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# Petrographic insights into phosphate formation in L and LL ordinary chondrites

Valentine Megevand, Alexander N Krot, Adrian J Brearley, François Guyot, Sylvain Bernard, Elena Dobrică

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Valentine Megevand, Alexander N Krot, Adrian J Brearley, François Guyot, Sylvain Bernard, et al. Petrographic insights into phosphate formation in L and LL ordinary chondrites. Geochimica et Cosmochimica Acta, 2025, 408, pp.153 - 173. 10.1016/j.gca.2025.08.023. hal-05336751

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# Petrographic insights into phosphate formation in L and LL ordinary chondrites

Valentine Megevand<sup>a, *, ID</sup>, Alexander N. Krot<sup>b, ID</sup>, Adrian J. Brearley<sup>c, ID</sup>, François Guyot<sup>a, ID</sup>, Sylvain Bernard<sup>a, ID</sup>, Elena Dobrică<sup>b, ID</sup>

## ARTICLE INFO

Associate Editor: Martin Robert Lee

**Keywords:**

Phosphates

Apatites

Merrillites

Ordinary chondrites

L chondrites

LL chondrites

Thermal metamorphism

## ABSTRACT

The origin and formation mechanisms of phosphate minerals in L and LL ordinary chondrites remain an open question. Here we report detailed quantitative data on the abundances, grain sizes and compositions of apatite and merrillite in L, LL and L/LL chondrites, covering a total analysed area of ~2730 mm². Our results show that the abundances of phosphate minerals are slightly higher in L than in LL chondrites and reveal distinct petrographic differences between merrillite and apatite. Merrillite abundance is anti-correlated with Fe, Ni metal content and increases with petrologic type, as does grain size – indicating grain coarsening processes – supporting the formation of this mineral during thermal metamorphism. In contrast, apatite shows no clear trend in abundance or grain size with petrologic type. Although chlorine contents are similar between apatites from L and LL chondrites, the small dispersion of Cl concentration and its decrease with petrologic type in LL chondrites indicate that apatites in these chondrites undergo equilibration with increasing thermal metamorphism, a pattern not observed in L chondrites. Although the Cl contents in apatites are fairly homogeneous across the samples, scarce F-rich apatites have been observed in some meteorites, leading to significant variability in Cl#. However, due to low concentrations and analytical challenges, we argue that comparisons between different OC groups should primarily be based on Cl concentrations. Furthermore, apatite and merrillite have different grain size distributions in L chondrites, but similar ones in LL chondrites, where they also occur as close assemblages, suggesting a petrogenetic relationship between apatite and merrillite in LL chondrites, but not in L chondrites. Overall, the present study suggests that phosphates did not have exactly the same history in L and LL chondrites.

## 1. Introduction

Ordinary chondrites (OCs) are stony chondritic meteorites representing around 60% of all known meteorites and accounting for 38% of recorded falls in mass (Walton et al., 2021; Grady, 2022). OCs are divided into three groups: the H chondrites (high total iron content), the L chondrites (low total iron content, exhibiting lower metal content but higher content of iron oxide in silicates), and the LL chondrites (low total iron contents as well as Low metallic iron relative to total iron, exhibiting the highest silicate iron contents). OCs exhibit a wide range of thermal metamorphism (petrologic types 3–6; Van Schmus and Wood, 1967), as shown by fine-grained matrix recrystallization, chondrule mesostasis devitrification, chondrule-matrix integration, and secondary phases (e.g., phosphate, plagioclase, diopside, and chromite, Van

Schmus and Wood, 1967; Huss et al., 2006). The corresponding metamorphic temperatures, estimated based on these mineralogical and petrographic features, likely range from ~260 °C for the lowest petrologic types (e.g., Semarkona LL3.00; Alexander et al., 1989) to ~950 °C at the onset of melting in type 6 chondrites (e.g., Dodd, 1981).

OCs also show evidence of fluid activity (Krot et al., 2025), including the presence of phyllosilicates (Alexander et al., 1989), sulfide-magnetite assemblages (Huss, 1979), carbide-magnetite assemblages (Krot et al., 1997), carbides (Keller, 1998), bleached chondrules (Grossman et al., 2000), hydrated amorphous materials (Dobrică et al., 2019; Dobrică and Brearley, 2020a), iron-rich olivine (Krot et al., 2004; Doyle et al., 2015; Dobrică and Brearley, 2020b; Dobrică et al., 2022), Fe oxidation (McSween and Labotka, 1993), plagioclase alteration features (e.g., zoning, micropores, alteration lamellae, albitization, and

<footnote><sup>a</sup> Muséum National d'Histoire Naturelle, Sorbonne Université, CNRS UMR 7590, Institut de Minéralogie, Physique des Matériaux et Cosmochimie, Paris, France</footnote>

<footnote><sup>b</sup> Hawai'i Institute of Geophysics and Planetology, School of Ocean, Earth Science, and Technology, University of Hawai'i at Manoa, Honolulu, HI, USA</footnote>

<footnote><sup>c</sup> Department of Earth and Planetary Sciences, MSC03-2040, University of New Mexico, Albuquerque, NM, USA</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: valentine.megevand@mnhn.fr (V. Megevand).</footnote>
formation of K-feldspars; Lewis et al., 2022) and phosphates (Jones
et al., 2014, 2016; Lewis and Jones, 2016). Aqueous fluids in OCs likely
originated from the melting of accreted water ice, forming phyllosili-
cates and/or hydrated amorphous materials, which were later dehy-
drated during thermal metamorphism, contributing to the fluid present
in higher petrologic types (~3.6 and above; Rubin, 2005; McCubbin and
Jones, 2015; Lewis et al., 2022).

Phosphate minerals recorded the thermal, shock metamorphic, and aqueous histories of the parent bodies of OCs. Detailed studies of ocurrences, associations, textures and chemistry of phosphate minerals have been conducted on a limited number of OCs (i.e., 8H, 7 L and 6 LL, Dobrică et al. 2025; Jones et al., 2014; Jones et al., 2016; Lewis and Jones, 2016; Li and Hsu, 2018; McCubbin et al., 2023; Wu and Hsu, 2019). In OCs, which generally contain less than 1 vol% of phosphates (McSween et al., 1991; Gastineau-Lyons et al., 2002, Jones et al., 2014, 2016; Lewis and Jones, 2016), these minerals are interpreted as secondary phases that formed during thermal metamorphism and/or fluid-rock interaction processes (e.g., Brearley and Jones, 1998; Jones et al., 2014, 2016; Lewis and Jones, 2016). Consistently, Lewis and Jones (2016) reported an increase in the abundance of phosphate minerals with increasing petrologic type, but this remains to be confirmed as most of the variation remains within error bars.

Plus, a number of other questions remain, in particular regarding the
exact petrogenetic relationships between the different phosphate min-
erals reported so far in OCs, the most common being apatite [a volatile-
rich phosphate – Ca₅(PO₄)₃X, with X = Cl⁻, F⁻, or OH⁻] and merrillite [a
volatile-free phosphate – Ca₁₈Na₂Mg₂(PO₄)₁₄] (e.g., Hughes et al., 2006;
McCubbin and Jones, 2015). Apatites in non-brecciated OCs are
generally Cl-rich, although their Cl/F values are variable and tend to
decrease from H to LL chondrites (Jones et al., 2014, 2016; Lewis and
Jones, 2016; McCubbin et al., 2023). In contrast, apatite compositions in
brecciated OCs exhibit greater variability, with a limited number of
meteorites containing F-rich apatite (Jones et al., 2016; Lewis and Jones,
2016; Li and Hsu, 2018; Wu and Hsu, 2019). Merrillites are relatively
chemically similar across all groups, regardless of the petrologic type
(Lewis and Jones, 2016). The average grain size of apatite is larger than
that of merrillite; but merrillite typically has a larger maximum grain
size than apatite (Lewis and Jones, 2016). Despite the fact that phos-
phates are thought to be able to form primarily in the gas nebula when
phosphide begins to oxidize to phosphate, especially fluorapatite (at ~
850 K), phosphates in OCs are instead viewed as products of meta-
morphic and metasomatic processes occurring on parent bodies (Benner
et al., 2025; Martínez and Brearley, 2024; McCubbin et al., 2023; Pasek,
2019; Rubin and Grossman, 1985). Within OCs, merrillite would have
formed via the solid-state oxidation of elemental phosphorus in metal
and/or mobilization of P₂O₅ from chondrule mesostasis, whereas
apatite, except for neocrystallized apatite formed during shock meta-
morphism, would have formed either by direct precipitation from a fluid
with high Cl/F ratios or via interface-coupled dissolution-reprecipitation
reactions between merrillite and a halogen-bearing fluid (Ahrens, 1970;
Zanda et al., 1994; Jones et al., 2014, 2016; McCubbin and Jones, 2015;
Lewis and Jones, 2016).

Here, we conducted a petrographic study of phosphate minerals in OCs, including a larger number of distinct meteorites and refining modal abundances of apatite and merrillite over larger areas of analysis, incorporating also quantitative measurements of their grain size distributions in L and LL chondrites. A specific focus was made on apatite as a direct indicator of the presence of volatile species and of the relative abundances of halogens (e.g., Harlov, 2015; McCubbin and Jones, 2015). We specifically targeted L and LL chondrites in the present study since they generally contain a higher halogen concentration compared to H chondrites (e.g., Brearley and Jones, 2018).

## 2. Materials and methods

2.1. *Samples*

Table 1 shows the list of the 15 L, LL, and L/LL chondrites (19 distinct sections) investigated in this study. Note that Bjurböle was classified as an L/LL by Kalllemeyn et al. (1989) and Rubin (1990) based on its mean olivine composition (Fa26.2, L - Fa23–25.8 and LL – Fa26.6–32.4; Rubin, 1990); however, Scott et al. (1985) and more recently Bischoff et al. (2018) classified it as a L4. Most of the samples analyzed are from the University of Hawai'i (UH) meteorite collection, except Bjurböle and Paragould sections, which were obtained from the American Meteorite Laboratory (AML) collection. The meteorites were selected based on two primary considerations. Firstly, for representativeness, different petro- logic types and shock stages were selected based on classifications from previous studies (Kalllemeyn et al., 1989; Stöffler et al., 1991; Rubin, 1992, 1994, 2004; Scott et al., 1993; Bennett and McSween, 1996; Rubin et al., 1997; Friedrich et al., 2004; Gattacceca et al., 2005; Bischoff et al., 2018; Gail and Trieloff, 2019; Schrader and Zega, 2019). All meteorites were classified as equilibrated OCs (EOCs). Secondly, only fall samples were selected to limit the extent of terrestrial weathering and the po- tential associated bias. Most of the analytical characterizations were carried out either on polished thick sections embedded in epoxy (apart from St-Séverin and Soko Banja, for which only thin sections were available). Subsequently, eight thin sections were produced from the polished thick sections (see Table 1) by Spectrum Petrographics Inc. for optical microscopy characterization of their shock level.

## 2.2. Scanning electron microscopy

All meteorites were characterized using scanning electron micro-
scopy (SEM) with a FEI Helios 660 dual-beam focused ion beam SEM
(FIB-SEM) instrument at the Advanced Electron Microscopy Center
(AEMC), University of Hawai'i at Mānoa (UHM). Each section was
mapped using energy-dispersive X-ray spectroscopy (EDS) at 30 kV and
3.2nA to locate and identify phosphate minerals, using an Oxford In-
struments X-Max 80 detector and AZtec software. The spatial resolution
of the resulting maps ranged from approximately 1 to 2.5 µm px⁻¹,
depending on the sample size. Additionally, detailed images and EDS
maps of phosphates were acquired using the backscattered electron
(BSE) and EDS detectors.

We quantified the phosphate abundances (vol.%), grain number, and size distribution on each EDS map, using pixel counting in ImageJ, based on P X-ray maps for total phosphates and Cl X-ray maps to distinguish apatite from merrillite. Additionally, a similar procedure was applied to calculate the Fe,Ni metal abundances (using Fe X-ray maps) as this phase is considered the primary host of phosphorus (Zanda et al., 1994). For each section, the number and the size of the phosphates were determined using the “Analyze Particles” function in ImageJ. A minimum particle size of 5-pixel units was applied to exclude noise. Grain sizes in micrometers were estimated from the areas determined by pixel counting (a), using the formula r = √(a/π), assuming a circular shape for the grains (though this assumption may not always hold). Therefore, throughout this study, the grain size will refer to the grain’s radius, assuming a circular shape. The minimum grain size detected was 3.5 µm. This approach allowed for the calculation of the distribution of phosphate grains (in grains per mm²), and grain size distributions for both phosphates in each meteorite. If multiple sections of the same meteorite were available (see Table 1), the entire quantification process was carried out on all sections and the results were averaged.

