
returned samples, and consequently giving the
opportunity to study both mineralogy and organic
matter.

IR spectroscopy studies on RIs are sparse. Near-IR
spectroscopy in reflectance has been performed on CAIs
from CV and CO chondrites (Cloutis & Gaffey, 1993;
Rajan & Gaffey, 1984; Sunshine et al., 2008). A few other

studies have performed mid to far-IR spectroscopy in
transmittance on CAIs from CV and CO chondrites
(Posch et al., 2007; Morlok et al., 2008 and 2014).
However only two studies have performed MIR
spectroscopy in reflectance on CAIs. Hamilton and
Connolly (2012) imaged CAIs from CV chondrites to
demonstrate the utility of reflectance spectroscopy for the
non-destructive characterization of chondritic
components, while Morlok et al. (2020) made spot
measurements from 20 to 100 micrometers in size on CAIs
from CV and CO chondrites to study the IR features of
the main CAI minerals. Unlike previous studies, our aim
here was to develop a detection method for RIs by
performing fine-scale imaging over large fields of view and
characterize the RIs of space return samples, while using a
MIR database of CAI minerals and machine-learning
algorithms.

CM chondrites hold on average 1.21 vol% CAIs
(Hezel et al., 2008), and more generally 5 vol% of RIs
(Scott et al., 1996). Most CAIs in CM chondrites are
<150 µm in size (Hezel et al., 2008; MacPherson &
Davis, 1994). Having abundant RIs and small CAIs, CM
chondrites are thus useful meteorites to develop a robust
RI detection method.

CM chondrites are hydrated meteorites that
underwent incipient to extensive aqueous alteration on
their asteroidal parent body (McSween Jr, 1979; Rubin
et al., 2007; Suttle et al., 2021). They are aggregates of RIs,
chondrules and metal grains bound by a fine-grained
matrix. The matrix is the main component and, in CM
chondrites, is mostly made of aqueous alteration products
such as carbonates, tochilinite-cronstedtite intergrowths
(TCIs) and hydrated phyllosilicates. To cover a range of
variously altered objects of affinity with CM chondrites, we
studied sections from Murchison of petrologic type CM2.5
(Rubin et al., 2007) and Paris of average petrologic type
CM2.7 (Marrocchi et al., 2014; Rubin, 2015), although
different lithologies with various degrees of alteration have
been observed in the latter (Hewins et al., 2014).
Considering the rarity of unaltered CM chondrites and the
possible genetic link between CO and CM chondrites, we
also studied the Colony CO3.05 chondrite (Grossman &
Brearley, 2005).

The most common group of CAIs found in CM
chondrites are spinel-pyroxene inclusions (MacPherson,
2014). These inclusions have a layered structure with a core
of spinel ± perovskite ± hibonite, a layer of Fe-rich
phyllosilicates and finally a rim of diopside. Some of them
can be rimmed by forsterite. CM chondrites also contain
rare hibonite-rich CAIs classified by Ireland (1988, 1990) as
spinel-hibonite spherules, blue hibonite aggregates and
platy hibonite crystals (SHIBs, BAGs, and PLACs,
respectively). Another type of hibonite-rich CAI called
hibonite-pyroxene/glass spherules was described by Ireland
et al. (1991). Finally, rare CAIs from CM chondrites contain grossite (Hewins et al., 2014; Kimura et al., 2020; Simon et al., 1994) or corundum (Bar-Matthews et al., 1982; MacPherson et al., 1984; Simon et al., 2002). CAIs in CM chondrites recorded the aqueous alteration of the parent body through mineralogical transformations such as a proposed loss of melilite in favor of Fe-rich phyllosilicates (Elmaleh et al., 2015; Rubin, 2007). Unlike most CAIs in aqueously altered CM chondrites, less altered CO chondrites such as Colony still contain melilite-bearing CAIs (Zhang et al., 2020).

Here, a spectral database of the main CAI minerals was first acquired in the MIR range to obtain the IR features of CAI minerals. We developed a new method to detect small RIs down to tens of micrometers accounting for up to 96.5% of the RIs in heterogeneous carbonaceous CM and CO chondrites using MIR hyperspectral imaging. For a better characterization of the detected RIs, they were subsequently analyzed individually by IR reflectance spectroscopy in spot mode over a larger spectral range to fully determine their mineralogy. They were also analyzed by MIR hyperspectral imaging with a greater spatial sampling/resolution to assess their OH content. We further investigated the OH content of RIs to be able to classify hydrated versus anhydrous RIs. The ultimate aim of the study was, in a context of initial non-destructive/invasive characterization of samples returned from space missions, to be able to rapidly detect RIs and characterize their mineralogy and water content to provide their first-order characterization as a guide for the selection and allocation processes of RI-containing space samples.

# MATERIALS AND METHODS

## Samples Description

Polished and carbon-coated sections of Murchison (Murchison 2), Paris (4029sp2), and Colony (3325sp2) were loaned by the Museum National d'Histoire Naturelle in Paris for this study. The surface areas of the maps of Murchison, Paris and Colony for this work are 8.04, 8.18, and 8.67 mm², respectively. The three sections were coated with 15–20 nm of carbon before IR and SEM measurements.

In our case, the thin coating deposited on sections does not significantly affect IR measurements (Figure S1). IR and SEM measurements can therefore be combined. The samples were measured under ambient conditions for all IR measurements.

Murchison is a partially altered CM chondrite. Although it is a breccia composed of several clasts/lithologies, it has been assigned an overall petrologic subtype 2.5 on the Rubin et al. (2007) scale.

Murchison consists of abundant matrix bonding chondrules, RIs and a few Fe-Ni metal grains together. The matrix is mainly composed of hydrous phyllosilicates and TCIs, with 1–2 vol% carbonates and 0.55 vol% sulfides (Rubin et al., 2007). Olivine and pyroxene in chondrules are unaltered contrary to the glassy mesostasis that has been replaced by hydrated phyllosilicates (Rubin et al., 2007).

Paris is composed of several more or less altered lithologies, with one of the least altered lithologies among CM chondrites. Paris was assigned an average petrologic subtype 2.7 by Marrocchi et al. (2014) and Rubin (2015) on the scale of Rubin et al. (2007). The matrix is mainly composed of hydrous phyllosilicates and TCIs with abundance varying with the extent of alteration. The least altered lithology of Paris still contains amorphous silicates in the matrix and abundant metal grains (Hewins et al., 2014; Leroux et al., 2015).

Colony is one of the least metamorphosed CO chondrites and was assigned a petrologic subtype 3.05 by Grossman and Brearley (2005). However, Colony was extensively weathered on Earth before its recovery (Rubin et al., 1985). This extensive weathering on earth has not affected the detection of RIs as primary minerals are still preserved in Colony. As a CO chondrite, Colony is made up of less matrix and more chondrules than Paris and Murchison. It still contains unaltered glassy mesostasis within chondrules (Rubin et al., 1985).

## Standards Description

We first built a database composed of MIR reflectance spectra of reference minerals; hereafter referred to as “standards.” These standards were used as references for the detection of RI phases in the meteorite sections. The standards include (1) terrestrial and synthetic minerals similar to CAI minerals and (2) meteoritic minerals from well-preserved RIs from Colony for internal comparisons when terrestrial and meteoritic minerals are likely to differ (notably hibonite). The standards come from various sources including the mineral collections of the Museum National d'Histoire Naturelle (Paris, France) and Sorbonne University (Paris, France). The standards were embedded in epoxy and mounted in aluminum rings and then polished.

In most cases, pyroxenes either of meteoritic or terrestrial origin are not pure diopside but are Ca-rich clinopyroxenes with different Al-Ti-enrichments. In the following, it is referred to as diopside for simplicity or Al-Ti-diopside when its composition deviates significantly from diopside. The fine-grained diopside standard in Colony meteorite refers to a diopside found inside a fine-grained CAI and not a fine-grained diopside itself. Also, three spectra were acquired from different areas
(edge, intermediate zone, and core) of coarse-grained Ca-pyroxenes from 3529-Z, a type B CAI from the Allende CV chondrite (Aléon, 2016). These pyroxenes present a progressive enrichment in Al and Ti towards their core. The standards, their origin (terrestrial, meteoritic or synthetic) and their locality are listed in Figure S2.

These spectra were acquired under ambient conditions in single spot mode with a ThermoFisher Continuum FT-IR microscope coupled to a mercury cadmium telluride (MCT) detector installed at the SMIS beamline of the SOLEIL synchrotron. We collected a background reference on a pure gold foil before each measurement. Terrestrial standards were measured with a ×32 objective, a physical aperture of 60 × 60 µm and the globar source of the instrument. Meteoritic hibonite and pyroxene standards were measured with the synchrotron source and using a physical aperture of 40 × 40 and 20 × 20 µm, respectively. Each standard spectrum is taken over the entire MIR range from 4000 to 400 cm⁻¹ (2.5–25 µm) with a spectral resolution of 4 cm⁻¹ and is the average of 512 coadded scans.

## Hyperspectral Imaging for Detection

We subsequently acquired high-definition reflectance hyperspectral maps of the Murchison, Paris, and Colony polished sections to test the efficiency of our spectral database and methods for accurately detecting RIs. These maps were acquired with an Agilent Cary 670/620 microscope system (SMIS beamline of the SOLEIL synchrotron) coupled to a LN2-cooled MCT focal plane array (FPA) detector of 128 × 128 pixels resized to 64 × 64 pixels under ambient conditions and using the internal globar source of the instrument. To choose the best measurement conditions, we had to compromise between (1) measurement time, (2) size of the mapped area, (3) minimum measurement configuration required to detect RIs, and (4) volume of mapping data that can be stored and processed.