2.3. Electron microprobe analysis

The chemical composition of phosphates and silicates (olivines and
low-Ca pyroxenes) were measured using the JEOL JXA-8500F field
<table><caption>**Table 1**
Sample list.</caption><thead><tr><th>Meteorite</th><th>Section</th><th>Group & Petrologic type (literature)</th><th>Shock stage (literature)</th><th>Weathering</th><th>Section surface (mm<sup>2</sup>)<sup>a</sup></th><th>SEM characterization</th><th>EPMA characterization</th></tr></thead><tbody><tr><td colspan="8"><strong>L/LL Ordinary Chondrites</strong></td></tr><tr><td>Bjurbôle</td><td>AML-H171</td><td>L/LL4<sup>a,d</sup>, L4<sup>h,i</sup></td><td>S1-S2<sup>a,d,j</sup></td><td>W1</td><td>111</td><td>Thick & thin section</td><td>No</td></tr><tr><td colspan="8"><strong>L Ordinary Chondrites</strong></td></tr><tr><td rowspan="2">Tennasilm</td><td>UH-0300.1</td><td rowspan="2">L4<sup>b,d</sup></td><td rowspan="2">S2-S3<sup>b,d</sup></td><td>W1</td><td>201</td><td>Thick section</td><td>Yes</td></tr><tr><td>UH-0300.2</td><td>W1</td><td>192</td><td>Thick section</td><td>Yes</td></tr><tr><td>Saratov</td><td>UH-0025.1</td><td>L4<sup>b,e</sup></td><td>S2-S4<sup>b,e</sup></td><td>W1</td><td>44</td><td>Thick section</td><td>No</td></tr><tr><td>Atarra</td><td>UH-0168.1</td><td>L4<sup>b</sup></td><td>S3<sup>b</sup></td><td>W1</td><td>118</td><td>Thick section</td><td>Yes</td></tr><tr><td rowspan="2">Bald Mountain</td><td>UH-0200.1</td><td rowspan="2">L4<sup>b,d</sup></td><td rowspan="2">S4<sup>b,d</sup></td><td>W0</td><td>27</td><td>Thick section</td><td>Yes</td></tr><tr><td>UH-0200.3</td><td>W0</td><td>76</td><td>Thick section</td><td>Yes</td></tr><tr><td>Ausson</td><td>UH-0176.1</td><td>L5<sup>a,b,e</sup></td><td>S2-S3<sup>a,b,e,k</sup></td><td>W1</td><td>72</td><td>Thick & thin section</td><td>Yes</td></tr><tr><td>Kyushu</td><td>UH-0180.1</td><td>L6<sup>b</sup></td><td>S5<sup>b</sup></td><td>W1</td><td>56</td><td>Thick & thin section</td><td>Yes</td></tr><tr><td>Alfianello</td><td>UH-0166.1</td><td>L5<sup>a,d</sup>, L6<sup>l</sup></td><td>S5<sup>a,d</sup></td><td>–</td><td>25</td><td>Thick section</td><td>Yes</td></tr><tr><td rowspan="2">Modoc</td><td>UH-0155.7</td><td rowspan="2">L6<sup>b,e</sup></td><td rowspan="2">S2-S4<sup>b,e</sup></td><td>W0</td><td>74</td><td>Thick & thin section</td><td>Yes</td></tr><tr><td>UH-0155.8</td><td>W2</td><td>49</td><td>Thick section</td><td>Yes</td></tr><tr><td colspan="8"><strong>LL Ordinary Chondrites</strong></td></tr><tr><td rowspan="2">Hamlet</td><td>UH-0205.5</td><td rowspan="2">LL3<sup>a,l</sup>, LL4<sup>d,m</sup>, LL5/6<sup>l</sup></td><td rowspan="2">S3<sup>a,d</sup></td><td>W1</td><td>336</td><td>Thick & thin section</td><td>Yes</td></tr><tr><td>UH-0205.4</td><td>W1</td><td>188</td><td>Thick section</td><td>Yes</td></tr><tr><td>Paragould</td><td>AML-95</td><td>LL5<sup>c,d</sup></td><td>S4-S5<sup>c,d</sup></td><td>W0</td><td>213</td><td>Thick & thin section</td><td>Yes</td></tr><tr><td rowspan="2">Soko Banja</td><td>UH-0193.6</td><td rowspan="2">LL4<sup>g</sup></td><td rowspan="2">S3<sup>g</sup></td><td>–</td><td>159</td><td>Thin section</td><td>Yes</td></tr><tr><td>UH-0250.2</td><td>W0</td><td>514</td><td>Thick & thin section</td><td>Yes</td></tr><tr><td>St Séverin</td><td>UH-0069.1</td><td>LL6<sup>f,g</sup></td><td>S2<sup>f,g</sup></td><td>–</td><td>174</td><td>Thin section</td><td>No</td></tr><tr><td>Dhurmsala</td><td>UH-0239.5</td><td>LL6<sup>a,d</sup></td><td>S3<sup>a,d</sup></td><td>W1</td><td>101</td><td>Thick & thin section</td><td>Yes</td></tr></tbody></table>

References: a. Stöffler et al., 1991; b. Friedrich et al., 2004; c. Rubin, 1992; d. Rubin, 1994; e. Gail and Trieloff, 2019; f. Rubin, 2004; g. Schrader and Zega, 2019; h. Kallemeyn et al., 1989; i. Bischoff et al., 2018; j. Rubin et al., 1997; k. Bennett and McSween, 1996; l. Scott et al., 1993; m. Gattacceca et al., 2005.

* The section surface represents the combined area (sum) of thick and thin sections when both were available, reflecting the total analyzed surface area. This approach accounts for variations in phosphate abundances between sections, which likely represent distinct portions of the sample.

emission electron microprobe in the Department of Earth Sciences, UHM. For phosphates, F, Cl, Na, Fe, P, Ca, and Mg were quantified at 15 kV accelerating voltage, 20nA beam current, and 5 µm spot diameter. Fluorine Kα was measured using a LDE1 detector crystal, a synthetic multi-layer crystal with large d-spacing, recommended for high-quality F measurements (Potts and Tindle, 1989), Cl Kα was measured using a PETH detector crystal. For silicates, we quantified K, Ti, Al, Mn, Na, Ca, Cr, Si, Fe, and Mg at 20 kV accelerating voltage, 30nA beam current, and 5 µm spot diameter. For both phosphates and silicates, a 30 s peak counting (plus 30 s background counting) was applied. A linear background regression was applied for all elements, except for F, where an exponential background regression was used. For phosphate measurements, the primary standards used were San Carlos olivine for Mg and Fe, Wilberforce apatite for Ca, P and F, and scapolite for Na and Cl. We used fluorapatite USNM 104021 to monitor the reproducibility and accuracy of the primary standards, with standard measurements intercalated every ~50 unknown analyses. Major elements (>1 %) like P and Ca have relative errors of about ±0.5 %, while minor elements (<1 %) like Cl, Na, Fe and Mg have relative errors of about ±0.01 %. Fluorine abundance measurements are consistent over time and present relative errors of ±0.2 –0.6 % depending on the session when the data were collected and the specific corrective factor that was applied. For silicate

measurements, the primary standards used were San Carlos olivine for Si, Mg, Fe, K-anorthoclase for K, Na, anorthite USNM 137041 for Ca, Al, ilmenite USNM 96189 for Ti, Mn, and chromite USNM 117075 for Cr. Additionally, olivine USNM 2566 (Fo83) and Cr-augite USNM 164905 were used to monitor the reproducibility and accuracy of the data, which are overall consistent with previous published values within the margin of errors. Minor elements such as Ti, Cr, Mn and Na have relative errors of about ±0.05 %, while major elements have relative errors around ±0.1 for Ca and Fe, and ±0.5 % for Al and Si.

Apatite and merrillite stoichiometries were calculated following the method proposed by Ketcham (2015). The ideal formula for apatite is Ca₅(PO₄)₃X with stoichiometry calculated on a 13-anion basis in which the total X-site anion sum should be 1. It is commonly assumed that the “other” anions besides Cl⁻ and F⁻ in the apatite X-site is OH⁻. However, Jones et al. (2014) showed that within chlorapatites from LL chondrites, this site is probably filled by alternative anions like S²⁻ or others (CO₃²⁻, O²⁻, Br⁻ and I⁻, Pan and Fleet, 2002) rather than the OH⁻ anion. These “other” anions will be thus referred here as the “missing component”, calculated as the difference 1-(Cl + F). Cation substitutions are also common in the apatite structure. For example, Na⁺ and Mg²⁺ can occupy Ca1 sites, while Fe²⁺ can substitute at either Ca1 or Ca2 sites (Pan and Fleet, 2002). Therefore, when small amounts of Na, Fe, and Mg (total < 2
atoms per formula unit, pfu) were measured in apatite, these elements were considered to occupy octahedral sites as does Ca. The ideal formula for merrillite is Ca₁₈Na₂Mg₂(PO₄)₁₄, with stoichiometry calculations performed on a 56-oxygen basis. In this structure, Ca, Na, and Mg occupy octahedral sites, while P occupies tetrahedral sites (Shearer et al., 2015). Ideally, merrillite should have 22 cations (22 atoms pfu) in the octahedral sites and 14 cations (14 atoms pfu) in the tetrahedral sites. However, a known cation substitution in merrillite involves Fe²⁺ substituting for Mg²⁺ in octahedral sites (FeVI₃Mg-site ⇌ MgVI₃Mg-site; Shearer et al., 2015). Therefore, Fe is also included in the calculation of the total octahedral site occupancy.

Silicate composition measurements were used to check whether the petrologic types of the analysed meteorite fragments were consistent with those attributed in the literature to other fragments of the same meteorite. To do this, we measured multiple low-Ca pyroxenes in all sections, except Bjurböle, Saratov and St. Séverin, where the condition of the section did not allow accurate electron microprobe measurements. Additionally, we analysed olivines in two meteorites (Tennasilm – section UH-0300.1 and Hamlet – sections UH-0205.5 and UH-0205.4) with the lowest petrologic types to refine their classification. For each silicate type, approximately 35 grains, randomly distributed within the sections, were analysed to ensure representativeness of the data. For each analysis point, ferrosilite (Fs) or fayalite (Fa) values were calculated. The mean and standard deviation σ of Fs and Fa values were then determined for each section. Following the approach described by Grossman (2011) in an unpublished whitepaper for the Meteorological Society's Nomenclature Committee, σ-Fa was used to distinguish between unequilibrated (petrologic types 3.0 to 3.9) and equilibrated OCs (petrologic types > 4), while σ-Fs was used among equilibrated OCs, to distinguish between petrologic type 4 and 5–6.

## 3. Results

### 3.1. Petrologic types

As a first step we verified the petrologic type classification for most of the analyzed fragments to ensure an accurate assessment of their degree of thermal metamorphism (Table 2). Fig. 1 shows the mean Fs in low-Ca pyroxenes against the standard deviation. For meteorites where more than one section was analyzed, averages of all sections are plotted,

except for Hamlet, whose sections (UH-0205.5 and UH-0205.4) are plotted separately due to the large differences observed in silicate compositions. The dashed horizontal lines show the boundaries between different petrologic types as a function of the threshold set by Grossman (2011) using the σ-Fs in low-Ca pyroxene. The vertical error bars represent the uncertainty in the σ-Fs values, calculated as a function of the number of measurements (N) using $1/\sqrt{2(N)}$ (Grossman, 2011).

According to the degree of equilibration of pyroxene and olivine, only four meteorites (Tennasilm, Kyushu, Modoc, and Dhurmsala, Table 2) retained the same petrologic classification as reported in previous studies. Both sections of Hamlet were reclassified as petrologic type 3 based on σ-Fs (UH-0205.5 – 7.8 σ-Fs and UH-0205.4 – 5.0 σ-Fs) since the σ-Fs threshold between petrologic type 3 and 4 is 5 σ-Fs. To refine the petrologic type in these different fragments of Hamlet we used the σ-Fa of olivine (see Grossman, 2011). Therefore, using both σ-Fs and σ-Fa the two sections of Hamlet were reclassified as petrologic type 3.5–3.6 for UH-0205.5 and 3.8–3.9 for UH-0205.4. Hamlet should thus be considered as an unequilibrated OC (UOC).