Hyperspectral maps for RI detection were collected with the ×15 objective at standard magnification, which gives a projected pixel size of 5.5 µm. Hyperspectral maps are mosaics of IR images formed from several combined tiles. A tile consists of a 64 × 64 pixels image (352 µm field of view) with, for each pixel, an IR spectrum in the range 4000–800 cm⁻¹ (2.5–12.5 µm) having a spectral resolution of 4 cm⁻¹ and acquired from the average of 32 coadded scans. Each hyperspectral map was acquired during approximately 1 hour and a half and covers a surface area of around 8 mm². We collected a background on a gold mirror for calibration of reflectance measurements before each hyperspectral map. Note that hyperspectral maps used for RI detection will be referred to as “hyperspectral

detection maps” for less confusion with other hyperspectral imaging performed in this study.

## FT-IR Spectroscopy for the Characterization of the Detected RIs

Eleven RIs detected in the Murchison section were further analyzed to determine their mineralogy and assess their water content. The mineralogical characterization of these 11 RIs was carried out using two measurement methods: (1) spectra obtained by k-means clustering on hyperspectral maps and (2) single spot mode measurements. The measurement of the OH content in RIs was carried out using spectra obtained by clustering on hyperspectral maps.

## Clustering on Hyperspectral Maps of RIs

### Configuration of Acquisition

We acquired IR reflectance hyperspectral maps of each RI and its close environment. These measurements were collected with a ×25 objective at standard magnification, which provides a 3.3 µm projected pixel size (212 µm field of view), in order to improve the spatial sampling and resolution thanks to a larger numerical aperture (NA). The NA equals 0.81 for the ×25 objective against 0.62 for the ×15 objective. We used the unsupervised machine-learning algorithm called k-means++ clustering (Arthur & Vassilvitskii, 2007; Likas et al., 2003) on the hyperspectral map of each RI to cluster the pixels that only correspond to RI minerals. With this method, we finally obtain one average spectrum for each RI. The advantage of clustering on hyperspectral maps is that the entire surface of a RI can be sampled, whereas a measurement in spot mode allows only a partial sampling of a RI if the spot is smaller than the RI, or implies mixing of a RI with the surrounding matrix if the spot is larger than the RI (Figure 1).

### Mineralogical Characterization

The 1300–800 cm⁻¹ spectral range was used to determine the mineralogy of each of the 11 RIs. We especially looked at the Si-O stretching bands around 1000 cm⁻¹ (Figure 1) to decipher the mineralogy of silicates. Metal-O stretching bands of oxides also occur in the MIR range but mainly under the range limit of the FPA detector (i.e., under 800 cm⁻¹). Therefore, we additionally made single spot measurements with the full MIR range (4000–400 cm⁻¹) to confirm the mineralogy of oxides (see section Single Spot Mode).

### OH Content Characterization

The major water feature in our spectral range of study is the 3-µm band corresponding to -OH and H₂O vibrational modes. This band is actually the results of
<figure><img src="image_2.png" /><figcaption>FIGURE 1. Example on calcium–aluminum-rich inclusion (CAI-06) of the differences between hyperspectral imaging and single spot mode acquisitions. Hyperspectral imaging can fully sample the CAI, whereas the single spot mode can only partially sample it. Due to a specular reflectance configuration (close to backscattering), both hyperspectral imaging and single spot mode favor surface scattering over volume scattering regime. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

several contributions, which in order of decreasing energy
can be simplified as follows (Bishop et al., 1994;
Kuligiewicz et al., 2015): (1) metal-OH vibrations in
hydrated minerals around 2.7–2.8 µm (3700–3570 cm⁻¹),
(2) symmetric and asymmetric stretch of bound water
around 2.76–2.84 µm (3620–3520 cm⁻¹), (3) symmetric and
asymmetric stretch of adsorbed H₂O around 2.9–2.99 µm
(3450–3340 cm⁻¹), and (4) Fermi resonant second bending
overtone of H₂O at about 3.1 µm (3225 cm⁻¹).

Terrestrial weathering can contribute to the OH/H₂O bands in two ways: (1) water adsorbed on the mineral surface and (2) water absorbed in phyllosilicate interlayers, resulting in a more prominent H₂O stretching and bending band. Terrestrial weathering, however, does not contribute to the metal-OH band. Since we are comparing CAIs from the same meteorite (i.e., Murchison), we can assume that they are all homogeneously affected by terrestrial weathering.

We used the metal-OH band to assess the OH
content within RIs linked to the formation of
phyllosilicates. We specifically focused on the metal-OH
band because it was the most prominent water feature in
RI spectra. The peak positions and shape of the -OH and
H₂O vibrational modes were used to obtain information

about the type of phyllosilicates, and therefore on the
nature of hydration in CAIs.

The determination of the absolute water or OH
content based on reflectance IR spectroscopy is
challenging (Beck et al., 2018). Indeed, reflectance does
not probe the absorption properties of a given mineral in
a linear way. However, we used the metal-OH band area
to obtain a scale of the relative OH content between the
11 detected RIs. Measuring the area of the metal-OH
band is equivalent to measuring the amount of OH
trapped and bound in octahedral layers of phyllosilicates,
and in a way, the abundance of these phyllosilicates.

In detail, we assessed the relative OH content by
calculating the logarithm of the area of the metal-OH band
between 3500 and 3800 cm⁻¹ (2.85–2.63 µm) on the mean
spectrum of each RI obtained by clustering on
hyperspectral maps. This range also includes the H₂O
stretching band (3620–3520 cm⁻¹). We observe that the
H₂O bending band near 1600 cm⁻¹ is weak, indicating that
corresponding H₂O stretching is likely weak as well.
Consequently, the contribution of the H₂O stretching band
is expected to be minimal compared to the metal-OH
band in the 3500–3800 cm⁻¹ range. We calculated the
logarithm because, in our configuration, this band is
detected in a regime of volume scattering (Figure 1). The area of the band corresponds to the area between the reflectance spectrum and the continuum. The position of the continuum was approximated using the rubber band function in Python. The rubber band function identifies the maxima (for volume scattering) in the spectrum, which are assumed to lie on or near the baseline. A contour is then interpolated between these points, as if a rubber band were pulled between them. This interpolated line is then subtracted from the original signal, thus eliminating the background trend.

## Single Spot Mode

The 11 detected RIs were also analyzed in spot mode over the entire MIR range (4000–400 cm⁻¹). These IR measurements were performed with the configuration used for terrestrial standard measurements except that the physical aperture was 50 × 50 µm. The measurements in spot mode provide a wider spectral range for the mineralogical characterization. However, contrary to clustering on hyperspectral maps, measurements in spot mode cannot sample the entire surface of a RI due to the restricted size and shape of the physical aperture required for this kind of measurements (Figure 1).

## FEG-SEM: BSE Images and EDS Analyses

Backscattered electron (BSE) images and spot energy dispersive X-ray spectroscopy (EDS) analyses were acquired at 15 keV and 3 nA with a Clara TESCAN field emission gun scanning electron microscope (FEG-SEM) coupled with two Bruker Xflash EDS spectrometers (spectral resolution of 123 eV) in antagonist positions and working in conventional mode (under ~10⁻⁴ Pa in the analysis chamber), installed at Institut de Minéralogie, Physique des Matériaux et Cosmochimie (IMPMC), Museum National d'Histoire Naturelle (MNHN), Paris. Each EDS detector has a collector surface of 30 mm² (60 mm² in total with both detectors).

False color MgCaAl X-ray maps in red–green–blue color scheme of the whole sections were used to locate all RIs within the area delimited by hyperspectral detection map of the sections. The surface area of each RI was determined on SEM images with the ImageJ software. The efficiency of the detection method was determined as the ratio between the surface area of RIs detected in IR (hereafter, detected RIs) and the surface area of all RIs found by SEM.

BSE images and EDS analyses were systematically acquired on the detected RIs in Murchison (1) to better understand the accuracy and the limits of our detection method in IR and (2) to compare the results of the mineralogical characterization in SEM with those of IR spectroscopy.

## Detection Method

### Basic Processing for Detection

The detection of typical CAI minerals in IR maps is based on supervised and unsupervised machine learning using our standard spectra database. The detection method was developed on Murchison for spinel and diopside, while the detection of hibonite and gehlenite was developed on Colony where both minerals were present (Figure 2). The Paris section was subsequently used as an unknown sample for validation of the detection method.

For data processing, we used the open-source software Quasar (Toplak et al., 2017, 2021) to design workflows aiming at visualizing raw data, doing pre-processing and using machine-learning algorithms.

Several pre-processing steps were carried out on the hyperspectral detection maps in order to ease the detection of RIs:

1. Pixels from metal grains (kamacite–taenite) may appear as false positives due to saturation caused by the high reflectivity of kamacite combined with very low signal to noise ratio of the detectors around 800 cm⁻¹. The saturation could thus be confused with reflective features of spinel and hibonite around 800 cm⁻¹. Using a threshold, we therefore removed the pixels corresponding to metal grains from the maps by keeping only pixels with a reflectance under 50% at 800 cm⁻¹.

2. We reduced the spectral range of the map data and kept only the spectral range that characterizes the CAI minerals. Attention was paid to keep the Christiansen feature of the minerals in the chosen spectral range. The ranges kept for each mineral were as follows: 800–940 cm⁻¹ for hibonite and spinel; 840–1170 cm⁻¹ for diopside; and 820–1100 cm⁻¹ for gehlenite.