Additionally, several meteorites (Atarra, Bald Mountain, and Soko Banja) described in the literature as petrologic type 4 were reclassified as petrologic type 5 based on the low standard deviation values of Fs (<1.7 σ-Fs). According to the method described by Grossman (2011), no threshold for the distinction between petrologic type 5 and 6 was made. However, we observed that most meteorites classified as petrologic type 6 in previous studies (Modoc, Kyushu, and Dhurmsala) show very low σ-Fs values (0.2–0.3), indicating a higher degree of equilibration (Fig. 1). Therefore, since both Alfianello and Ausson fragments contain similar σ-Fs values (0.2 and 0.3 σ-Fs, respectively), we reclassified them as petrologic type 6. Although we do not establish a strict σ-Fs threshold to distinguish between petrologic types 5 and 6, a series of Fisher's tests were performed on the variance of the electron microprobe data sets of low-Ca pyroxenes for each meteorite (see section 1 in Supplementary Material). The resulting comparison matrix (Fig. S1 in Supplementary Material) confirmed significant differences (within a 95 % confidence interval) between the variances of the two groups of meteorites having distinct σ-Fs values (0.8–1.4 σ-Fs – Atarra, Bald Mountain, Soko Banja, and Jelica; 0.2–0.3 σ-Fs – Ausson, Kyushu, Alfianello, Modoc, and Dhurmsala). This shows that the first group has greater compositional heterogeneity (0.8–1.4 σ-Fs), indicating less equilibration, and supports a reassessment as petrologic type 5, while the second group, with lower

<table><caption>**Table 2**
Petrologic types based on σ-Fs and σ-Fa values calculated from electron microprobe measurements on silicates.</caption><thead><tr><th rowspan="2">Meteorite</th><th rowspan="2">Section</th><th colspan="3">Low Ca-pyroxenes</th><th colspan="3">Olivines</th><th rowspan="2">Petrologic type (this study)</th></tr><tr><th>N</th><th>Mean Fs</th><th>σ-Fs</th><th>N</th><th>Mean Fa</th><th>σ-Fa</th></tr></thead><tbody><tr><td colspan="9"><strong>L Ordinary Chondrites</strong></td></tr><tr><td rowspan="2">Tennasilm</td><td>UH-0300.1</td><td>39</td><td>17.9</td><td>2.0</td><td rowspan="2">41</td><td rowspan="2">22.8</td><td rowspan="2">0.3</td><td>4</td></tr><tr><td>UH-0300.2</td><td>41</td><td>19.2</td><td>3.0</td><td>4</td></tr><tr><td rowspan="3">Atarra</td><td>UH-0168.1</td><td>38</td><td>19.2</td><td>1.1</td><td rowspan="3"></td><td rowspan="3"></td><td rowspan="3"></td><td>5</td></tr><tr><td>UH-0200.1</td><td>37</td><td>19.4</td><td>0.8</td><td>5</td></tr><tr><td>UH-0200.3</td><td>35</td><td>19.6</td><td>1.4</td><td>5</td></tr><tr><td rowspan="2">Ausson</td><td>UH-0176.1</td><td>39</td><td>20.5</td><td>0.3</td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td>6</td></tr><tr><td>UH-0180.1</td><td>40</td><td>20.7</td><td>0.2</td><td>6</td></tr><tr><td>Alfianello</td><td>UH-0166.1</td><td>39</td><td>20.7</td><td>0.2</td><td rowspan="2"></td><td rowspan="2"></td><td rowspan="2"></td><td>6</td></tr><tr><td>Modoc</td><td>UH-0155.7</td><td>38</td><td>20.7</td><td>0.2</td><td>6</td></tr><tr><td></td><td>UH-0155.8</td><td>38</td><td>21.0</td><td>0.2</td><td></td><td></td><td></td><td>6</td></tr><tr><td colspan="9"><strong>LL Ordinary Chondrites</strong></td></tr><tr><td>Hamlet</td><td>UH-0205.5</td><td>39</td><td>15.0</td><td>7.8</td><td>40</td><td>18.1</td><td>11.1</td><td>3.5–3.6</td></tr><tr><td></td><td>UH-0205.4</td><td>35</td><td>21.1</td><td>5.0</td><td>50</td><td>27.1</td><td>5.4</td><td>3.8–3.9</td></tr><tr><td>Paragould</td><td>AML-95</td><td>38</td><td>18.5</td><td>4.1</td><td></td><td></td><td></td><td>4</td></tr><tr><td>Soko Banja</td><td>UH-0193.6</td><td>37</td><td>22.6</td><td>1.1</td><td></td><td></td><td></td><td>5</td></tr><tr><td>Jelica</td><td>UH-0250.2</td><td>23</td><td>25.1</td><td>1.3</td><td></td><td></td><td></td><td>5</td></tr><tr><td>Dhurmsala</td><td>UH-0239.5</td><td>39</td><td>22.3</td><td>0.3</td><td></td><td></td><td></td><td>6</td></tr></tbody></table>

The ferrosilite (Fs) and fayalite (Fa) values correspond to the measured pyroxenes and olivines, respectively, representing the iron content relative to the sum of Fe and Mg in atomic percent. The number of electron microprobe measurements conducted on distinct crystals from each section is represented by N. The petrologic type of this study is deducted from the σ-Fs and σ-Fa values according to the classification proposed by Grossman (2011).
<figure><img src="image_5.png" /><figcaption>Fig. 1. Standard deviation of ferrosilite number (σ-Fs) versus mean ferrosilite number (Fs) in low-Ca pyroxenes from L (squares) and LL (diamond) chondrites. Black dashed lines represent the σ-Fs threshold between the different petrologic types according to Grossman (2011). For meteorites where multiple sections were analyzed, averages of all sections are plotted, except for Hamlet (sections UH-0205.5 and UH-0205.4), which are plotted separately due to large differences observed in the silicate compositions.</figcaption></figure>

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

Phosphate minerals and Fe, Ni metal abundances (vol.%) and number of phosphate grains per mm².

<table><thead><tr><th rowspan="2">Meteorite</th><th rowspan="2">Group & Petrologic type</th><th colspan="2">Volume Abundances %</th><th rowspan="2">Merrillite</th><th rowspan="2">Fe.Ni-metal</th><th colspan="2">Number of grain per mm²</th></tr><tr><th>Phosphate total</th><th>Apatite</th><th>Apatite</th><th>Merrillite</th></tr></thead><tbody><tr><td colspan="8"><strong>L/LL Ordinary Chondrites</strong></td></tr><tr><td>Bjurbole*</td><td>L/LL4.L4**</td><td>2.48</td><td>0.04</td><td>2.44</td><td>1.29</td><td>0.6</td><td>4.5</td></tr><tr><td colspan="8"><strong>L Ordinary Chondrites</strong></td></tr><tr><td>Tennasilm*</td><td>L4</td><td>0.26</td><td>0.13</td><td>0.13</td><td>3.83</td><td>0.9</td><td>4.1</td></tr><tr><td>Saratov</td><td>L4**</td><td>0.22</td><td>0.00</td><td>0.22</td><td>3.32</td><td>0.0</td><td>5.7</td></tr><tr><td>Atarra</td><td>L5</td><td>0.43</td><td>0.37</td><td>0.05</td><td>3.02</td><td>1.7</td><td>2.0</td></tr><tr><td>Bald Mountain*</td><td>L5</td><td>0.28</td><td>0.03</td><td>0.25</td><td>3.09</td><td>0.4</td><td>4.0</td></tr><tr><td>Ausson*</td><td>L6</td><td>0.54</td><td>0.20</td><td>0.33</td><td>2.81</td><td>1.1</td><td>3.8</td></tr><tr><td>Kyushu*</td><td>L6</td><td>0.29</td><td>0.08</td><td>0.21</td><td>2.07</td><td>0.7</td><td>1.5</td></tr><tr><td>Alfianello</td><td>L6</td><td>0.37</td><td>0.21</td><td>0.16</td><td>3.48</td><td>0.5</td><td>2.2</td></tr><tr><td>Modoc*</td><td>L6</td><td>0.51</td><td>0.26</td><td>0.26</td><td>3.15</td><td>1.1</td><td>3.0</td></tr><tr><td><strong>Average L OCs</strong></td><td></td><td><strong>0.36</strong></td><td><strong>0.16</strong></td><td><strong>0.20</strong></td><td><strong>3.10</strong></td><td><strong>0.8</strong></td><td><strong>3.3</strong></td></tr><tr><td colspan="8"><strong>LL Ordinary Chondrites</strong></td></tr><tr><td>Hamlet*</td><td>LL3.5-LL3.9</td><td>0.16</td><td>0.05</td><td>0.11</td><td>1.68</td><td>0.6</td><td>3.3</td></tr><tr><td>Paragould*</td><td>LL4</td><td>0.17</td><td>0.05</td><td>0.11</td><td>1.50</td><td>2.2</td><td>5.7</td></tr><tr><td>Soko Banja</td><td>LL5</td><td>0.25</td><td>0.10</td><td>0.15</td><td>1.20</td><td>2.0</td><td>3.8</td></tr><tr><td>Jelica*</td><td>LL5</td><td>0.34</td><td>0.17</td><td>0.17</td><td>0.77</td><td>4.1</td><td>2.1</td></tr><tr><td>St Séverin</td><td>LL6**</td><td>0.30</td><td>0.03</td><td>0.27</td><td>0.60</td><td>0.3</td><td>2.2</td></tr><tr><td>Dhurmsala</td><td>LL6</td><td>0.37</td><td>0.09</td><td>0.27</td><td>0.21</td><td>1.2</td><td>1.7</td></tr><tr><td><strong>Average LL OCs</strong></td><td></td><td><strong>0.27</strong></td><td><strong>0.08</strong></td><td><strong>0.18</strong></td><td><strong>0.99</strong></td><td><strong>1.7</strong></td><td><strong>3.2</strong></td></tr></tbody></table>

<footnote>* For the sections marked with an asterisk, abundance data corresponds to the average of the abundances calculated on all available sections.</footnote>

<footnote>** Petrologic type from literature (Kallmeyn et al., 1989; Stöffler et al., 1991; Rubin, 1994, 2004; Friedrich et al., 2004; Bischoff et al., 2018; Gail and Trieloff, 2019; Schrader and Zega, 2019).</footnote>
$\sigma$-Fs values (0.2–0.3 $\sigma$-Fs), will be considered petrologic type 6 in this study. Additionally, the fragment analyzed in this study from Jelica and Paragould were also reclassified based on the $\sigma$-Fs distribution in low-Ca pyroxenes as petrologic type 5 and 4, respectively (Fig. 1, Table 2).

## 3.2. Petrography

### 3.2.1. Abundances and grain size distribution

We measured the abundances and distributions of phosphate minerals in 19 sections of 15 meteorites (8 L, 6 LL, and one L/LL), covering a total area of approximately 2730 mm² (Table 1). On average, L chondrites contain a higher abundances of phosphate minerals than LL chondrites (0.36 ± 0.12 vol% and 0.27 ± 0.09 vol%, respectively, with large standard deviations due to variations across petrologic types). Their maximum surface abundances do not exceed ~0.5 vol% and ~0.4 vol% for L and LL chondrites, respectively (Table 3). The only exception is the Bjurböle (L/LL4), which exhibits an abundance of phosphate minerals of 2.48 vol% due to the presence of a large merrillite grain of 0.8 × 2.6 mm² in size. Otherwise, the highest abundances in phosphate minerals are observed in chondrites classified as petrologic type 6 (L6 Ausson – 0.54 vol% and LL6 Dhurmsala – 0.37 vol%, Table 3). Among phosphate minerals, merrillites are, on average, 1.4 times (in Ls) to 2 times (in LLs) more abundant than apatites. However, in some L chondrites (Atarra, Alfianello), apatite is the dominant phosphate mineral or as abundant as merrillites (Tennasilm, Modoc). For example, apatite is up to 7 times more abundant than merrillites in Atarra. On the other hand, no apatite was identified in the Saratov section.

Additionally, we measured the abundance of Fe,Ni metal in these meteorites, since previous studies have suggested that P is originally hosted in metal grains before equilibration during thermal metamorphism (Zanda et al. 1994). When averaging the abundances of Fe, Ni metal within the same petrologic type for each OC group, we observe a slight decrease in abundance as thermal metamorphism increases (Table 3). Petrologic type 4 L chondrites contain, on average, 3.6 ± 0.3 vol% Fe, Ni metal compared to 2.9 ± 0.6 vol% Fe, Ni metal in petrologic type 6. Similarly, among LL chondrites, Hamlet, which has the lowest petrologic type (3.5–3.9), contains 1.68 vol% Fe, Ni metal, while the two meteorites reclassified as petrologic type 6 contain, on average, the lowest amount of Fe, Ni metal (0.4 ± 0.3 vol%).

Fig. 2 shows the abundance of phosphate minerals (total phosphate minerals, merrillite, and apatite) versus Fe, Ni metal abundance and the correlations between the two phases. First, within each chondrite group (L and LL), the Fe, Ni metal abundance decreases with the increase of abundance in phosphate minerals and the petrologic type (Fig. 2a). We refer to this trend as the intragroup correlation. For example, in low petrologic types such as Hamlet (LL3.5–3.9), the Fe, Ni metal abundance is much higher (1.68 vol% Fe,Ni) and the phosphate abundance much lower (0.16 vol% total phosphate) compared to meteorites of high petrologic types such as Dhurmsala (LL6 – 0.21 vol% Fe,Ni metal and 0.37 vol% total phosphate). This correlation is stronger in LL chondrites compared with L chondrites (LL – R² = 0.92, and L – R² = 0.03, Fig. 2a). However, if we exclude Kyushu, which contain the lowest abundance of metal (2.1 vol% Fe, Ni metal) among all L chondrites, the correlation between abundances of phosphate and metal phases in L chondrites is stronger (R² = 0.42, Fig. 2a). Second, a systematic intergroup pattern is observed between L and LL chondrites. At equivalent petrologic types, higher abundances of phosphate minerals coincide with higher Fe, Ni metal abundances (e.g., both phosphate and Fe, Ni metal contents are higher in L4 meteorites than in the LL4 Paragould: 0.24 ± 0.02 vol% vs. 0.17 vol% for phosphate minerals and 3.6 ± 0.3 vol% vs. 1.5 vol% for Fe,Ni metal). However, if we analyze the merrillite and apatite abundances separately (Fig. 2b–c), the correlations are less clear. In L chondrites, large variations in merrillite and apatite abundances within the same petrologic type prevents us from identifying a significant correlation of increasing abundances with petrologic type. In LL chondrites, apatite abundances do not show any correlation with petrologic type either. However, merrillites in LL chondrites show a clear increase in abundance from petrologic types 3.5 to 6 (0.11 vol% in both sections

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Abundances (vol.%) of Fe, Ni-metal versus total phosphate minerals (a), merrillites (b) and apatites (c) in L (squares) and LL (diamonds) chondrites. Data are color-coded according to the petrologic types assigned in this study. The colored areas highlight the distribution of values for each petrologic type within each group, and linear regressions are plotted separately for L and LL meteorites. Meteorite abbreviations: Alf – Alfianello, Atr – Atarra, Aus – Ausson, Bal – Bald Mountain, Dhu – Dhurmsala, Hml – Hamlet, Jel – Jelica, Kyu – Kyushu, Mod – Modoc, Par – Paragould, Sar – Saratov, Sev – St. Séverin, Sok – Soko Banja, Ten – Tennasilm.</figcaption></figure>
of petrologic types 3.5–4 to 0.27 vol% in the two sections of petrologic type 6, Fig. 2b, Table 3).