3. The detection of spinel and hibonite requires to keep the end of the spectral range provided by the instrument (i.e., close to 800 cm⁻¹), which is also the range with the worst signal to noise ratio. As such, the spectra were smoothed for the detection of spinel and hibonite in order to improve their detection (Gaussian smoothing with standard deviation equal to 5).

4. Two identical minerals in the same meteorite section may have different peak intensities because of their respective orientation in three dimensions and/or because of defocusing if the section is not strictly flat. We thus normalized the spectra (area normalization) to lessen the effect of peak intensity and defocusing. Area normalization function divides each spectrum of hyperspectral maps by its integral from 0.
<figure><img src="image_3.png" /><figcaption>FIGURE 2. (a) Euclidean spectral distance map on Colony to the spinel standard between 800 and 940 cm⁻¹. The detected objects are highlighted with black arrows for spinel and purple arrows for hibonite. (b) Clustering map on Colony with each pixel belonging to a cluster associated with an average spectrum and a color. All pixels with a Euclidean distance higher than 0.032 were removed before clustering. (c) Average spectrum of each cluster between 940 and 800 cm⁻¹, and (d) their attribution to a mineral or a mix. Clusters 6, 8, and 9 were removed from these plots to ease the visualization. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

After these pre-processing steps, we used machine learning, such as the nearest neighbor method, with the Orange spectroscopy toolbox of Quasar to detect typical CAI minerals. The nearest neighbor method is a supervised machine learning tool used to find features in a dataset that are close to a reference. For each pixel in the hyperspectral detection maps, a Euclidean spectral distance (Equation 1) is calculated between the sample spectrum and that of the given reference.

$$ \text{Euclidean distance} = \sqrt{\sum_{i=1}^{n} (x_i - y_i)^2} \quad (1) $$

In Equation (1), x is the reflectance of the reference spectrum at a given wavelength (i) and y is the reflectance of a pixel spectrum at the same wavelength (i).

Here, we calculated the Euclidean distance to our standard spectra. Standard spectra can be either external references (terrestrial or synthetic standard) or internal references (meteoritic standard). After calculating the distances between spectra from each pixel and a mineral standard, a Euclidean spectral distance map is produced

where pixels with the smallest distance (blue spots) are those whose spectral features are closest to the mineral used as reference.

Due to spectral similarities between several minerals, the detection of hibonite, spinel, and diopside needs an additional processing step. On the contrary, gehlenite can be detected on its own with just one processing step as it exhibits more characteristic features in the range 1100–800 cm⁻¹.

### Additional Step for Diopside

After applying all the steps of the basic processing, we obtained a Euclidean distance map to the fine-grained diopside reference between 840 and 1170 cm⁻¹ on Murchison. However, diopside in RIs and enstatite in chondrules exhibit close spectral features, which results in almost the same Euclidean distance to the fine-grained diopside reference. To distinguish diopside from enstatite and obtain a Euclidean spectral distance map to diopside only, (1) we kept the pixels that have a Euclidean distance inferior to 0.012 from the first nearest neighbor processing step between 840 and 1170 cm⁻¹ because higher values of
Euclidean distance correspond to pixels with a spectrum that significantly differ from those of pyroxenes and (2) we ran a second time the nearest neighbor method in a restricted spectral range, between 920 and 1040 cm⁻¹, as diopside exhibits its main spectral differences with enstatite in this range.

## Additional Step for Spinel and Hibonite

Colony was used to build the method for distinguishing spinel from hibonite. Following the basic processing described above, we obtained a map of the Euclidean distance to the Colony spinel standard (Figure 2a). It should be noted that (1) the Euclidean distance is calculated between 940 and 800 cm⁻¹, a range where the signal-to-noise ratio is low and (2) the clouds of blue-red pixels close to each other on the map are very noisy pixels corresponding to polished section pullouts and are therefore not RIs. Because spinel, hibonite and gehlenite show spectral similarities in the 940 and 800 cm⁻¹ range (i.e., an increase in reflectance), the Euclidean distance map to the spinel standard (Figure 1b) actually shows detections of these three minerals. Note that we are not particularly aiming at detecting gehlenite here, as it can be easily detected on its own with basic processing. We just need to separate it from hibonite and spinel detections.

We therefore used k-means++ clustering (Arthur & Vassilvitskii, 2007; Likas et al., 2003) to distinguish hibonite from spinel. In such unsupervised machine learning method, the algorithm finds similarities inside the data set without many inputs. K-means clustering was already successfully used in a few recent studies of carbonaceous chondrites (Dionnet et al., 2022; Hewins et al., 2021). To separate the detection of spinel from that of hibonite: (1) We kept the pixels that have a Euclidean distance inferior to 0.032 because higher values of Euclidean distance correspond to pixels with a spectrum that significantly differ from those of hibonite and spinel. (2) We used k-means++ algorithm with 10 clusters ($k=10$), 10 re-runs and 300 iterations as inputs. We finally obtained a clustering map (Figure 2b) where each pixel belongs to one cluster. The average spectrum of each cluster (Figure 2c) allows to define which mineral is associated with which cluster (Figure 2d) and then know which RIs contain hibonite and/or spinel. Cluster C3 is identified as hibonite, C7 and C2 as spinel, and C4 and C5 as a spinel-gehlenite mix. C6, C8, and C9 are clusters associated with noisy pixels. They were thus removed from Figure 2. The C10 cluster and marginally the C1 cluster are associated with pixels that represent the transition between the RIs and the matrix. These transition pixels between two mineralogical phases are due to diffraction and the limited pixel size.

# RESULTS

## IR Spectra of CAI Minerals

Standard spectra of individual CAI minerals are shown in Figures 3 and 4. The positions of their characteristic features are given in Table 1, with the exception of peaks below 800 cm⁻¹ for silicates, as they do not allow easy discrimination between minerals, and of peaks under 500–450 cm⁻¹ for oxides, as spectra under this wavelength have a low signal to noise ratio. Note that the standard spectra in Figures 3 and 4 are not normalized, hence we observe that the peaks of oxides, nepheline, and sodalite are more intense than those of anorthite, gehlenite, åkermanite, and Ca-pyroxenes.

The feature positions in mineral spectra from previous studies discussed hereafter may sometimes differ slightly from those shown here. They are possibly due to (1) a different geometry (mostly reflection vs. transmission mode and also specular vs. diffuse reflectance), (2) a different sample preparation (polished section vs. powder), and (3) slightly different compositions.

### Corundum

The Longido corundum exhibits a plateau between 845 and 650 cm⁻¹ and two peaks at 618 and 583 cm⁻¹. Zeidler et al. (2013) observed similar spectral features in a synthetic corundum, except for a single band instead of the two peaks at 618 and 518 cm⁻¹ observed here (Figure 3). Although the spectrum of different corundums is not expected to change much, some slight shifts in their spectrum could be attributed to variations in trace element content in natural corundum.

### Perovskite

Morlok et al. (2020) measured meteoritic perovskite in reflectance, but the spectrum is featureless in the MIR range. However, the Gardiner Complex perovskite exhibits a plateau between 710 and 585 cm⁻¹ (Figure 3).

### Hibonite

The MIR features of Colony hibonite are quite different from those of Madagascar hibonite (Figure 3). Colony hibonite exhibits many spectral features including a broad band between 855 and 795 cm⁻¹ (with a maximum at 810 cm⁻¹) and many single peaks at 920, 740, 710, 625, 600, 557, and 523 cm⁻¹. Interestingly, the Colony hibonite spectrum shows features similar to those of the synthetic hibonite from Hofmeister et al. (2004) even if the latter was measured in absorption. The only differences between these two hibonites are (1) the peak at 920 cm⁻¹ in Colony hibonite that is not observed in the synthetic hibonite and
<figure><img src="image_4.png" /><figcaption>FIGURE 3. Standard spectra in reflectance (%) in the range 1400 to 400 cm⁻¹ of oxides and silicates used for the detection of RIs. Spectra are offset for clarity and not normalized. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

(2) the two peaks at 625 and 600 cm⁻¹ correspond actually to three separated peaks in the synthetic hibonite. By contrast, Madagascar hibonite has less features with two broad bands (1) from 850 to 750 cm⁻¹ (with a maximum at 785 cm⁻¹) including a shoulder a 730 cm⁻¹ and (2) from 620 to 550 cm⁻¹ (with a maximum at 600 cm⁻¹).

### Spinel

The Vietnam spinel exhibits broad bands between 750 to 700 and 560 to 490 cm⁻¹ with maxima at 730 and 525 cm⁻¹, respectively. It also presents a shoulder feature at 835 cm⁻¹ and a narrow peak at 590 cm⁻¹. All these features correspond to band positions of terrestrial spinels measured in reflectance by Chihara et al. (2000).

### Gehlenite and Åkermanite

Gehlenite and åkermanite (Al- and Mg-rich endmembers of the melilite solid solution, respectively) show many characteristic features in the MIR range that make them easily detectable. Our synthetic gehlenite shows features at 1060, 1046, 1020, 980, 913, 895, 856, and 809 cm⁻¹, that match band positions of synthetic gehlenite of Chihara et al. (2007) and meteoritic gehlenite of Morlok et al. (2014) both measured in absorption, except for the 1046 cm⁻¹ peak that was not described previously. Our synthetic åkermanite exhibits peaks at 1073, 1016, 979, 941, and 852 cm⁻¹, and two shoulders at 993 and 908 cm⁻¹ that match band positions of synthetic åkermanite of Chihara et al. (2007) measured in
<figure><img src="image_5.png" /><figcaption>FIGURE 4. Evolution of Ca-pyroxene spectra with Al and Ti enrichments in reflectance (%) within the range 1400 to 400 cm⁻¹. (a) Standard diopside spectra (terrestrial 71-98, fine-grained and Al-Ti-rich diopside in Colony) and (b) Ca-pyroxene spectra in 3529-Z type B CAI. The vertical dashed gray lines are centered on the maximum of reflectance peaks for the fine-grained diopside in Colony. The compositions are given in wt% of Al₂O₃ and TiO₂. Spectra are offset for clarity. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

absorption except for the 1073 cm⁻¹ feature that rather peaks around 1042 cm⁻¹ in the latter study.