The number of phosphate grains per mm² is similar between L and LL chondrites, averaging about 4 grains/mm². Merrillite grains are generally two to three times more abundant than apatite grains (L – avg. 3.3 ± 1.4 merrillite/mm², 0.8 ± 0.5 apatite/mm²; LL – 3.2 ± 1.4 merrillite/mm², 1.7 ± 1.3 apatite/mm², Table 3), except in Jelica (LL5), where apatite grains are twice as abundant as merrillite grains (4.1 vs. 2.1 grain/mm²). With the exception of the anomalously large merrillite grain in Bjurböle (L/LL4) mentioned earlier, the largest phosphate grains are observed in Jelica (LL5). This meteorite contains merrillite grains reaching up to 0.6 × 0.8 mm² and apatite grains approximately 0.3 × 0.5 mm² in size. Fig. 3 shows the grain size distribution of both phosphates (merrillite – red and apatite – blue) across the different meteorite sections, shown as a box and whisker plot. In this plot, each box shows where the middle 50% of the grain sizes are distributed (i.e. the interquartile range IQR), with the central horizontal line indicating the median. The whiskers extend to the semi-interquartile limits (i.e. the smallest and largest values within 1.5 times the IQR). Note that the largest grains, including those observed in Jelica (LL5) and Bjurböle (L/LL4), were included in the calculation of the statistic but are not shown in the plot as they fall outside of the semi-interquartile limits. We observe that most merrillite grains in the meteorites analysed have grain sizes that do not exceed ~10 µm, with a small increase in size with the petrologic type. However, apatites show a high variability in size, with no clear pattern observed as a function of the petrologic type. A notable difference is observed between apatites in L and LL chondrites. In LL chondrites the apatites are smaller and less variable in size than in L chondrites. For example, while most apatites in LL chondrites have sizes smaller than 10 µm, the middle 50% of the apatite grains in Alfianello (L6) range between ~10 and 30 µm.

Fig. 4 shows the proportion of large grains (grain radius >10 µm) as a function of the number of grains per mm² for total phosphates, merrillite, and apatite in EOCs. Hamlet was not plotted in this graph since both

sections were reclassified as UOCs. The proportion of large grains (>10 µm) was selected as the parameter to monitor grain size across different meteorites, rather than the mean grain size, because its statistical error can be calculated similarly to that of the number of grains per mm². The samples classified as petrologic type 4 exhibit a higher number of total phosphate grains per mm² (4–6 grains/mm²) and a smaller proportion of large grains (10–25%) compared to meteorites classified as petrologic type 6, which show a lower number of grains per mm² (2–5 grains/mm²) but a higher proportion of large grains (25–35%, Fig. 4a–b). Meteorites classified as petrologic type 5 fall between these values. LL chondrites tend to have a lower proportion of large apatite grains despite a varying number of grains per mm² (Fig. 4c). All LL chondrites cluster towards the lower part of the y-axis (blue background), indicating that larger apatite grains are less frequent in these meteorites. L chondrites, on the other hand, show a broader range in the proportion of large apatite grains and tend to occupy the upper region of the plot, suggesting that larger apatite grains are more common in these meteorites (orange background).

### 3.2.2. Textural characteristics of phosphate minerals and mineralogical associations

Apatites show diverse morphologies from irregular (Fig. 5a and Fig. 6c, e) to more compact and euhedral (Fig. 5e and Fig. 6g), and are often highly fractured (e.g., Fig. 5a, c, e and Fig. 6a, e). Most apatites with irregular boundaries and various mineral inclusions (silicates – most common, and Fe, Ni metal, and sulfides – rare) are observed in meteorites with low petrologic types (3.5–4, e.g., in Tennasilm L4, Fig. 5a and Paragould LL4, Fig. 6c), but are absent in type 6 chondrites, where compact, euhedral grains with sharp boundaries and fewer inclusions dominate (e.g., Fig. 5e – Modoc L6 and Fig. 6g – St. Séverin LL6). Some apatites in highly metamorphosed OCs share triple-junction boundaries with olivine (Fig. 6g – St. Séverin LL6). Veins are also present (up to 200–300 µm in length and 10–20 µm in width) in three meteorites (Bald Mountain – UH-0200.3, Fig. 5c, Hamlet – Fig. 6a, and Paragould).

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Grain size distributions (µm, interquartile range, IQR) for merrillites (red) and apatites (blue) across the different meteorite sections within L, LL, and L/LL chondrites. Grain size refers to the grain's radius, assuming a circular shape as explained in Section 2.2. Each box represents the middle 50% of the grain sizes (the largest grains are not represented), the interquartile range (i.e., the limits of the first and third quartiles), with the central horizontal line indicating the median. The whiskers, extending to the semi-interquartile limits, show the general spread of the data. Meteorites are ordered by increasing petrologic types among L (orange background), LL (blue background), and L/LL (white background) chondrites.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Proportion (%) of large grains (grain radius > 10 µm) versus the number of grains per mm² for total phosphate minerals (a), merrillites (b), and apatites (c) for each meteorite. Note that axis scales are different in different panels. Data are color-coded according to the petrologic types. Error bars represent the propagation of statistical uncertainty on the number of grains N, assuming a Poisson distribution, and are calculated as $σ_N = \sqrt{N}$. Apatites have different proportions depending on the meteorite groups (L – orange background, LL – blue background). Meteorite abbreviations: Alf – Alfianello, Atr – Atarra, Aus – Ausson, Bal – Bald Mountain, Dhu – Dhurmsala, Jel – Jelica, Kyu – Kyushu, Mod – Modoc, Par – Paragould, Sar – Saratov, Sev – St. Séverin, Sok – Soko Banja, Ten – Tennasilm.</figcaption></figure>

Merrillites with irregular morphologies are more common in lower petrologic types (L4 – Tennasilm, Fig. 5b and L5 – Bald Mountain, Fig. 5d) and compact euhedral grains are dominant in highly meta-morphosed samples, especially in L chondrites (L6 – Modoc, Fig. 5g). This pattern is less pronounced in LL chondrites (Fig. 6). Merrillite veins are also observed in only three meteorites, which are not necessarily the same as those containing apatite veins. Bald Mountain and Hamlet contain both merrillite and apatite veins, while Soko Banja has merrillite veins and Paragould contains apatite veins. Silicate inclusions are commonly observed within merrillites, whereas metal-sulfide inclusions are occasional. Unlike apatite, merrillite commonly associates with metals, sulfides, and chromites in all groups and petrologic types analyzed. In low petrologic types, merrillite often occurs as rims around metal or sulfide. For example, in Hamlet, we identified a merrillite rim of ~10 µm wide surrounding a 80 µm diameter Fe,Ni metal nodule (Fig. 6b). Merrillite can also form spherical inclusions within metals or sulfides, with small inclusions (~5 µm diameter) within Fe,Ni metal and larger grains (~10 – 20 µm diameter) near sulfide edges (L4 Tennasilm, Fig. 5b). A similar assemblage is observed in LL4 Paragould (Fig. 6d), where a 65 × 120 µm² merrillite grain is partially embedded in metal grains. Merrillite inclusions persist in L5 but disappear in LL5 and higher petrologic types. By petrologic type 6, merrillites are no longer embedded in metal-sulfide assemblages (LL6 St. Séverin, Fig. 6h).

Apatite-merrillite assemblages are common in LL chondrites (Fig. 7) but rather rare in L chondrites (only one occurrence in the Modoc (L6) section-red arrow on Fig. 5e). These assemblages, show diverse morphologies across the LL chondrites sections such as, irregular shapes (Fig. 7a–b), veins (Fig. 7c–d), rims (Fig. 7f), and lamellae (Fig. 7e). In LL3.5–3.6 Hamlet and LL4 Paragould, the phosphate boundaries appear highly irregular (Fig. 7a–d). These assemblages may contain small Fe, Ni-metal and chromite grains (Fig. 7b) or silicate and sulfide inclusions as observed in Paragould (Fig. 7d). Large apatite-merrillite assemblages have been identified in Jelica. For example, a large merrillite grain (120 × 140 µm², Fig. 7e) containing subparallel lamellae of apatite (up to ~ 15 µm wide) and a thin (~5 µm wide) apatite rim was observed. Additionally, Fig. 7f shows a 120 × 180 µm² merrillite grain with a continuous apatite rim (20–30 µm wide), except where it interfaces with a chromite grain partially embedded in the phosphate assemblage. In LL6

St-Séverin, apatite-merrillite assemblages are less common and occur as adjacent individual grains with less irregular boundaries (Fig. 7g–h).

### 3.3. Chemical composition of phosphate minerals

#### 3.3.1. Apatites

Average apatite compositions for major elements were calculated for each meteorite section from individual measurements made on several grains obtained by electron microprobe analysis (Table 4, individual EPMA measurements are available in the Supplementary Material Table S2). Calcium alone does not fully maintain a 5:3 stoichiometry with phosphorus because small amounts of Na, Fe, and Mg (<2) also occupy octahedral sites (Pan and Fleet, 2002; Ketcham, 2015). Taking this into account, the cation balance is close to the ideal apatite formula Ca₅(PO₄)₃(Cl,F,X), with octahedral cations (Ca, Na, Fe and Mg) ranging from 5.01 to 5.06 atoms pfu and tetrahedral cations (P) ranging from 2.99 to 3.03 atoms pfu. Within anionic sites, Cl and F are abundant (0.89–0.99), with the missing component ranging from 0.01 to 0.11 atoms per formula unit. Therefore, the Cl# values, calculated as Cl# = Cl/ (Cl + F) × 100 (with Cl and F in at.%), provide a simplified representation of the variations between the two dominant anions. Most apatites in L and LL chondrites have high Cl# values between 82 and 91, demonstrating that they are Cl-rich. However, some meteorites exhibit lower Cl# values: L5 Bald Mountain (Cl# = 57 ± 3), LL3.5 – 3.6 Hamlet (Cl# = 65 ± 16, UH-0205.5), and LL6 Dhurmsala (Cl# = 19 ± 3), indicating an enrichment in fluorine. The corresponding F contents range from 1.21 ± 0.62–1.46 ± 0.12 wt% in Bald Mountain and Hamlet (UH-0205.5), and 2.94 ± 0.08 wt% in Dhurmsala.

Fig. 8 shows the composition of apatites (ternary diagrams of Cl-X-F anionic compositions and Cl# values) in L and LL chondrites color-coded as a function of their petrologic type (Fig. 8a, b, d). Additionally, we plotted all the apatite compositions and data from previous studies (Jones et al., 2014; Lewis and Jones, 2016) color-coded as a function of their OC groups (L – orange and LL – blue symbols and areas, Fig. 8c). In L chondrites, most compositions range from 0.7 to 0.9Cl atoms pfu, except for several F-rich grains in L5 Bald Mountain (0.35–0.45F atoms pfu). Only a small variation is observed toward the X apex (0–0.2 atoms pfu). Apart from Bald Mountain, the overlap across different meteorites
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Backscattered electron images (panels a–f) and two insets (panels a and e) showing energy dispersive X-ray spectroscopy maps of phosphates minerals (apatites – left column and merrillite – right column) of L chondrites from each petrologic type (a–b – Tennasilm, L4; c–d – Bald Mountain, L5; e – f – Modoc, L6). Apatite in Tennasilm is irregular in shape and contains numerous silicate inclusions (see EDS map); however, it is more compact, euhedral and lacks silicates and metal-sulfide inclusions in Modoc (panel e). Veins of apatite are present in Bald Mountain (panel c). Merrillite morphologies vary from irregular in shape in Tennasilm (panel b) to more compact morphologies in Bald Mountain and Modoc (panels d and f). Numerous merrillite inclusions (red arrows) are also visible within the sulfide (panels b and d). Boundaries of phosphates are highlighted by yellow dashed lines when necessary. Minerals abbreviations: ap – apatite, chr – chromite, Fe, Ni – Fe, Ni metal, Fsp – feldspar or feldspathic glass, mer – merrillite, and sulf – sulfide.</figcaption></figure>

is significant, with only L5 Atarra showing slightly Cl-enriched compositional (0.85–0.9Cl atoms pfu). In LL chondrites, similarly to L chondrites most apatite compositions also fall between 0.7 and 0.9Cl atoms pfu. While a slight shift toward the Cl apex is observed from petrologic type 3.5 to 5; however, it is not statistically significant (see high standard deviations of Cl#, Table 4). All the measured Dhurmsala apatites are F-rich and their fluorine compositions fall around 0.80 ± 0.02 atoms pfu. The lower Cl# value of Hamlet (Cl# = 65 ± 16, UH-0205.5) mentioned earlier (Table 4) is explained by the presence of one outlier (blue circles, Fig. 8b), corresponding to one apatite with a Cl content of approximately 0.52 atoms pfu. Apart from the F-rich apatite measurements, the remaining apatite compositions are Cl-rich. The apatites in Paragould and Jelica exhibit slightly more variability in their compositions (0.70–0.87Cl atoms pfu and 0.72–0.85Cl atoms pfu, respectively), while the apatite compositions in Soko Banja are homogeneous (0.78–0.83Cl atoms pfu). However, these patterns should be interpreted cautiously as the number of measurements varies across sections. The relative halogen compositions of apatites in individual meteorites are shown in Fig. 8d. The mean Cl# values are shown with error bars representing 1σ, while group averages are indicated by vertical lines (solid-

colored lines include all apatite measurements in each OC group; dashed colored lines exclude all six outlier apatites that are F-rich, hereafter referred to as outliers; black dashed lines avg. for each OC group from previous studies Jones et al., 2014; Lewis and Jones, 2016).