### Anorthite, Nepheline, and Sodalite

Hachijo anorthite spectrum shows features at 1180, 1125, 1026, 988, 950, 927, and 903 cm⁻¹ that approximately match band positions of synthetic anorthite measured in absorption by Chihara and Koike (2017). Also, the Hachijo anorthite spectrum is similar to the California anorthite spectrum measured in reflectance reported in Nash and Salisbury (1991).

Posch et al. (2007), who measured CAIs in absorption, attributed a 990-cm⁻¹ strong band to nepheline that, here, Bancroft nepheline also exhibits but closer to 1010 cm⁻¹. Bancroft nepheline additionally has two peaks at 1096 and 1045 cm⁻¹ that were not observed in Posch et al. (2007). The spectrum of the meteoritic nepheline measured in reflectance from Morlok et al. (2020) exhibits the three peaks we also observe in the terrestrial Bancroft nepheline.

Morlok et al. (2008), who measured CAIs in absorption, attributed a 980-cm⁻¹ strong band, and two

features at 735 and 670 cm⁻¹ to sodalite. Although these features are also observed in the spectrum of Bancroft sodalite, the strong band at 980 cm⁻¹ is changed here into a strong band with a maximum at 1005 cm⁻¹ and a shoulder at 983 cm⁻¹. The spectrum of the meteoritic sodalite measured in reflectance from Morlok et al. (2020) also exhibits this strong band around 1000 cm⁻¹ associated with its shoulder peak around 980 cm⁻¹.

### Ca-Pyroxene Spectra with Different Al-Ti Enrichments

The three clinopyroxene spectra shown in Figure 4a exhibit a similar spectral shape that is characteristic of diopside in general. However, the Lherz 71-98 diopside shows a weak band between the two peaks at 1095 and 955 cm⁻¹ that is absent from the spectra of Colony diopsides. The Lherz 71-98 diopside exhibits peaks at 1095, 955, 923, and a shoulder at 885 cm⁻¹. By contrast, the fine-grained diopside from Colony exhibits IR features at 1080, 960, 918, and 880 cm⁻¹ that match band positions of the synthetic diopside measured in absorption from Koike et al. (2000) and the meteoritic
<table><caption>TABLE 1. Characteristic MIR features between 1400 and 400 cm⁻¹ for each mineral standard.</caption><thead><tr><th>cm<sup>-1</sup></th><th>Corundum<br>Longido<br>Al<sub>2</sub>O<sub>3</sub></th><th>Perovskite<br>Gardiner<br>Complex<br>CaTiO<sub>3</sub></th><th>Hibonite<br>Colony<br>(meteorite)<br>CaAl<sub>12</sub>O<sub>19</sub></th><th>Hibonite<br>Madagascar<br>CaAl<sub>12</sub>O<sub>19</sub></th><th>Spinel<br>Vietnam<br>MgAl<sub>2</sub>O<sub>4</sub></th><th>Synthetic<br>åkermanite<br>Ca<sub>2</sub>MgSi<sub>2</sub>O<sub>7</sub></th><th>Synthetic<br>gehlenite<br>Ca<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub></th><th>Anorthite<br>Hachijo<br>CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub></th></tr></thead><tbody><tr><td>1150–1100</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>1180*<br>1125</td></tr><tr><td>1100–1050</td><td></td><td></td><td></td><td></td><td></td><td>1073</td><td>1060*</td><td></td></tr><tr><td>1050–1000</td><td></td><td></td><td></td><td></td><td></td><td>1016</td><td>1046<br>1020</td><td>1026</td></tr><tr><td>1000–950</td><td></td><td></td><td></td><td></td><td></td><td>993*<br>979</td><td>980</td><td>988<br>950*</td></tr><tr><td>950–900</td><td></td><td></td><td>920</td><td></td><td></td><td>941<br>908*</td><td>913</td><td>927<br>903*</td></tr><tr><td>900–800</td><td><strong>845 to 650</strong></td><td></td><td><strong>875 to 780</strong><br>(max 810)</td><td><strong>850 to 750</strong><br>(max 785)</td><td><strong>835*</strong><br>(max 730)</td><td>852</td><td>895 856 809</td><td></td></tr><tr><td>800–700</td><td>618</td><td><strong>710 to 585</strong></td><td>740<br>710</td><td>730*</td><td><strong>750 to 700</strong><br>(max 730)</td><td></td><td></td><td></td></tr><tr><td>700–600</td><td></td><td></td><td>625<br>600</td><td><strong>620 to 550</strong><br>(max 600)</td><td>590<br><strong>560 to 490</strong><br>(max 525)</td><td></td><td></td><td></td></tr><tr><td>600–500</td><td>583</td><td></td><td>557<br>523</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Note: The corresponding spectra can be seen in Figures 3 and 4. The position of the feature is noted in cm⁻¹. Please note that the actual composition of each standard is shown in Figure S4; here, it simply indicates the close but ideal composition of each mineral. Underline = plateau. A plateau is a broad band with a flat top. No maximum of reflectance can be determined for such a feature. Bold = broad band. A broad band is a peak that is larger than 50 cm⁻¹. The edges of the broad bands noted here are where the reflectance begins to fall rapidly. A maximum of reflectance for each broad band in hibonite and spinel is noted as they have a curved top. Asterisk* = shoulder peak. A shoulder is a peak that is not sufficiently intense to be individualized, and therefore appears as a contribution close to a more intense peak. The peak and shoulder positions are attributed by the position of the maximum of reflectance.

diopside measured in absorption from Posch et al. (2007) and Morlok et al. (2014). The Al-Ti-rich diopside from Colony exhibits clear peaks at 1078, 943, and 884 cm⁻¹.

Although the three Ca-pyroxene spectra from 3529-Z with different Al-Ti enrichments (from edge to core) shown in Figure 4b share some common peaks with diopside, they also exhibit different spectral shapes. Note that Ca-pyroxenes from 3529-Z are well crystallized with different crystalline orientations and show sector zoning. The edge of pyroxene exhibits peaks at 1087, 968, and 915 cm⁻¹. The edge have similar peak positions with the fine-grained diopside, and they have close Al-Ti composition. The area between the edge and the core exhibits peaks at 1094, 1000, 958, and 850 cm⁻¹. The core exhibits peaks at 1060, 940, and 882 cm⁻¹. Therefore, we observe that Ca-pyroxene spectra with different Al-Ti enrichments have different peak positions.

## Detections of RIs in the Maps of Murchison, Paris, and Colony

We used the method developed for the detection of gehlenite, diopside, spinel, and hibonite on the maps of Murchison, Paris and Colony, and the results of these detections are Euclidean spectral distance maps shown in Figures 5–7. The detection of gehlenite is only shown for

Colony as gehlenite was not detected in the maps of Murchison and Paris.

### Murchison Section

Using the spinel detection method, we found nine objects (Figure 5a) and using the diopside detection method we found 11 objects (Figure 5b) in the map of Murchison. One of the detected objects was found by using both spinel and diopside detection methods. The SEM MgCaAl X-ray map shows that these 19 detected objects are indeed RIs (Figure 5c). In total, we detected 19 RIs for an area of 8.04 mm² in Murchison, accounting for 2.36 RIs/mm².

We found 28 RIs in the MgCaAl X-ray map of Murchison against 19 with the detection method in IR. SEM examinations show that among the nine RIs that were not detected in IR (true negatives), only four have the shape of a RI and are recognizable as so. These four RIs are phyllosilicate-rich. The last five undetected RIs are small fragments ranging from 15 to 30 µm in size.

### Paris Section

Using the spinel detection method, we found 12 objects (Figure 6a) in the map of Paris. We also found seven objects with the diopside detection method (Figure 6b). Two of the detected objects were found by both spinel and diopside detection methods. The SEM
<table><thead><tr><th>Nepheline<br>Bancroft<br>Na<sub>3</sub>KAl<sub>4</sub>Si<sub>4</sub>O<sub>16</sub></th><th>Sodalite<br>Bancroft<br>Na<sub>4</sub>Si<sub>3</sub>Al<sub>3</sub>O<sub>12</sub>Cl</th><th>Diopside<br>Lherz<br>CaMgSi<sub>2</sub>O<sub>6</sub></th><th>Fine-grained<br>diopside<br>colony<br>(meteorite)</th><th>Al-Ti-rich<br>diopside<br>colony<br>(meteorite)</th><th>Edge of<br>Ca-pyroxene<br>3529-Z<br>Allende<br>(meteorite)</th><th>In<br>between<br>3529-Z<br>Allende<br>(meteorite)</th><th>Core of<br>Ca-pyroxene<br>3529-Z<br>Allende<br>(meteorite)</th><th>Fosterite<br>San Carlos<br>(Mg,Fe)<sub>2</sub><br>SiO<sub>4</sub></th></tr></thead><tbody><tr><td>1096</td><td></td><td>1095</td><td>1080</td><td>1078</td><td>1087</td><td>1094<br>1000</td><td>1060</td><td></td></tr><tr><td>1045</td><td>1005</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1045</td></tr><tr><td>1010</td><td>983*</td><td>955<br>923</td><td>970<br>918</td><td>943</td><td>968<br>915</td><td>958</td><td>940</td><td>982<br>942</td></tr><tr><td></td><td></td><td>885*</td><td>880*</td><td>884</td><td></td><td>850</td><td>882</td><td>837</td></tr></tbody></table>

MgCaAl X-ray map shows that one object found using the spinel detection method is not a RI (false positive) and that the other 16 detected objects are indeed RIs (Figure 6c). In total, we detected 16 RIs for an area of 8.18 mm² in Paris, accounting for 1.96 RIs mm⁻².