### 3.3.2. *Merrillite*

The average compositions of merrillites in the meteorites investigated are summarized in Table 5 (individual EPMA measurements are available in the Supplementary Material Table S3). Overall, merrillite compositions in L and LL chondrites are homogeneous in Na and Mg (Fig. 9, Table 5). However, some variations in Fe relative to Mg are observed both within and between different sections, although most measurements from different meteorites overlap, and no clear patterns emerge (Fig. 9, Table 5). Additionally, the Mg# index, calculated for average merrillite compositions as Mg# = Mg/(Mg + Fe) × 100 (with Mg and Fe in at%), ranges from 80 to 91 (except in L/LL4 Bjurböle where Mg# 54, Table 5), showing no significant variation across L and LL sections or petrologic types.
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Backscattered electron images of phosphate minerals (apatites – left column and merrillite – right column) of LL chondrites from each petrologic type (a–b – Hamlet, LL3.8–3.9; c–d – Paragould, LL4; e–f – Soko Banja, LL5; g–h – St. Séverin, LL6). One apatite vein, confined in between the olivine crystals of a barred chondrule was identified in Hamlet (panel a). Most apatites have irregular morphologies in petrologic types 3–5 (panels a, c, and e); however, they are compact, euhedral having triple junction edges with the associated silicates in higher petrologic types (red crosses in LL6, St. Séverin, panel g). Examples of merrillite morphologies (rim – panel b, irregular morphologies – panel d, and more compact crystals – panels f and h) are shown in the right column. Boundaries of phosphates are highlighted by yellow dashed lines when necessary. Minerals abbreviations: ap – apatite, chr – chromite, Fe, Ni – Fe, Ni metal, Fsp – feldspar or feldspathic glass, mer – merrillite, px – pyroxene, and sulf – sulfide.</figcaption></figure>

**4. Discussion**

The data collected for the present study (i.e. abundances, petrographic relationships, compositions) allow discussing the processes responsible for the formation of phosphate minerals in OCs of various petrologic types. Naturally, such a thorough discussion can only be carried out if the petrologic types of the sections under investigation are

accurately assigned, which is where we begin below.

**4.1. Reassessment of petrologic types**

Only four of the meteorite sections investigated in the present study can be assigned to the petrological type indicated for them in the literature (Tennasilm, Kyushu, Modoc, and Dhumrsala), the others required
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Backscattered electron images of apatite/merrillite assemblages in LL chondrites with different petrologic type (a-b – Hamlet, LL3.5-3.6; c-d – Paragould, LL4; e-f – Jelica, LL5; g-h – St. Séverin, LL6). Merrillite with variable morphologies such as, irregular shapes (panels a-d), veins (panels c-d), rims (panel f), and lamellae (panel e) are shown. Most boundaries between phosphates are highly irregular in shape in samples from petrologic types LL3.5-5 (panels a-f); however, they become more compact in petrologic types LL6 (panels g-h). Borders between apatite and merrillite are highlighted with yellow dashed lines when necessary. Minerals abbreviations: ap – apatite, chr – chromite, Fe, Ni – Fe, Ni metal, Fsp – feldspar or feldspathic glass, mer – merrillite, px – pyroxene, and sulf – sulfide.</figcaption></figure>

reclassification (Fig. 1 – Table 2). This is likely related to the significant fraction of breccias among OCs, with LL being more brecciated than L (L – 18–23%; and LL – 58–79%; Binns, 1967; Scott et al., 1989; Bischoff et al., 2018). In addition, some of the OC breccias are genomict breccias containing cognate clasts – fragments related to the host rock but of different petrologic type (Bischoff et al., 2006), which justifies the necessity of a preliminary verification of the degree of thermal metamorphism of each analyzed fragment.

Here, we reclassified four L chondrites (Atarra, Bald Mountain,

Ausson, Alfianello), and four LL chondrites (Hamlet, Paragould, Soko Banja, and Jelica). Several of these reclassified samples were previously described as breccias, including Paragould, Soko Banja, and Jelica (Fredriksson et al., 1968; Gopalan and Wetherill, 1969; Chou and Cohen, 1973; Grady 2000; Wilkison et al., 2003; Bischoff et al., 2018). Paragould was described as a highly shocked and brecciated chondrite (Fredriksson et al., 1968; Chou and Cohen, 1973). We reclassified the analyzed fragment as petrologic type 4, although all previous studies describe it as petrologic type 5 (Rubin, 1992; Rubin, 1994). Soko Banja
<table><caption>**Table 4**
Average apatite compositions (calculated from individual electron microprobe measurements on distinct apatite grains). Values are given in oxides (wt.%) and corresponding formula were calculated based on 13 anions (O, F, Cl, missing component X).</caption><thead><tr><th rowspan="3"></th><th colspan="2">L Ordinary Chondrites</th><th colspan="2">Atarra (L5)</th><th colspan="2">Bald Mountain (L5)</th><th colspan="2">Ausson (L6)</th><th colspan="2">Modoc (L6)</th><th colspan="2">Kyushu (L6)</th><th colspan="2">Alfianello (L6)</th><th colspan="2">Hamlet (LL3.5-3.6)</th><th colspan="2">Paragould (LL4)</th><th colspan="2">Jelica (LL5)</th><th colspan="2">Soko Banja (LL5)</th><th colspan="2">Dhurmsala (LL6)</th></tr><tr><th colspan="2">Tennasilm (L4)</th><th colspan="2">mean</th><th colspan="2">mean</th><th colspan="2">mean</th><th colspan="2">mean</th><th colspan="2">mean</th><th colspan="2">mean</th><th colspan="2">mean</th><th colspan="2">mean</th><th colspan="2">mean</th><th colspan="2">mean</th><th colspan="2">mean</th></tr><tr><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th><th>σ</th></tr></thead><tbody><tr><td>F</td><td>0.41</td><td>0.06</td><td>0.30</td><td>0.02</td><td>1.46</td><td>0.12</td><td>0.54</td><td>0.13</td><td>0.43</td><td>0.12</td><td>0.57</td><td>0.10</td><td>0.48</td><td>0.11</td><td>1.21</td><td>0.62</td><td>0.54</td><td>0.18</td><td>0.54</td><td>0.12</td><td>0.38</td><td>0.08</td><td>2.94</td><td>0.08</td></tr><tr><td>Cl</td><td>5.40</td><td>0.14</td><td>5.87</td><td>0.11</td><td>3.65</td><td>0.12</td><td>5.35</td><td>0.14</td><td>5.28</td><td>0.15</td><td>4.98</td><td>0.13</td><td>5.24</td><td>0.27</td><td>4.20</td><td>0.88</td><td>5.03</td><td>0.38</td><td>5.50</td><td>0.26</td><td>5.45</td><td>0.06</td><td>1.31</td><td>0.19</td></tr><tr><td>Na2O</td><td>0.42</td><td>0.02</td><td>0.36</td><td>0.01</td><td>0.28</td><td>0.05</td><td>0.36</td><td>0.02</td><td>0.48</td><td>0.02</td><td>0.41</td><td>0.03</td><td>0.38</td><td>0.07</td><td>0.37</td><td>0.05</td><td>0.61</td><td>0.08</td><td>0.36</td><td>0.03</td><td>0.34</td><td>0.02</td><td>0.18</td><td>0.02</td></tr><tr><td>FeO</td><td>0.69</td><td>0.61</td><td>0.45</td><td>0.25</td><td>0.79</td><td>0.22</td><td>0.57</td><td>0.34</td><td>0.46</td><td>0.20</td><td>0.52</td><td>0.22</td><td>0.38</td><td>0.18</td><td>0.63</td><td>0.09</td><td>0.76</td><td>0.08</td><td>0.33</td><td>0.10</td><td>0.39</td><td>0.22</td><td>0.42</td><td>0.02</td></tr><tr><td>P2O5</td><td>40.89</td><td>0.54</td><td>40.80</td><td>0.45</td><td>40.91</td><td>0.56</td><td>40.34</td><td>0.50</td><td>40.99</td><td>0.33</td><td>41.01</td><td>0.19</td><td>40.82</td><td>0.16</td><td>40.94</td><td>0.30</td><td>40.95</td><td>0.25</td><td>40.44</td><td>0.27</td><td>40.60</td><td>0.30</td><td>41.32</td><td>0.35</td></tr><tr><td>CaO</td><td>52.79</td><td>0.67</td><td>52.41</td><td>0.64</td><td>52.38</td><td>0.34</td><td>52.59</td><td>1.02</td><td>52.55</td><td>0.39</td><td>53.11</td><td>0.67</td><td>53.01</td><td>0.71</td><td>52.80</td><td>0.44</td><td>51.80</td><td>0.41</td><td>53.09</td><td>0.46</td><td>53.19</td><td>0.53</td><td>54.26</td><td>0.31</td></tr><tr><td>MgO</td><td>0.19</td><td>0.36</td><td>0.13</td><td>0.10</td><td>0.31</td><td>0.22</td><td>0.36</td><td>0.31</td><td>0.06</td><td>0.04</td><td>0.07</td><td>0.07</td><td>0.05</td><td>0.03</td><td>0.10</td><td>0.08</td><td>0.31</td><td>0.05</td><td>0.04</td><td>0.02</td><td>0.04</td><td>0.01</td><td>0.15</td><td>0.03</td></tr><tr><td>-O(Cl + F)</td><td>1.39</td><td>0.03</td><td>1.45</td><td>0.03</td><td>1.44</td><td>0.03</td><td>1.43</td><td>0.04</td><td>1.37</td><td>0.05</td><td>1.36</td><td>0.02</td><td>1.38</td><td>0.05</td><td>1.46</td><td>0.06</td><td>1.36</td><td>0.03</td><td>1.47</td><td>0.02</td><td>1.39</td><td>0.03</td><td>1.53</td><td>0.01</td></tr><tr><td>Total oxide<sup>a</sup></td><td>99.39</td><td>0.64</td><td>98.86</td><td>0.68</td><td>98.34</td><td>0.27</td><td>98.68</td><td>1.01</td><td>98.88</td><td>0.50</td><td>99.30</td><td>0.48</td><td>98.98</td><td>0.57</td><td>98.79</td><td>0.44</td><td>98.64</td><td>0.54</td><td>98.83</td><td>0.57</td><td>99.00</td><td>0.59</td><td>99.04</td><td>0.49</td></tr><tr><td colspan="22">Formula based on 13 anions</td></tr><tr><td>F</td><td>0.11</td><td>0.02</td><td>0.08</td><td>0.01</td><td>0.40</td><td>0.03</td><td>0.15</td><td>0.03</td><td>0.12</td><td>0.03</td><td>0.16</td><td>0.03</td><td>0.13</td><td>0.03</td><td>0.33</td><td>0.17</td><td>0.15</td><td>0.05</td><td>0.15</td><td>0.03</td><td>0.11</td><td>0.02</td><td>0.80</td><td>0.02</td></tr><tr><td>Cl</td><td>0.80</td><td>0.02</td><td>0.87</td><td>0.02</td><td>0.54</td><td>0.02</td><td>0.79</td><td>0.03</td><td>0.78</td><td>0.02</td><td>0.73</td><td>0.02</td><td>0.77</td><td>0.04</td><td>0.62</td><td>0.13</td><td>0.74</td><td>0.05</td><td>0.81</td><td>0.04</td><td>0.81</td><td>0.01</td><td>0.19</td><td>0.03</td></tr><tr><td>Na</td><td>0.07</td><td>0.00</td><td>0.06</td><td>0.00</td><td>0.05</td><td>0.01</td><td>0.06</td><td>0.00</td><td>0.08</td><td>0.00</td><td>0.07</td><td>0.00</td><td>0.07</td><td>0.01</td><td>0.06</td><td>0.01</td><td>0.10</td><td>0.01</td><td>0.06</td><td>0.00</td><td>0.06</td><td>0.00</td><td>0.03</td><td>0.00</td></tr><tr><td>Fe</td><td>0.05</td><td>0.05</td><td>0.03</td><td>0.02</td><td>0.06</td><td>0.02</td><td>0.04</td><td>0.03</td><td>0.03</td><td>0.02</td><td>0.04</td><td>0.02</td><td>0.03</td><td>0.01</td><td>0.05</td><td>0.01</td><td>0.06</td><td>0.01</td><td>0.02</td><td>0.01</td><td>0.03</td><td>0.02</td><td>0.03</td><td>0.00</td></tr><tr><td>P</td><td>3.01</td><td>0.02</td><td>3.02</td><td>0.03</td><td>3.01</td><td>0.03</td><td>2.99</td><td>0.02</td><td>3.02</td><td>0.01</td><td>3.01</td><td>0.02</td><td>3.01</td><td>0.02</td><td>3.01</td><td>0.01</td><td>3.03</td><td>0.01</td><td>2.99</td><td>0.01</td><td>3.00</td><td>0.02</td><td>2.99</td><td>0.01</td></tr><tr><td>Ca</td><td>4.92</td><td>0.04</td><td>4.90</td><td>0.05</td><td>4.88</td><td>0.04</td><td>4.93</td><td>0.05</td><td>4.91</td><td>0.02</td><td>4.94</td><td>0.04</td><td>4.95</td><td>0.05</td><td>4.91</td><td>0.00</td><td>4.84</td><td>0.03</td><td>4.97</td><td>0.03</td><td>4.98</td><td>0.03</td><td>4.97</td><td>0.00</td></tr><tr><td>Mg</td><td>0.02</td><td>0.05</td><td>0.02</td><td>0.01</td><td>0.04</td><td>0.03</td><td>0.05</td><td>0.04</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.01</td><td>0.01</td><td>0.04</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.01</td><td>0.00</td><td>0.02</td><td>0.00</td></tr><tr><td>Total</td><td>8.98</td><td>0.03</td><td>8.98</td><td>0.05</td><td>8.98</td><td>0.04</td><td>9.02</td><td>0.03</td><td>8.95</td><td>0.02</td><td>8.96</td><td>0.03</td><td>8.97</td><td>0.02</td><td>8.99</td><td>0.02</td><td>8.96</td><td>0.01</td><td>9.02</td><td>0.02</td><td>8.98</td><td>0.03</td><td>9.02</td><td>0.02</td></tr><tr><td>Octahedral total<sup>b</sup></td><td>5.06</td><td>0.06</td><td>5.01</td><td>0.06</td><td>5.02</td><td>0.06</td><td>5.08</td><td>0.04</td><td>5.03</td><td>0.03</td><td>5.05</td><td>0.04</td><td>5.05</td><td>0.05</td><td>5.03</td><td>0.02</td><td>5.04</td><td>0.02</td><td>5.06</td><td>0.03</td><td>5.07</td><td>0.03</td><td>5.05</td><td>0.01</td></tr><tr><td>Missing component<sup>c</sup></td><td>0.09</td><td>0.02</td><td>0.05</td><td>0.02</td><td>0.06</td><td>0.02</td><td>0.06</td><td>0.02</td><td>0.10</td><td>0.03</td><td>0.11</td><td>0.01</td><td>0.09</td><td>0.03</td><td>0.05</td><td>0.03</td><td>0.11</td><td>0.02</td><td>0.04</td><td>0.01</td><td>0.09</td><td>0.02</td><td>0.01</td><td>0.01</td></tr><tr><td>Cl/F<sup>d</sup></td><td>7.15</td><td>1.20</td><td>10.56</td><td>0.57</td><td>1.35</td><td>0.15</td><td>5.52</td><td>1.29</td><td>7.27</td><td>2.65</td><td>4.82</td><td>0.88</td><td>6.24</td><td>1.70</td><td>2.26</td><td>1.19</td><td>6.92</td><td>7.13</td><td>5.74</td><td>1.26</td><td>7.93</td><td>1.50</td><td>0.24</td><td>0.04</td></tr><tr><td>Cl#<sup>e</sup></td><td>87</td><td>2</td><td>91</td><td>0</td><td>57</td><td>3</td><td>84</td><td>3</td><td>87</td><td>3</td><td>82</td><td>3</td><td>86</td><td>3</td><td>65</td><td>16</td><td>83</td><td>5</td><td>85</td><td>3</td><td>88</td><td>2</td><td>19</td><td>3</td></tr><tr><td>n<sup>f</sup></td><td>19</td><td></td><td>6</td><td></td><td>3</td><td></td><td>5</td><td></td><td>15</td><td></td><td>5</td><td></td><td>11</td><td></td><td>3</td><td></td><td>8</td><td></td><td>16</td><td></td><td>7</td><td></td><td>2</td><td></td></tr></tbody></table>