## Colony Section

Using the spinel detection method, we found 14 objects (Figure 7a) in the map of Colony. The clustering method shows that among these 14 objects, 13 are spinel-bearing and one is hibonite-bearing (Figure 2). We also found four objects using the diopside detection method (Figure 7b). Finally, we found two objects based on the gehlenite detection method (Figure 7c). Two of the detected objects were found by both spinel and diopside detection and two other objects were found by both spinel and gehlenite detection. The SEM MgCaAl X-ray map attests that these 16 detected objects are indeed RIs (Figure 7d). In total, we detected 16 RIs for an area of 8.67 mm² in Colony, accounting for 1.84 RIs/mm².

## IR Mineralogical Characterization of RIs in Murchison

Following their detection, 11 RIs from Murchison were studied in detail in the MIR range. They

were analyzed in spot mode from 4000 to 400 cm⁻¹ and by clustering on hyperspectral maps from 4000 to 800 cm⁻¹. The 11 studied RIs can be grouped into four groups based on their MIR features (Figure 8): spinel-rich (1), diopside-rich (2), phyllosilicates-diopside-rich (3), and olivine-diopside-rich (4). These groups are indicative of the main mineralogical phases of the observed RIs.

1. The IR spectra of CAI-14, 13, and 08 exhibit a clear spinel feature near 835 cm⁻¹ in Figure 8a. In addition, the spectra of CAI-14 and 13 exhibit weak diopside features. Although CAI-14, 13, and 08 may contain minor phases, their spectra in Figure 8b show characteristic spinel features, which are two strong broad bands around 750–700 and 560–490 cm⁻¹, and shoulders at 835 and 590 cm⁻¹. Therefore, CAI-14, 13, and 08 belong to the spinel-rich group.

2. CAI-10 and 09 belong to the diopside-rich group as their spectra exhibit clear diopside peaks at 1080, 970, and 918 cm⁻¹ with a shoulder at 880 cm⁻¹ (Figure 8b). The diopside features for CAI-10 around 1080, 918, and 880 cm⁻¹ are somewhat weaker (Figure 8a), meaning it probably also bears minor phases.

3. The IR spectra of CAI-03, 07, 05, 06, and 01 exhibit a strong peak at 970 cm⁻¹, weaker peaks around 1080, 970 cm⁻¹.
Murchison meteorite

<figure><img src="image_6.png" /></figure>

FIGURE 5. Correspondence between the Euclidean spectral distance map to spinel standard, to fine-grained diopside standard and the scanning electron microscopy (SEM) X-ray MgCaAl map of Murchison (1760 × 4928 µm). (a) Euclidean spectral distance map to the spinel standard between 800 and 940 cm⁻¹. The refractory inclusions (RIs) that are detected based on spinel are highlighted with black arrows. Gray pixels (pixels without value) are either out of the section or metal grains. (b) Euclidean spectral distance map to fine-grained diopside standard between 920 and 1040 cm⁻¹. The RIs that are detected based on diopside are highlighted with white arrows. Gray pixels (pixels without value) are outside the section, metal grains or removed pixels with a Euclidean distance superior to 0.032. (c) The corresponding false colors SEM X-ray MgCaAl (RGB) map of Murchison. Arrows with combined black and white colors highlight RIs that are detected based on both spinel and diopside. (Color figure can be viewed at wileyonlinelibrary.com)

FIGURE 6. Correspondence between the Euclidean spectral distance map to spinel standard, to fine-grained diopside standard and the scanning electron microscopy (SEM) X-ray MgCaAl map of Paris (3872 × 2112 µm). (a) Euclidean spectral distance map to the spinel standard between 800 and 940 cm⁻¹. The RIs that are detected based on spinel are highlighted with black arrows. The pink arrow highlights the false positive detection. Gray pixels (pixels without value) are pixels out of the section or metal grains. (b) Euclidean spectral distance map to fine-grained diopside standard between 920 and 1040 cm⁻¹. The RIs that are detected based on diopside are highlighted with white arrows. Gray pixels (pixels without value) are outside the section, metal grains or removed pixels with a Euclidean distance superior to 0.032. (c) The corresponding SEM X-ray MgCaAl (RGB) map of Paris. Arrows with combined black and white colors highlight RIs that are detected based on both spinel and diopside. (Color figure can be viewed at wileyonlinelibrary.com)
Paris meteorite

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>
Colony meteorite

<figure><img src="image_10.png" /></figure>

FIGURE 7. Correspondence between the Euclidean spectral distance map to spinel standard, to fine-grained diopside standard, to gehlenite standard and the scanning electron microscopy (SEM) X-ray MgCaAl map of Colony (3520×2464 µm). (a) Euclidean spectral distance map to the spinel standard between 800 and 940 cm⁻¹. The RIs that are detected based on spinel are highlighted with black arrows. The purple arrow highlights a hibonite detection. Gray pixels (pixels without value) are pixels out of the section or metal grains. (b) Euclidean spectral distance map to fine-grained diopside standard between 920 and 1040 cm⁻¹. The RIs that are detected based on diopside are highlighted with white arrows. Gray pixels (pixels without value) are outside the section, metal grains or removed pixels with a Euclidean distance superior to 0.032. (c) Euclidean spectral distance map to the gehlenite standard between 820 and 1100 cm⁻¹. The RIs that are detected based on gehlenite are highlighted with blue arrows. (d) The corresponding SEM X-ray MgCaAl (RGB) map of Colony. Arrows with combined black and white colors highlight RIs that are detected based on both spinel and diopside standards. Arrows with combined black and blue colors highlight RIs that are detected based on both spinel and gehlenite standards. (Color figure can be viewed at wileyonlinelibrary.com)
<figure><img src="image_11.png" /></figure>

FIGURE 8. (a) Scaled infrared (IR) spectra of the 11 studied refractory inclusions (RIs) in Murchison collected by clustering on the hyperspectral maps in the range 3900–3100 and 1300–800 cm⁻¹. Vertical dashed lines indicate the positions of mineral features for diopside in gray, spinel in black and olivine in dark red. The turquoise dashed vertical line corresponds to the maximum absorption of the metal-OH band, except for CAI-02. The blue dashed vertical line corresponds to the adsorbed water feature. (b) Scaled IR spectra of the 11 studied RIs in Murchison collected in spot mode in the range 1500–400 cm⁻¹. Vertical dashed lines indicate the positions of mineral features for diopside in gray, spinel in black and olivine in dark red. Spectra are offset for clarity. (Color figure can be viewed at wileyonlinelibrary.com)

<figure><img src="image_12.png" /></figure>

918 cm⁻¹, and a shoulder at 880 cm⁻¹ that correspond to diopside features (Figure 8a). However, the 970-cm⁻¹ peak is still strong compared to other diopside features that are weaker. We know from our data that Fe-phyllosilicates exhibit mainly one strong peak, which is also located around 970 cm⁻¹ (Figure 9). Therefore, if we combine the

spectrum of diopside and Fe-phyllosilicates, we can obtain those of CAI-03, 07, 05, 06, and 01, indicating these CAIs belong to the phyllosilicates-diopside-rich group.

4. The IR spectrum of CAI-02 exhibits clear diopside features at 1080, 918, and 880 cm⁻¹ (Figure 8). We can also observe the contribution of two thin peaks at 982
and 839 cm⁻¹ that we attribute to olivine (Figure S1). In addition, the fact that the two features of diopside around ~900 cm⁻¹ are more prominent than usual (i.e., diopside alone) is due to the contribution of a strong olivine band in this range (Figure S1). Therefore, CAI-02 belongs to the olivine-diopside-rich group as its MIR spectrum shows both the characteristic features of olivine and diopside.

## SEM Observations of RIs in Murchison

All CAIs discussed hereafter are shown in Figure S2. SEM observations revealed that CAI-01, 03, 05, 06, 07, 08, 09, 10, 13, and 14 are fine-grained CAIs, ranging in size from 40 to 290 µm, with different proportions of spinel, Fe-phyllosilicates and (Al-Ti-) diopside. CAI-03 is composed of Fe-phyllosilicates with minor spinel and a thin rim of diopside. CAI-05 and 06 are composed of small perovskite-spinel cores surrounded by Fe-phyllosilicates and a 10- to 20-µm thick diopside rim (see CAI-06 in Figure 10). CAI-07 is composed of large nodules of Fe-phyllosilicates enclosed by diopside rims, with small spinels scattered throughout the phyllosilicate mass. CAI-01 is an aggregate of several small nodules (10–20 µm) containing spinel-perovskite cores surrounded by Fe-phyllosilicates, themselves enclosed by a diopside rim. CAI-10 is an aggregate of 5–10 µm wide nodules that contain Al-diopside and diopside only. CAI-09 is a compact (Al-Ti-) diopside mass enclosing minor spinel and Fe-phyllosilicates (Figure 10). CAI-13 and 14 are composed of a large spinel core surrounded by a thin layer of Fe-phyllosilicates (few µm), themselves enclosed by a 10-µm thick diopside rim (see CAI-13 in Figure 10). CAI-08 looks like CAI-13 and 14 except it has no diopside rim and a thicker layer of Fe-phyllosilicates (10 to 15 µm). And finally, unlike others, CAI-02 is composed of spinel-diopside nodules enclosed by a forsterite mass (Figure 10). CAI-02 is thus different from other fine-grained CAIs and looks more like an AOA.

## Water Features in RIs from Murchison

Here, we describe the characteristics of the different IR water features in RIs from Murchison. In Figure 8a, most RIs exhibit a minimum of reflectance (i.e., maximum of absorption in the volume scattering regime) at 3625 cm⁻¹ (2.75 µm) which correspond to the metal-OH band of phyllosilicates within CAIs (Figure 9). This band seems to be more prominent in the RI spectra, where the peak intensities of spinel and diopside are weak.

As expected, the group of phyllosilicates-diopside-rich RIs shows a more pronounced metal-OH band

than the group of spinel-rich RIs. Note that the group of the diopside-rich RIs also shows a weak metal-OH band. Unlike others, the olivine-diopside-rich RI (called CAI-02) shows a weak metal-OH band around 3700 cm⁻¹ rather than at 3625 cm⁻¹, which could be related to a different type of phyllosilicates in this RI.

The H₂O stretching vibration around 2.9 µm in RIs was weak or absent (Figure 8a), attesting of limited H₂O adsorbed on the surface of minerals.

## DISCUSSION

### Suitable Standards for Detection?

In the following, we discuss the choice of the standards that should be representative of the mineral assemblages found in RIs. The quality of this choice will, indeed, affect the efficiency of their detection.

#### Corundum

The variations in composition between meteoritic and terrestrial corundum are limited and are due to variations in their content in minor or trace elements. Therefore, the IR spectra of corundum of terrestrial or meteoritic origin is expected to be similar. Longido corundum contains little Fe and Cr (0.31 wt% Fe₂O₃ and 0.97 wt% Cr₂O₃; Figure S4). Meteoritic corundum can contain up to 1 wt% FeO and <1 wt% in TiO₂, CaO, MgO, and SiO₂ (Bar-Matthews et al., 1982; Krot et al., 2001; Makide et al., 2013; Simon et al., 2002). The synthetic corundum studied by Zeidler et al. (2013) is representative of meteoritic/astronomical corundum and exhibits close spectral features with Longido corundum, which can therefore be considered representative of meteoritic/astronomical corundum and is thus a suitable standard for detection.

#### Perovskite

Perovskite in RI are almost pure CaTiO₃. The Gardiner Complex perovskite contains Ca and Ti as major elements, and should therefore have spectral similarities with the pure CaTiO₃. However, minor modifications in Gardiner Complex perovskite involve substitutions of Ca by larger atoms such as Sr and Ce, and of Ti by smaller (Al) and larger (Nb) atoms (Figure S4). This should probably result in a CaTiO₃ perovskite spectrum with shifted peak positions and minor spectral differences.

#### Hibonite

Hofmeister et al. (2004) pointed out that hibonites from Madagascar are not good IR analogs of
<figure><img src="image_13.png" /></figure>

FIGURE 9. Raw mid-infrared spectrum of phyllosilicates in the range 3900–800 cm⁻¹ extracted from the CAI-06. The turquoise dashed vertical line corresponds to the maximum absorption of the metal-OH band at 3625 cm⁻¹. The blue dashed vertical line corresponds to the adsorbed water feature. The brown vertical dashed lines correspond to the phyllosilicate features at 970, 925, and 818 cm⁻¹. (Color figure can be viewed at wileyonlinelibrary.com)

meteoritic hibonites due to metamictization and significant substitution in their crystal lattice. Most likely, the spectrum of Madagascar hibonite has weaker and broader peaks than meteoritic hibonite due to the loss of crystal structure associated with metamictization. Our data agree with this conclusion as the spectrum from Colony hibonite is largely distinct from the one of Madagascar hibonite. Like Hofmeister et al. (2004), we observed a shift toward higher wavelengths in Madagascar hibonite of about ~25 cm⁻¹ (Figure 3) that can be attributed to substitutions by larger atoms (Ca by Ce and La, and Al by Ti, Mg, Zn and Fe; Figure S4) in the lattice. Also note that among all the standard spectra collected here, terrestrial hibonite is the one that diverges the most from its meteoritic counterpart.

The Colony hibonite has a composition close to the pure CaAl₁₂O₁₉ with minor substitution of Al by Mg, Ti and Fe (Figure S4) like many meteoritic hibonites. Like the spectrum of synthetic hibonite obtained by Hofmeister et al. (2004), the Colony hibonite spectrum can be considered as a suitable standard for the IR study

<figure><img src="image_14.png" /></figure>

FIGURE 10. RGB false color X-ray maps (MgCaAl) superimposed with BSE images of four of the RIs studied in IR in Figure 8. Each RI is a representative of its class based on mid-infrared features (phyllosilicate-diopside-rich, diopside-rich, spinel-rich, and olivine-diopside-rich). (Color figure can be viewed at wileyonlinelibrary.com)
of hibonites in meteorites and astronomical environments.

## Gehlenite and Åkermanite

Meteoritic melilite is quite different from terrestrial melilite found in slags and ultramafic volcanic rocks, which contain common and abundant Fe and Na-rich components. In meteorites, melilite is restricted to RIs and, with the exception of two reports of ferro-åkermanite (Aléon et al., 2022; Shollenberger et al., 2022), is a solid solution restricted to the Mg and Al endmembers (åkermanite and gehlenite, respectively). As a result, our synthetic Mg and Al melilite endmembers provide a better comparison with meteoritic melilites than terrestrial ones.

## Spinel

The terrestrial spinel from Vietnam measured here is almost pure $MgAl_2O_4$ like those found in RIs from unheated meteorites. The Vietnam spinel contains 0.39 wt % FeO (Figure S4), which is still consistent with spinel from metasomatized inclusions from CV chondrites (Connolly Jr et al., 2003; Connolly Jr & Burnett, 1999). Therefore, Vietnam spinel can be considered as a good analog for spinels in RIs.

## Anorthite, Nepheline, and Sodalite

Because Hachijo anorthite is close to the calcium endmember of the plagioclase solid solution (0.49 wt% $Na_2O$; Figure S4) and contains only a small amount of Fe (0.6 wt% FeO; Figure S4), it should be a good analog of meteoritic anorthite, which is commonly the pure anorthite endmember in RIs (Brearley & Jones, 1998; and references therein). It can therefore be considered a suitable standard for their detection.

The spectra of Bancroft nepheline and sodalite show many similarities with meteoritic nepheline and sodalite spectra of Morlok et al. (2020). This supports the idea that our standards are good analogs of their meteoritic counterparts and suitable standards for detection.

Sodalite, nepheline, and anorthite were not used in the present study, neither for detection nor for characterization of RIs, because none of these three minerals are found in RIs from our sections of Murchison, Paris, and Colony. However, their IR spectra can be used to study RIs in chondrites that commonly show such minerals (e.g., CV chondrites, CO chondrites with higher grade of thermal metamorphism).

## Diopside

The feature positions of the Colony fine-grained diopside correspond to the peak positions of the synthetic diopside of Koike et al. (2000) and the meteoritic

diopside of Posch et al. (2007) and Morlok et al. (2014). The Colony fine-grained diopside is a suitable standard for the detection of diopside in RIs as it was directly acquired in a fine-grained CAI in Colony.

## Evolution of the Spectra of Ca-clinopyroxene with Al-Ti Enrichments

The composition of Ca-pyroxenes from RIs evolves from diopside ($CaMgSi_2O_6$) to Al and/or Ti-rich Ca-pyroxene (kushiroite and grossmanite; Ma et al., 2010), where Al and Ti mainly substitute for Si and Mg. These substitutions should cause peak position to shift but in a non-trivial way because the atoms in diopside, namely Si and Mg, are simultaneously replaced by larger and smaller atoms (Al and Ti) in Al-Ti-enrich Ca-pyroxenes; Mg is larger than Al and Ti, while Si is smaller than Al and Ti. In addition, the differences in spectral shapes and peak intensities between Ca-pyroxenes spectra could be attributed to variable crystallographic orientations and crystal zoning of pyroxenes within the 3529-Z inclusion.

It is best to be cautious when using diopside as a reference for Al-Ti-diopside because some discrepancies may occur in the spectra. More data over a wider range of Al and Ti composition in Ca-pyroxenes would certainly further constrain the evolution of IR features along with Al and Ti enrichments.

## Terrestrial Versus Synthetic Versus Meteoritic/Astronomical RI Minerals

The minerals found in RIs (i.e., meteoritic) are mainly restricted to specific endmember or solid solution composition. A similar assumption can be made for RI minerals in astronomical environments, as they are expected to resemble meteoritic ones. Our synthetic standards, which are endmembers devoid of minor and trace element (Figure S4), are more similar to minerals found in meteorites or astronomical environment.

However, terrestrial standards of RI minerals can contain different minor and trace element substitutions (e.g., Ce, La, and Th for Madagascar hibonite; Figure S4). These substitutions in the crystal lattice by larger or smaller atoms lead to shifts in peak positions. Therefore, spectra of terrestrial standards of RI minerals may sometimes significantly differ from those of their synthetic and meteoritic/astronomic counterparts (e.g., Madagascar vs. Colony hibonite in Figure 3).