a. Total oxide corresponds to the sum of oxides minus -O(Cl + F). b. Octahedral total corresponds to the sum of Na, Fe, Ca and Mg atomic abundances. c. Missing component (X) calculated by difference: 1 - (Cl + F), not included in the total. d. Atomic ratio. e. Cl# = Cl/(Cl + F) × 100, atomic. f. Number of electron microprobe measurements conducted on distinct grains from the meteorite sections. For meteorites where multiple sections were analyzed, only the last number of the section was included in the table for identification (see Table 3 for more details).
a L OCs

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

c

<figure><img src="image_13.png" /></figure>

b LL OCs

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

d

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

**Fig. 8.** Ternary representation of atomic proportions of Cl-F-X in apatites in L chondrites (a) and LL chondrites (b) as a function of petrologic type and OC group (c). Each point represents a single analysis. Chlorine and fluorine were measured directly, and the “X” apex was calculated by difference [$X = 1 - (F + Cl)$]. Each atom proportion ranging from 0 to 1, is given considering X,F,Cl atoms pfu. Analyses of apatites from L and LL chondrites of previous studies are added in panel c for comparison (Jones et al., 2014; Lewis and Jones, 2016; Li and Hsu, 2018; McCubbin et al. 2023; Wu and Hsu, 2019). The shaded areas represent a smoothed distribution of the single measurements (from both this study and literature), providing a visual overview of how the compositions are spread among L chondrites (orange shades) and LL chondrites (blue shades). This visualization highlights regions of higher concentration and overall dispersion in the data. Panel d shows the mean Cl# values [($Cl# = Cl/(Cl + F) \times 100$] with Cl and F in atoms pfu for individual OCs. Error bars represent ± 1-σ. Vertical lines represent the average Cl# per group and their associated standard deviation for all meteorites (solid-colored lines including all apatites measured in each group; dashed colored lines exclude the F-rich apatites, the outliers; and black dashed lines avg. for each OC group from previous studies Jones et al., 2014; Lewis and Jones, 2016; Li and Hsu, 2018; McCubbin et al. 2023; Wu and Hsu, 2019).
<table><caption>**Table 5**
Average merrillite compositions (calculated from individual electron microprobe measurements on distinct merrillite grains). Values are given in oxides (wt.%) and corresponding formula were calculated based on 56 oxygens.</caption><thead><tr><th rowspan="3"></th><th colspan="6">L Ordinary Chondrites</th><th colspan="6">LL Ordinary Chondrites</th><th colspan="6">L/LL Ordinary Chondrites</th></tr><tr><th colspan="2">Tennasilm (L4)</th><th colspan="2">Atarra (L5)</th><th colspan="2">Bald Mountain (L5)</th><th colspan="2">Ausson (L6)</th><th colspan="2">Modoc (L6)</th><th colspan="2">Kyushu (L6)</th><th colspan="2">Alfianello (L6)</th><th colspan="2">Hamlet (LL3.5-3.6)</th><th colspan="2">Paragould (LL4)</th><th colspan="2">Jelica (LL5)</th><th colspan="2">Soko Banja (LL5)</th><th colspan="2">Dhurmsala (LL6)</th><th colspan="2">Bjurböle (L/LL4)</th></tr><tr><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th><th>mean</th><th>σ</th></tr></thead><tbody><tr><td>F</td><td>0.06</td><td>0.11</td><td>0.02</td><td></td><td>0.03</td><td>0.02</td><td>0.06</td><td>0.01</td><td></td><td>0.01</td><td>0.01</td><td></td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.02</td><td>0.03</td><td>0.01</td><td>0.02</td><td>0.14</td><td></td><td>0.03</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.01</td></tr><tr><td>Cl</td><td>0.00</td><td>0.00</td><td>0.01</td><td></td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td></td><td>0.00</td><td>0.06</td><td></td><td>0.19</td><td>0.00</td><td></td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.01</td><td>0.01</td><td>0.01</td><td></td><td>0.01</td><td>0.00</td><td>0.00</td><td>0.01</td><td>0.00</td></tr><tr><td>Na2O</td><td>2.79</td><td>0.07</td><td>2.70</td><td></td><td>0.06</td><td>2.79</td><td>0.11</td><td>2.62</td><td></td><td>0.07</td><td>2.68</td><td></td><td>0.07</td><td>2.64</td><td></td><td>0.05</td><td>2.74</td><td>0.06</td><td>2.76</td><td>0.08</td><td>2.65</td><td>0.07</td><td>2.68</td><td></td><td>0.04</td><td>2.72</td><td>0.06</td><td>2.79</td><td>0.04</td><td>2.63</td></tr><tr><td>FeO</td><td>0.76</td><td>0.27</td><td>0.92</td><td></td><td>0.23</td><td>0.99</td><td>0.58</td><td>1.88</td><td></td><td>1.70</td><td>0.97</td><td></td><td>0.39</td><td>0.77</td><td></td><td>0.32</td><td>1.00</td><td>0.43</td><td>1.31</td><td>0.61</td><td>1.04</td><td>0.18</td><td>0.60</td><td></td><td>0.15</td><td>0.70</td><td>0.08</td><td>0.92</td><td>0.56</td><td>5.20</td></tr><tr><td>P2O5</td><td>46.32</td><td>0.36</td><td>45.77</td><td></td><td>0.28</td><td>45.95</td><td>0.49</td><td>45.22</td><td></td><td>1.06</td><td>45.52</td><td></td><td>0.50</td><td>46.04</td><td></td><td>0.36</td><td>45.90</td><td>0.36</td><td>45.65</td><td>0.39</td><td>45.76</td><td>0.24</td><td>45.66</td><td></td><td>0.22</td><td>45.54</td><td>0.21</td><td>45.94</td><td>0.40</td><td>44.07</td></tr><tr><td>CaO</td><td>46.32</td><td>0.27</td><td>45.66</td><td></td><td>0.26</td><td>45.84</td><td>0.50</td><td>44.98</td><td></td><td>1.06</td><td>46.07</td><td></td><td>0.30</td><td>46.19</td><td></td><td>0.36</td><td>45.78</td><td>0.48</td><td>45.81</td><td>0.49</td><td>45.97</td><td>0.18</td><td>46.23</td><td></td><td>0.15</td><td>46.28</td><td>0.18</td><td>46.37</td><td>0.38</td><td>42.80</td></tr><tr><td>MgO</td><td>3.60</td><td>0.08</td><td>3.67</td><td></td><td>0.03</td><td>3.60</td><td>0.09</td><td>3.67</td><td></td><td>0.10</td><td>3.54</td><td></td><td>0.13</td><td>3.60</td><td></td><td>0.21</td><td>3.65</td><td>0.12</td><td>3.58</td><td>0.15</td><td>3.45</td><td>0.05</td><td>3.50</td><td></td><td>0.04</td><td>3.59</td><td>0.14</td><td>3.65</td><td>0.06</td><td>3.36</td></tr><tr><td>-O(Cl + F)</td><td>0.03</td><td>0.05</td><td>0.01</td><td></td><td>0.01</td><td>0.01</td><td>0.02</td><td>0.01</td><td></td><td>0.01</td><td>0.02</td><td></td><td>0.05</td><td>0.00</td><td></td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.01</td><td></td><td>0.01</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.01</td></tr><tr><td>Total oxide<sup>a</sup></td><td>99.83</td><td>0.62</td><td>98.73</td><td></td><td>0.44</td><td>99.19</td><td>0.52</td><td>98.39</td><td></td><td>0.40</td><td>98.83</td><td></td><td>0.48</td><td>99.25</td><td></td><td>0.34</td><td>99.06</td><td>0.49</td><td>99.14</td><td>0.54</td><td>98.89</td><td>0.31</td><td>98.75</td><td></td><td>0.34</td><td>98.83</td><td>0.34</td><td>99.67</td><td>0.44</td><td>98.07</td></tr><tr><td colspan="33">Formula based on 56 oxygens</td></tr><tr><td>F</td><td>0.07</td><td>0.13</td><td>0.02</td><td></td><td>0.03</td><td>0.03</td><td>0.07</td><td>0.01</td><td></td><td>0.01</td><td>0.01</td><td></td><td>0.02</td><td>0.00</td><td></td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.02</td><td>0.03</td><td>0.01</td><td>0.02</td><td>0.16</td><td></td><td>0.04</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.01</td></tr><tr><td>Cl</td><td>0.00</td><td>0.00</td><td>0.00</td><td></td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.01</td><td></td><td>0.00</td><td>0.04</td><td></td><td>0.12</td><td>0.00</td><td></td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.01</td><td>0.00</td><td></td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>Na</td><td>1.93</td><td>0.04</td><td>1.90</td><td></td><td>0.04</td><td>1.95</td><td>0.07</td><td>1.86</td><td></td><td>0.04</td><td>1.88</td><td></td><td>0.05</td><td>1.84</td><td></td><td>0.04</td><td>1.92</td><td>0.04</td><td>1.94</td><td>0.05</td><td>1.86</td><td>0.05</td><td>1.88</td><td></td><td>0.03</td><td>1.91</td><td>0.04</td><td>1.94</td><td>0.03</td><td>1.89</td></tr><tr><td>Fe</td><td>0.23</td><td>0.08</td><td>0.28</td><td></td><td>0.07</td><td>0.30</td><td>0.18</td><td>0.58</td><td></td><td>0.53</td><td>0.30</td><td></td><td>0.12</td><td>0.23</td><td></td><td>0.10</td><td>0.30</td><td>0.13</td><td>0.40</td><td>0.19</td><td>0.32</td><td>0.06</td><td>0.18</td><td></td><td>0.05</td><td>0.21</td><td>0.02</td><td>0.28</td><td>0.17</td><td>1.61</td></tr><tr><td>P</td><td>14.03</td><td>0.04</td><td>14.03</td><td></td><td>0.02</td><td>14.03</td><td>0.07</td><td>13.96</td><td></td><td>0.16</td><td>13.97</td><td></td><td>0.07</td><td>14.04</td><td></td><td>0.06</td><td>14.03</td><td>0.03</td><td>13.98</td><td>0.06</td><td>14.02</td><td>0.03</td><td>13.99</td><td></td><td>0.02</td><td>13.97</td><td>0.03</td><td>13.98</td><td>0.05</td><td>13.83</td></tr><tr><td>Ca</td><td>17.76</td><td>0.11</td><td>17.71</td><td></td><td>0.09</td><td>17.71</td><td>0.15</td><td>17.58</td><td></td><td>0.20</td><td>17.90</td><td></td><td>0.11</td><td>17.82</td><td></td><td>0.08</td><td>17.71</td><td>0.12</td><td>17.75</td><td>0.14</td><td>17.83</td><td>0.05</td><td>17.93</td><td></td><td>0.05</td><td>17.97</td><td>0.04</td><td>17.85</td><td>0.09</td><td>17.00</td></tr><tr><td>Mg</td><td>1.92</td><td>0.04</td><td>1.98</td><td></td><td>0.02</td><td>1.94</td><td>0.05</td><td>1.99</td><td></td><td>0.08</td><td>1.91</td><td></td><td>0.07</td><td>1.93</td><td></td><td>0.12</td><td>1.96</td><td>0.07</td><td>1.93</td><td>0.08</td><td>1.86</td><td>0.03</td><td>1.89</td><td></td><td>0.02</td><td>1.94</td><td>0.08</td><td>1.95</td><td>0.03</td><td>1.86</td></tr><tr><td>Total</td><td>35.95</td><td>0.11</td><td>35.92</td><td></td><td>0.03</td><td>35.95</td><td>0.11</td><td>35.99</td><td></td><td>0.25</td><td>36.00</td><td></td><td>0.11</td><td>35.87</td><td></td><td>0.08</td><td>35.92</td><td>0.04</td><td>36.02</td><td>0.11</td><td>35.99</td><td>0.05</td><td>36.03</td><td></td><td>0.04</td><td>36.00</td><td>0.04</td><td>36.01</td><td>0.08</td><td>36.21</td></tr><tr><td>Octahedral total<sup>b</sup></td><td>21.84</td><td>0.08</td><td>21.87</td><td></td><td>0.05</td><td>21.89</td><td>0.17</td><td>22.01</td><td></td><td>0.41</td><td>21.99</td><td></td><td>0.18</td><td>21.83</td><td></td><td>0.14</td><td>21.89</td><td>0.07</td><td>22.02</td><td>0.16</td><td>21.87</td><td>0.08</td><td>21.88</td><td></td><td>0.04</td><td>22.03</td><td>0.07</td><td>22.03</td><td>0.13</td><td>22.36</td></tr><tr><td>Mg#<sup>c</sup></td><td>90</td><td>3</td><td>88</td><td></td><td>3</td><td>87</td><td>6</td><td>80</td><td></td><td>15</td><td>87</td><td></td><td>4</td><td>90</td><td></td><td>3</td><td>87</td><td>5</td><td>84</td><td>6</td><td>86</td><td>2</td><td>91</td><td></td><td>2</td><td>90</td><td>1</td><td>88</td><td>6</td><td>54</td></tr><tr><td>n<sup>d</sup></td><td>21</td><td></td><td>9</td><td></td><td>29</td><td></td><td>3</td><td></td><td></td><td>13</td><td></td><td></td><td>11</td><td></td><td></td><td>9</td><td></td><td>14</td><td></td><td>13</td><td></td><td>12</td><td></td><td></td><td>11</td><td></td><td>35</td><td></td><td>1</td></tr></tbody></table>