## Database: Perspectives for Returned Space Samples and Remote Sensing Observations

This new robust database of standards makes the determination of characteristic features of each CAI mineral possible and, consequently, eases the
recognition of their features. Also, building such MIR reflectance database on a large panel of minerals will enable to detect RIs located in meteorite or asteroid grain sections.

As stated by Posch et al. (2007), the detection of Ca-Al-rich minerals in astronomical environments should only be a matter of increasing instrumental sensitivity or spatial/spectral resolution. Interestingly, the arrival of the James-Webb Space Telescope (JWST) is pushing back the boundaries of remote sensing IR observations. Using our database of standards for the interpretation of astronomical observations (e.g., from the JWST) could therefore bring new fields of possibilities concerning the discovery of refractory minerals in astronomical environments.

For example, this database could be used to detect CAI-rich primitive asteroids in the solar system (proposal of de Kleer et al., 2021). Indeed, the database combined with JWST data could shed light on the hypothesis that L-type asteroids contain up to 30% of CAI materials (Devogèle et al., 2018; Mueller et al., 2023; Sunshine et al., 2008). Additional care should be taken when comparing our database with remote sensing observations, in particular in the MIR where remote sensing data are usually acquired in emission. This can be critical in the case of fine dust. Overall, our results should be applicable to conditions with similar geometry and where surface scattering is still dominant (boulders or surfaces with large grain size), but care should be taken when comparing with surfaces where volume scattering dominates (e.g., smaller grain size).

Our database of standard spectra could also be used to detect refractory minerals or inclusions in other forming planetary systems. For example, olivine, pyroxene, and amorphous silicate dust have already been detected in protoplanetary disks around solar-mass stars (Furlan et al., 2011; Olofsson et al., 2009; Watson et al., 2008) and Hofmeister et al. (2004) used spectra of refractory minerals (e.g., hibonite) to detect their presence in a planetary system in formation. Also, caution is advised when comparing our database with remote sensing observations of protoplanetary disks, as these observations rely mainly on transmission spectroscopy, whereas our data are based on reflection spectroscopy. The band positions of CAI minerals may vary between transmission and reflection.

Still finding IR features of CAI minerals in protoplanetary disks around other stars would bring new constraints about the evolution of the disk in the early solar system. And most importantly, it would show that the process of RI formation is not specific to the solar system but is widespread around other forming protoplanetary disks.

## Efficiency and Limits of Detection

Using the developed detection method, we were able to detect on average two RIs per millimeter squared. However, the number of detected RIs per unit area can greatly vary because of the heterogeneous distribution and occurrence of RIs among chondrites.

To evaluate the proportion of detected RIs relative to the total RIs present in a section of meteorite and to evaluate the efficiency of detection, we calculated the area of the detected RIs relative to the area covered by all the RIs found on the SEM X-ray map of Murchison. We find an area ratio of detected CAIs to overall CAI content of 0.91 and an area ratio of RIs to overall RI content of 0.965 on Murchison. We were thus able to detect approximately 91% of the total CAI surface and 96.5% of the total RI surface, which is a satisfactory proxy for the total volume or mass of refractory material in the sample.

Nineteen out of 28 RIs were detected using Euclidean distance method on the Murchison map. SEM observations showed that the nine undetected RIs were not detected because they contain refractory minerals (other than phyllosilicates) whose sizes are roughly smaller than the IR diffraction limit (around 10 µm). In detail, the spatial resolution limit of our detection method differs from the IR diffraction limit and from mineral to mineral as it depends on the spectral contrast of individual minerals. The spectral contrast is high when the spectrum of a mineral has intense and/or very local bands, and/or bands that do not overlap with those of other minerals. See carbonates detection in Dionnet et al. (2018) for an additional explanation of this effect. Even if a mineral is smaller than the IR diffraction limit, it can be detected if it has a high spectral contrast. On the other hand, if a mineral has a low spectral contrast, a larger surface area of that mineral is required to be detected. For example, diopside and gehlenite have lots of relatively intense and narrow bands above 800 cm⁻¹ which is within the range of the FPA detector (4000–800 cm⁻¹). Therefore, diopside and gehlenite, which have a high spectral contrast, can be detected even if they are smaller than the IR diffraction limit. Indeed, SEM examination showed that RIs are efficiently detected by IR when the spatial extent of diopside or gehlenite is greater than 4–5 µm. On the contrary, hibonite and spinel (oxides) have their intense features below the range of the FPA detector (i.e., under 800 cm⁻¹). It means that they have a low spectral contrast in our range of study. Consequently, the detection limit for hibonite and spinel is closer to 8–10 µm, which corresponds to the IR diffraction limit. As hibonite and spinel are collectively detected due to similar spectral features in our studied spectral range, RIs are detected
when the spatial extent of hibonite-spinel together is greater than 8–10 µm.

Most studies focus on RIs larger than 10–20 µm (e.g., MacPherson & Davis, 1994; Rubin, 2007, 2015) because other objects are too small to be studied in detail by most of the traditional in-situ analyses methods (e.g., SIMS and EPMA). In addition, tiny fragments of RIs are generally not studied because the full context of the initial object is missing. Finally, although altered RIs enable the study of alteration on the parent body, pristine RIs are more extensively studied in order to understand nebular processes. Here, the nine undetected RIs are either phyllosilicate-rich or small fragments which size range from 15 to 30 µm. Therefore, our detection method is efficient to detect most RIs whose study will provide information about their origin.

## IR Spectroscopy Versus SEM Observations

The SEM-EDS examinations of the 11 RIs confirm their assignment to the mineralogical groups defined on the basis of their reflectance MIR features (Figure 8). Indeed, the mineralogy determined by MIR features corresponds to the mineralogy observed with greater resolution on X-ray maps of individual RIs (Figure 10). However, minor phases like perovskite and sometimes spinel observed by SEM in the RIs could not be clearly identified from the IR average spectra in Figure 8. Although spinel could not be identified in the average spectrum, it could in some cases (e.g., CAI-06) be observed in the individual RI hyperspectral map.

The SEM and IR observations of the 11 detected RIs are consistent with the previous descriptions of CAIs in CM chondrites (MacPherson, 2014), which showed that the most common group of CAIs found in CM chondrites are spinel-pyroxene inclusions. These inclusions are composed of a core of spinel ± perovskite mantled by Fe-rich phyllosilicates, themselves rimmed by diopside. It is also consistent with the observations that Fe-rich phyllosilicates are the most common alteration phase of CAIs in Murchison (MacPherson et al., 1983).

The fact that SEM investigations confirm the observations made in MIR is important for the study of returned space samples. Indeed, it means that with MIR spectroscopy only, it is possible to characterize the mineralogy of the RIs during initial characterization of priceless returned space samples without using any destructive method or methods requiring coating like SEM. In this way, returned space samples can be easily selected based on these analyses during calls of opportunity, in order to address specific scientific questions regarding RIs.

IR reflectance spectroscopy performed in spot mode in the whole MIR range (i.e., 4000–400 cm⁻¹) provides an

easy recognition of a larger panel of minerals as it probes both silicates and oxides. However, spot measurements cannot fully sample the surface of a RI as explained before. Although hyperspectral maps in the range 4000–800 cm⁻¹ do not provide information about oxides, they allow spatial location of each component and a better spatial sampling, and thus yield spectra that fully sample RIs in their entirety. Both hyperspectral imaging and spot measurements are thus complementary to fully study individual RI.

Although IR reflectance spectroscopy alone provides sufficient characterization of returned space samples in the preliminary phases, it is interesting to see the complementarity between SEM and IR spectroscopy more generally. SEM provides µm-scale resolved information on the mineralogy and chemical composition of anhydrous phases, while IR spectroscopy provides molecular information at the tens of microns scale such as the presence of organic matter, the mineralogy of anhydrous and hydrated phases, the crystallinity of the phases and their water content. The combination of these two methods would therefore enable a robust study of samples on several aspects. An interesting example discussed below is the case of water abundance and molecular state in RIs that could not have been constrained by SEM alone.

## Nature of Hydration in RIs

RIs in CM chondrites recorded the aqueous alteration of their parent body and can hold a significant amount of OH/H₂O within their phyllosilicates. We therefore explored the nature of hydration of the 11 detected RIs based on their IR spectra displayed in Figure 8a and the extracted IR spectrum of phyllosilicate from CAI-06 (Figure 9).

The occurrence of the metal-OH band attests to the presence of OH groups bounded with octahedral layers of phyllosilicates. In more detail, the metal-OH band seen in RI spectra closely resembles phyllosilicates spectra of the serpentine family in Calvin and King (1997), especially antigorite, lizardite, and greenalite spectra. In addition, in Calvin and King (1997), the metal-OH band of the Mg-endmembers serpentine (lizardite) peaks close to 2.7 µm (3700 cm⁻¹) while Fe-endmembers peak closer to 2.75 µm (3625 cm⁻¹). Because the metal-OH band of phyllosilicates in Murchison RIs also peak at 2.75 µm (Figures 8a and 9), they seem to be more closely related to a Fe-endmember of the serpentine family. Our measurements were carried out under atmospheric conditions and therefore RI spectra show the contribution around 2.9 µm (3450 cm⁻¹) of H₂O molecules adsorbed on the mineral surface (Figures 8a and 9). Still, this contribution remains weak compared to the metal-OH band.
Elmaleh et al. (2015) determined by transmission electron microscopy that the phyllosilicates in a CAI from Murray (CM chondrite) are Fe-rich phyllosilicates and specifically Fe-rich cronstedtite (serpentine group). The metal-OH band of the Fe-rich cronstedtite spectrum from Bates et al. (2020) differs from that observed in RI spectra (Figures 8a and 9). Cronstedtite exhibits a metal-OH band with a minimum of reflectance at 2.87 µm, while our RI spectra show a narrow peak with a minimum of reflectance at 2.75 µm. Although our study is consistent with the fact that the hydrated phase in Murchison RIs are iron-rich phyllosilicates, the exact nature of the phyllosilicates in CM chondrite RIs seems difficult to determine by IR.