a. Total oxide corresponds to the sum of oxides minus -O(Cl + F). b. Octahedral total corresponds to the P abundances. c. Mg# = Mg/(Mg + Fe) × 100, atomic. d. Number of electron microprobe measurements conducted on distinct grains from the section.
<figure><img src="image_18.png" /><figcaption>Fig. 9. Major element variation in merrillite in L chondrites (a) and LL chondrites (b) measured by electron microprobe. Each point represents a single analysis and Fe, Na, and Mg contents are given in atoms per formula unit (atoms pfu) on a 56-oxygens basis.</figcaption></figure>

is a polymict breccia (Gopalan and Wetherill, 1969; Grady 2000; Wilkison et al., 2003) described as petrologic type 4 (Grady 2000; Wilkison et al., 2003); however, we reclassified it as petrologic type 5 based on the heterogeneities observed in low-Ca pyroxene compositions (Fig. 1, Grossman, 2011). Additionally, previous studies have described Jelica as an LL4–6 breccia (Bischoff et al. 2018). In this study the degree of thermal metamorphism indicates that the analysed fragment is a petrologic type 5.

For the rest of the reclassified meteorite fragments (Atarra, Bald Mountain, Ausson, Alfianello, and Hamlet), previous studies show no clear evidence of brecciation. However, several studies have classified them as different petrologic types or reported larger chemical heterogeneities than expected for their metamorphic grade, suggesting that some of these samples may be breccias. For example, Taylor et al. (1987) observed a large heterogeneity in the Ni content of the Fe,Ni metal grains from Ausson, which was interpreted as possible evidence of brecciation. Similarly, Friedrich et al. (2004) found a lanthanum enrichment in Atarra, which they attributed to mineralogical heterogeneity, possibly indicating brecciation. Thus, Ausson and Atarra sections analyzed in this study may represent more equilibrated fragments, explaining their reclassification as petrologic types 6 and 5, respectively. In the case of Ausson, reclassification as petrologic type 6 would be consistent with the temperature recorded by the olivine-spinel assemblages (761 °C), which is higher than the temperature recorded by the L6 chondrite Glatton (712 °C, Kessel et al., 2007). Notably, early work by Dodd (1981) estimated peak temperatures for petrologic type 6 in the range of 750–950 °C. Later studies defined peak temperatures as <675 °C for type 3, 675–865 °C for types 4 and 5, and 865–1000 °C for type 6 (Harrison and Grimm, 2010; Scott et al., 2014). However, these authors agree that these peak temperature boundaries are approximate and may overlap in reality.

Both Alfianello and Hamlet were described as having different petrologic types in previous studies. Alfianello is generally classified as petrologic type 5 (Stöffler et al., 1991; Rubin, 1994); however, Bischoff

et al. (2018) recently suggested it as belonging to petrologic type 6, which is consistent with the reclassification in the present study. Similarly, Hamlet is commonly classified as petrologic type 4 (Rubin 1994; Gattacceca et al. 2005), but alternative classifications have been suggested, including type 3.9 (Scott et al., 1993) and type 5–6 (Bischoff et al., 2018). Furthermore, the low mean fayalite content of olivine in section UH-0205.5 (Fa$_{18.1}$, S.D. 11.1) suggests that it could be related to H chondrites (for EOCs, H – Fa$_{17.3–20.3}$ and LL – Fa$_{26.6–32.4}$; Rubin, 1990) rather than LL chondrites; however, this fragment is classified as an UOC with large variations in composition (σ-Fa 11.1), which does not allow for confirmation that this fragment is more similar to H chondrites than to LL chondrites.

This study highlights the limitations of relying solely on petrologic classifications from the literature, as many meteorites are brecciated. We demonstrate that a fragment-specific approach is essential for accurate interpretation. We therefore recommend evaluating the degree of thermal metamorphism in individual fragments, especially in studies where metamorphism plays a key role in data interpretation. Additionally, we propose using the σ-Fs and σ-Fa values of low-Ca pyroxenes and olivine as proxies for assessing mineralogical equilibration and petrologic type, following the approach described by Grossman (2011).

## 4.2. Phosphate production from metal

Despite important variations across samples from the same group (especially in L chondrites), the abundance of phosphate minerals appears to be slightly higher in L chondrites (0.36 ± 0.16 vol%) than in LL chondrites (0.27 ± 0.08 vol%). This is consistent with the previous studies of Lewis and Jones (2016). However, their reported values are slightly higher (0.54 vol% for L and 0.46 vol% for LL chondrites), likely due to differences in methodological sensitivity (e.g., pixel-counting resolution) or statistical biases linked to the representativeness of the area investigated [e.g., L – 934 mm² in this study vs. 375 mm² in Lewis and Jones (2016); LL – 1685 mm² in this study vs. 226.5 mm² in Jones
et al. (2014)]. The large standard deviation suggests significant sample variability, which limits the robustness of this pattern. However, if valid, it would be consistent with slightly higher bulk P contents in L chondrites than in LL chondrites [950 ppm vs. 850 ppm, according to Wasson and Kalllemeyn (1988), and 1030 ppm vs. 910 ppm, according to Lodders and Fegley (1998)].

In each chondrite group (L and LL chondrites) the overall abundance of phosphate minerals increases with petrologic type (Fig. 2a). Meteorites of lower petrologic types (3.5–4) contain less phosphate minerals (0.1–0.3 vol%) compared to those of higher petrologic types (0.3–0.6 vol % in petrologic type 6 meteorites), while meteorites of petrologic type 5 show intermediate values (Fig. 2a). Previous studies (Jones et al., 2014; Lewis and Jones, 2016) mentioned a similar trend, but reported only a slight increase in abundances of phosphate minerals with petrologic type, close to the error margin of 0.1 vol%. In contrast, our results provide clear evidence for an increase in the abundances of phosphate minerals with petrologic type. This evidence further supports the interpretation that phosphate formed as secondary phases during thermal metamorphism, as suggested by several studies (Huss, et al. 1981; Zanda, et al. 1994; Lewis and Jones, 2016).

To better understand the mechanisms of phosphate mineral formation, previous studies have investigated the relationship between phosphate and Fe, Ni metal abundance (Murrell and Burnett, 1983; Pasek, 2019; Zanda et al., 1994); however, significant discrepancies remain. Zanda et al. (1994) suggested that the formation of phosphate minerals is directly related to Fe, Ni metal availability, since it is considered the primary P reservoir in OCs. In contrast, Lewis and Jones (2016) found no correlation between phosphate and metal abundance in OCs, suggesting two possible explanations for this lack of correlation: (1) any systematic differences between groups may fall below the precision of the abundance calculation method, or (2) the chondrule mesostasis could serve as an additional phosphorus reservoir.

In this study, we observe two correlations between abundances of phosphate minerals and Fe, Ni metal (Fig. 2). First, an *intragroup* correlation, where Fe, Ni metal abundance decreases with increasing phosphate abundance and petrologic type (Fig. 2a). Second, an *inter-group* pattern (between L and LL chondrites), where, at equivalent petrologic type, higher phosphate abundance corresponds to higher Fe, Ni metal abundance. The Fe, Ni metal abundances measured here (L – 3.10 ± 0.52 vol% and LL – 0.99 ± 0.56 vol%) are consistent with previous studies, although the reported values vary depending on the study (L – 4.1 vol% and LL – 2 vol%, Vernazza et al. 2016; and L – 3 vol% and LL – 1.5 vol%; Scott and Krot, 2014). The discrepancies between previous studies and our results may be due to differences in methodological sensitivity or statistical biases related to sample representativeness. However, our dataset includes a larger number of samples and covers a greater surface area, which improves the robustness of our conclusions. Additionally, Grossman and Brearley (2005) found no significant difference in P abundance within chondrule mesostases between L and LL chondrites (avg. L – 0.53 wt% P₂O₅ and LL – 0.59 wt% P₂O₅, see Table 5 from Grossman and Brearley, 2005), ruling out the possibility that P in the chondrule mesostasis played a significant role in the difference in phosphate abundance between in L and LL chondrites.

The two correlations (intra- and intergroup) between phosphate and metal abundance provide strong constraints on the formation of phosphate minerals in OCs. The intragroup correlation supports the model of phosphate formation through metamorphic process via solid-state oxidation reactions probably in the presence of a fluid (Ahrens, 1970; McSween and Labotka, 1993; Jones et al. 2014; Lewis and Jones, 2016). The intergroup correlation further reinforces the idea that P is originally hosted in Fe, Ni metal, as L chondrites, which contain higher metal abundances, also exhibit greater abundances of phosphate minerals.

Additionally, we show that the slopes of the phosphate-metal correlation differ between L and LL chondrites, with LL chondrites exhibiting a stronger correlation and a steeper slope (Fig. 2a). In LL chondrites, the steep slope indicates that even a small increase in

phosphate abundance corresponds to a significant decrease in Fe, Ni metal, suggesting that phosphate formation is directly tied to metal abundance—likely due to their higher oxidation state and lower initial metal content (McSween and Labotka, 1993; Vernazza et al., 2016; Scott and Krot, 2014). This implies that a large amount of metal must be oxidized to produce a relatively small amount of phosphate, consistent with previous studies showing that only LL chondrites reach oxygen fugacities above the iron-wüstite buffer, which is necessary to oxidize metallic Fe to FeO (Osadchii et al., 2017). In L chondrites, the phosphate and Fe, Ni metal abundances are not as strongly correlated. The shallower slope of the phosphate-metal correlation in L chondrites may reflect their lower oxidation state compared to LL chondrites, suggesting a less direct relationship between metal abundance and phosphate formation. Since phosphides can still oxidize to phosphates under low oxygen fugacities, this could indicate that phosphate formation in L chondrites was primarily driven by the oxidation of phosphides inherited from primitive materials (Rubin and Grossman, 1985; Pasek, 2015; Anzures et al., 2024).