We also mentioned earlier that CAI-02 exhibits a weak metal-OH band but rather close to 2.7 µm (3700 cm⁻¹). Therefore, the nature of hydration in this specific RI is probably different from that in other RIs rich in phyllosilicates.

## Relative OH Content Related to Mineralogy in RIs

Phyllosilicates are the predominant OH-bearing phases in RIs. Thus, by using the metal-OH band to estimate relative OH content, we ultimately measure the proportion of phyllosilicates in RIs. Consequently, the deeper and wider the metal-OH band in an RI, the more phyllosilicates it contains compared to other presumed anhydrous minerals.

If we consider that the phyllosilicates in RIs were formed during the aqueous alteration of the CM chondrites on their parent body, measuring the amount of OH a RI contains allows us to evaluate the extent to which this RI is altered or not.

Figure 11 shows the area integration of the metal-OH band between 3500 and 3800 cm⁻¹ of the 11 RIs in Murchison. Four classes of RIs emerge in terms of OH content, which reflect distinct mineralogy (Figures 8a and 11).

* The first class (mainly anhydrous) only contains CAI-02, which is mainly composed of anhydrous phases such as diopside and olivine with minor spinel (Figure 10). CAI-02 contains almost no hydrous phase, which explains why the OH stretching band is weak (Figure 8a) and that this inclusion is OH-poor (Figure 11).

* The second class (moderately anhydrous) is composed of CAI-08 and 14, which are mainly composed of large spinel areas (Figure S2). The low abundance of phyllosilicates in CAI-14, as seen by SEM and the weak band around 1000 cm⁻¹ in Figure 8a explains why it contains little OH (Figure 11). CAI-08 has a 10–15 µm thick layer of

<figure><img src="image_15.png" /><figcaption>FIGURE 11. Area integration of the logarithm of the metal-OH band between 3500 and 3800 cm⁻¹ (arbitrary units) for each of the 11 refractory inclusions (RIs) in Murchison. The color for each RI corresponds to those in Figure 8. (Color figure can be viewed at wileyonlinelibrary.com)</figcaption></figure>

Fe-rich phyllosilicates enclosing a 60-µm large core of spinel. As such, it is composed mostly of spinel relative to phyllosilicates. This explains why this inclusion falls within the second class even though it is partially composed of a thick phyllosilicates layer.

* The third class (moderately hydrated) is more heterogeneous and is composed of CAI-09, 13, 03, 01, and 10. CAI-13 is classified as spinel-rich, but still exhibits weak diopside and spinel features in Figure 8a. The weak features are probably linked with the presence of phyllosilicates. Indeed, SEM examination showed that CAI-13 is mainly composed of spinel and diopside but still bears Fe-rich phyllosilicates (Figure 10). Although CAI-09 and 10 are classified as diopside-rich, they exhibit a metal-OH band in Figure 8a that attests to the presence of phyllosilicates in these two RIs. SEM observations showed that CAI-09 contains Fe-rich phyllosilicates (Figure 10) that contribute to the metal-OH band. However, the contribution to the metal-OH band of CAI-10 probably comes from the matrix of phyllosilicates filling the gaps between the aggregation of little nodules according to SEM observations (Figure S2). In addition, the minimum of reflectance in CAI-10 is blue-shifted, meaning it probably probes more Mg-rich serpentine like those of the matrix. CAI-01 and 03 are classified as phyllo-diopside-rich and therefore contain a moderate amount of phyllosilicates. Even though the third class is heterogeneous, all the RIs (except CAI-10) contain Fe-rich phyllosilicates according to SEM observations and their MIR features in Figure 8a. Therefore, they do hold a moderate amount of OH (Figure 11) and it confirms their attribution to a third class.
* The fourth class (hydrated) contains CAI-05, 07, and 06 which are classified as phyllo-diopside-rich in Figure 8. SEM observations showed that they are composed of large Fe-rich phyllosilicates areas in addition to the presence of a wide diopside rim (Figure S2). Therefore, the high abundance of phyllosilicates in these three inclusions attest of their hydrated nature (Figure 11) and thus their attribution to a fourth class. RIs from this class are less likely to provide information on their origin, but rather on the subsequent aqueous alteration that took place on the parent body.

Although we cannot quantify the absolute OH content with our geometry of acquisition, we can still estimate the relative OH content in RIs without any invasive or destructive measurements. In the case of extraterrestrial samples, we can thus determine the relative OH content in RIs in order to determine which one is altered or not. This will allow us to focus on interesting RIs to be analyzed in greater detail, whether studying RIs holding primitive information from the nebula or RI alteration under asteroidal conditions.

## CONCLUSION

In this study, we first built a database of standard spectra of CAI minerals in the MIR range to develop the detection of RIs in meteorites and to determine their mineralogy. This database provides a set of useful standards that are made publicly available for future IR studies. For example, the spectra from this database can be used for the detection of RIs for initial characterization of returned space samples or also for the comparison of their spectra with astronomical remote sensing observations.

We subsequently developed a non-destructive and non-invasive method to detect and study RIs in extraterrestrial samples by IR reflectance spectroscopy. This method is based on supervised machine learning developed on MIR hyperspectral maps of meteorite sections and can detect up to 96.5% of the RI content in a meteorite section. The method can detect RIs containing at least one mineral (other than phyllosilicate) whose size is greater than 4–5 µm for minerals with high spectral contrast or 8–10 µm for minerals with low spectral contrast.

Because the detection was developed on small RIs from CM chondrites, it can also be applied to any other meteorites containing larger RIs. Our detection method is not restricted to meteorites, but can also be applied to any rock samples (terrestrial, meteoritic and returned space samples) and for the detection of any other mineral, provided its reference spectrum is available. Even though

the detection method was developed on polished sections so far, it can be applied to non-polished sections as previous studies have shown that spectral signatures of minerals are efficiently detected on non-polished sections (Arribard, 2023; Brunetto et al., 2020; Dionnet et al., 2022, 2023), thus allowing the analytical method to be even less destructive.

The development of the detection method was followed by the study of 11 individual RIs in Murchison (1) on a short spectral range (4000–800 cm⁻¹) by clustering on hyperspectral maps and (2) on a wider spectral range (4000–400 cm⁻¹) with spot measurements. The study of individual RIs over a wider spectral range provides better mineralogical characterization of oxides, while clustering on hyperspectral maps enables the complete surface of each RI to be sampled. The relative OH content of the 11 RIs was also assessed, based on the area integration of their metal-OH band. RIs were classified on a scale from mainly anhydrous to hydrated.

In the actual context of sample-return missions, analytical methods for the initial characterization of precious space samples are highly needed. The detection method and the following mineralogical and hydration characterizations developed here using IR micro-spectroscopy provide a new non-destructive and non-invasive way to detect RIs and provide their first order characterization as a guide for the selection and allocation processes of RI-containing space samples.

**Acknowledgments**—The standards were acquired over time by donations from many contributors: the mineral collection of Sorbonne University (Paris, France), and the mineral collection of the Muséum National d'Histoire Naturelle (Paris, France) are especially thanked for providing numerous samples. J.-P. Lorand, G. Giuliani, C. Engrand and S. Mostefaoui provided Lherz diopside, Vietnam spinel, synthetic melilites, and Hachijo anorthite, respectively. We thank the meteorite collection of the Muséum National d'Histoire Naturelle for the loan of the Murchison, Paris and Colony sections. Sylvain Pont is warmly thanked for helping with the FEG-SEM of the Museum electron microscopy facility. Christophe Sandt is thanked for his help with FT-IR measurements on the SMIS beamline at the Synchrotron SOLEIL. Zélia Dionnet is thanked for adding our standard spectra to the SSHADE infrastructure. We would also like to thank Flavie Hamoumraoui-Rocher for the preliminary measurements used as a basis for this study. Finally, Yann Arribard, Stefano Rubino and Tania Le Pivert-Jolivet are thanked for their help with python codes and the Quasar software. The micro-spectroscopy measurements were supported by grants from Region Ile-de-France (DIMACAV) and SOLEIL.
*Data Availability Statement*—The data supporting this study's findings will be available on the SSHADE infrastructure (https://www.sshade.eu). Direct link to standard spectra : https://www.sshade.eu/data/EXPERIMENT_ZeD_20241126_01

*Editorial Handling*—Dr. A. J. Timothy Jull

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## SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article.

**Figure S1.** (a) Standard olivine spectrum with and without carbon coating in reflectance (%) in the range 1400 to 400 cm⁻¹. The spectrum is not normalized. (b) Characteristic MIR features between 1400 and 400 cm⁻¹ for olivine standard. The position of the feature is noted in cm⁻¹. (c) Weight percent oxide composition (wt%) of olivine standard determined by EPMA.

**Figure S2.** Name, origin and locality of mineral standards that we can usually find in RIs. Standards may be of terrestrial, meteoritic or synthetic origin.

**Figure S3.** RGB false color X-ray maps (MgCaAl) superimposed with BSE images of the eleven RIs studied in IR in Figure 8.

**Figure S4.** Weight percent oxide composition (wt%) of each mineral standard determined either by SEM-FEG or EPMA.