The increase in merrillite abundance in LL chondrites better correlates with decreasing Fe, Ni metal abundance than the total phosphate abundance, indicating that the correlation with metal abundance in LL chondrites is primarily driven by merrillite (Fig. 2b). This suggests that Fe, Ni metal availability controls the merrillite formation, rather than the overall phosphate formation. The frequent association of merrillite with metal-sulfide assemblages (Fig. 6) further supports a relationship between Fe, Ni metal and merrillite in LL chondrites. Similar petrological assemblages (i.e., merrillite with metal-sulfide) are also observed in L chondrites (Fig. 5), also suggesting a potential connection between Fe, Ni metal and merrillite. However, the absence of a strong correlation between Fe, Ni metal and merrillite abundances in L chondrites indicates that this connection is less exclusive, implying that additional processes may contribute to merrillite formation in these meteorites.

### 4.3. Thermal evolution of phosphates

Previous studies have described textural changes in phosphate minerals, indicating a progressive grain-size coarsening with increasing thermal metamorphism (Jones et al., 2014, 2016; Lewis and Jones, 2016). Similarly, we observe a transition in phosphate textures, from irregular grains to compact, euhedral grains with sharp boundaries, as the petrologic type increases in both L and LL chondrites. However, textural changes alone are insufficient to confirm whether a mineral phase is undergoing grain size coarsening – a process driven by a reduction of the total grain boundary area in the system, typically evidenced by both an increase in grain size and a decrease in grain number (Thompson and Rubie, 1985). We thus measured both the grain size and the number of grains per mm² across the meteorite sections. If coarsening occurred, the proportion of large grains should be inversely correlated with the number of grains per mm².

Results show that meteorites with petrologic types 4 contain a higher number of phosphate grains per mm² and a lower proportion of large grains compared to meteorites classified as petrologic type 6, with those from petrologic type 5 having intermediate values (Fig. 4a). This confirms that phosphates undergo grain coarsening with increasing degree of thermal metamorphism. This process is tightly related to merrillite for which most meteorites (except for Alfianello, Atarra, and Jelica) follow this pattern, i.e. where the number of grains per mm² decreases as the proportion of large grains increases (Fig. 4b). In contrast, the number and sizes of apatite grains do not show a clear correlation with the degree of thermal metamorphism (Fig. 4c). Thus, merrillites appear to record the petrographic effects of thermal metamorphism much more distinctly than apatite, suggesting that this process plays a more significant role in the evolution of merrillite.

Apatite-merrillite associations were observed in different types of meteorites such as chondrites (Jones et al., 2014; Jones et al., 2016; Krzesińska, 2017; Lewis and Jones, 2016; McCubbin et al., 2023; Rubin
and Jones, 2003), achondrites (McCubbin et al., 2021; Shearer et al., 2011; Zhang et al., 2023), planetary meteorites (e.g., Martian and lunar meteorites, Černok et al., 2019; Gross et al., 2013; Howarth et al., 2016; McCubbin et al., 2011, 2014; Patiño Douce and Roden, 2006; Shearer et al., 2015; Wu et al., 2024), but are rare on Earth (e.g., Patiño Douce et al., 2011). Some of these occurrences, in particular in igneous rocks, reflect co-crystallization of apatite and merrillite (e.g., Gross et al., 2013; Shearer et al., 2015), and most have been interpreted as resulting from the replacement of merrillite by apatite. This process is attributed to incomplete coupled dissolution-reprecipitation reactions resulting from the interaction with a late Cl-rich fluid, particularly in OCs (Jones et al., 2014, 2016; Krzesińska, 2017; Lewis and Jones, 2016; Shearer et al., 2011). In this study, apatite-merrillite associations are common in LL chondrites (Fig. 7); however, they are rare in L chondrites, the only occurrence having been observed in Modoc (L6). Consistently, similar

associations have been previously reported in LL3.9, L/LL4, and LL6 chondrites (Jones et al., 2014), but only in a few L chondrites so far (the L4 Santa Barbara, Lewis and Jones, 2016; and the in the impact-melt portion of NWA 7251, host part of the chondrite L4, S4, Li and Hsu, 2018). Our observations in LL chondrites support a replacement mechanism in which merrillite is progressively transformed into apatite: at the grain scale, relict merrillite is found enclosed within apatite (e.g., Fig. 7a) and a representative example is the assemblage observed in Jelica (LL5, S3), where a merrillite grain with a lobate margin is rimmed by a continuous apatite layer in contact with chromite (Fig. 7f). At the section scale, the similarity in the size distributions of merrillite and apatite (comparable variability and median values) is consistent with apatite inheriting its distribution from merrillite (Fig. 3). In L chondrites, however, the grain size distribution of apatite differs significantly from that of merrillite, with apatite grains being larger and showing greater

<figure><img src="image_19.png" /><figcaption>**Fig. 10.** Crystal size distribution (CSD) of merrillite and apatite grains and lognormal fit for Tennasilm (L4) and Paragould (LL4). a–b) Lognormal fit with a single component. c–d) Lognormal fit with two components. Plain lines and shaded associated areas represent the CSD of the different phosphate minerals and the associated uncertainties. Dashed lines represent the final lognormal fit for each CSD. Dotted lines represent the lognormal component curves when more than one lognormal component was used to fit the CSD.</figcaption></figure>
size variability (Fig. 3). These observations challenge the interpretation that apatite forms only from pre-existing merrillite and suggest the involvement of alternative or additional processes in the formation of apatite in L chondrites.

To further understand the differences between the populations of phosphate minerals in L and LL chondrites, we analysed the crystal size distributions (CSDs) of apatite and merrillite (Fig. 10). For this analysis, we selected only 2 meteorites (Tennasilm, L4 and Paragould, LL4) for their similar petrologic types and grain counts. Fig. 10 shows the CSDs of both sections as solid lines (merrillite – red, apatite – blue), with shaded areas representing the statistical uncertainty. Lognormal fits, a standard approach in mineralogical CSD analysis (Eberl et al., 1998, 2002; Kile et al., 2000), are shown as dashed lines. In both chondrite groups, the frequency of grains decreases as crystal size increases (Fig. 10). In Paragould (LL4), the CSD curves of apatite and merrillite largely overlap, which is consistent with the general distribution observed in Fig. 3 for LL chondrites, where both minerals have similar median sizes and variability. However, Tennasilm (L4) has distinct CSD curves for apatite and merrillite (Fig. 10). Apatite grain frequencies are lower than those of merrillite at smaller grain sizes (0.15 vs. 0.33 at 3 µm), but become slightly higher beyond ~9 µm, where the two curves intersect. At ~15 µm, the frequency of apatite grains reaches ~0.04, while the corresponding merrillite frequency drops to ~0.01. The single component lognormal distributions follow the general decreasing trend of the CSDs in both meteorites (Fig. 10a – b). However, they do not adequately fit the apatite CSD curve for crystal sizes larger than ~15 µm in Tennasilm, or the merrillite CSD curve between ~7–15 µm in Tennasilm. Similar deviations are observed for both minerals in Paragould (Fig. 10). To refine the analysis, two-component lognormal fits have been applied, modelling the CSDs as the sum of two lognormal distributions: individual components appear as dotted lines, while the resulting fit or envelope is still plotted as a dashed line (Fig. 10c-d). This approach provides a better fit to the observed CSD shapes for both minerals and meteorites. This observation suggests that the grain-size distributions in Paragould and Tennasilm results from two distinct grain populations within each phosphate mineral, which are either formed at different times (as suggested by Streitenberger and Zöllner, 2015) or governed by different growth mechanisms (as suggested by Eberl et al., 1998, 2002).

In Paragould (LL4), the first lognormal components of the apatite and merrillite CSDs overlap, while the second components are closely aligned, though the merrillite distribution appears slightly steeper than that of apatite (Fig. 10d). Given the statistical uncertainties, this difference may not be significant. In contrast, in Tennasilm (L4), none of the lognormal components overlap between apatite and merrillite (Fig. 10c). These observations suggest a different link between the two phosphate minerals in the two meteorites. In the case of Paragould (LL4), the two-by-two superposition - between apatite and merrillite - of the individual components of the fit could indicate that the formation of the two phosphate sub-populations is synchronous or results from similar formation processes. This would be consistent with a genetic link between the different apatite and merrillite populations. This does not appear to be the case for Tennasilm (L4), for which the distribution decompositions do not appear to be correlated between both minerals, reinforcing the idea mentioned above that apatite does not form exclusively from merrillite.

Such a detailed CSD analysis cannot be conducted on the other meteorites investigated here due to the variable and/or limited phosphate grain counts. Therefore, it remains difficult to generalize these interpretations to all L and LL chondrites. However, because apatite and merrillite have very different grain size distributions in most L chondrites (Fig. 3), similar patterns might be anticipated from CSD decompositions, which would suggest that phosphates have not been produced by the exact same processes in L and LL chondrites. Future research should focus on rare earth element (REE) measurements to better understand phosphate mineral formation in L and LL chondrites. Jones et al. (2014) observed Eu depletion in merrillites of LL chondrites

and a lesser negative Eu anomaly in apatites, supporting a petrogenetic relation between the two minerals. A similar study is needed for L chondrites. As a first step investigation, Raman spectroscopy could be used to screen REE luminescence patterns in apatite and merrillite grains (Fau et al., 2022). This technique was so far applied to phosphate minerals from Martian meteorite NWA 7533, revealing variations in REE enrichment that suggest distinct phosphate generations (Malarewicz et al., 2023).

### 4.4. Volatile content of apatites

Apatite is frequently identified in both chondritic and achondritic meteorites and is commonly used to monitor the volatile evolution of planetary bodies and small Solar System bodies (Boyce et al., 2014; Sarafian et al., 2013; Sarafian et al., 2017; McCubbin and Jones, 2015; Bellucci et al., 2017). In OCs, apatites have very low H₂O contents suggesting that the fluids with which they interact were dry (i.e., H₂O depleted) and halogen-rich (Jones et al., 2014; Brearley and Jones, 2018). Therefore, these fluids could have been derived from melt degassing, produced either by partial melting in the interior of OC parent bodies, or as a result of impact melting (Jones et al., 2014; Brearley and Jones, 2018).

Apatites in OCs are generally Cl-rich (Jones, et al. 2014, 2016; Lewis and Jones, 2016; McCubbin et al., 2023). This is consistent with the broadly ultramafic composition of OCs, as Cl-rich apatites are generally considered a fluid tracer in mafic to ultramafic rocks (Scott and Krot, 2014, Jones, et al. 2014, 2016; Harlov, 2015; Lewis and Jones, 2016), and with Cl being the most abundant halogen in chondritic meteorites (e.g., Brearley and Jones, 2018; Lodders and Fegley, 1998). Despite the bulk Cl concentrations in OCs showing high heterogeneities (L – 76–270 ppm Cl and LL – 130–200 ppm Cl; e.g., Brearley and Jones, 2018), with discrepancies observed even among measurements analyzing similar meteorites multiple times (i.e., Bruderheim, L6 – 66–137 ppm Cl; e.g., Brearley and Jones, 2018), Cl contents remain higher than bulk F concentrations in OCs (L: 41–100 ppm; LL: 63–70 ppm; Wasson and Kallemeyn, 1988; Lodders and Fegley, 1998). In the present study, the apatites analyzed exhibit a wide range of Cl# values (17.3–96.1Cl#), with a clear distinction between L and LL chondrites (L – avg. 82.2Cl#, 11.3 S.D.; LL – avg. 68.2Cl#, 28.8 S.D., Fig. 8d). However, most apatites from both groups plot relatively consistently within the same field in the ternary diagram (Cl-F-X anion, Fig. 8c), except for six individual F-rich apatites from Bald Mountain, Dhurmsala, and Hamlet, which are considered outliers. When these six outliers are excluded, the mean Cl# values in L and LL chondrites are no longer significantly different (L – avg. 86.3Cl#, 3.1 S.D.; LL – avg. 82.7Cl#, 5.8 S.D., Fig. 8d, dashed colored lines), similar to the Cl content of apatites, which also shows no significant difference between the two groups (L – avg. 5.4 wt% Cl, 0.3 S. D., N = 61; LL – avg. 5.3 wt% Cl, 0.4 S.D., N = 33, Fig. 6, Table 4 and Table S2). This result suggests that F-rich apatite outliers have a strong influence on Cl#, leading to large apparent Cl# heterogeneities, whereas the Cl content of apatite remains relatively homogeneous with well-defined similarities between L and LL chondrites. Such observations contrast with the conclusions of Lewis and Jones (2016), who reported that L chondrite apatites were slightly richer in Cl (avg. L: 5.2 wt % Cl, S.D. 0.1; Cl# 84.7; LL: 4.7 wt% Cl, S.D. 0.3; Cl# 76.1; see Fig. 8c–d), but are consistent with the global Cl# average when incorporating more recent literature data (avg. L: Cl# 78.7, S.D. 9; LL: Cl# 77.6, S.D. 4), particularly those reporting L-melt rocks with F-rich apatites (Li and Hsu, 2018; Wu and Hsu, 2019). Additionally, Stormer et al. (1993) showed that the F concentrations can change by as much as 100 % if the measurements are made on apatite crystals oriented with the c-axis perpendicular to the polished section. This effect is not as significant for Cl or crystals with oblique orientations (Stormer et al., 1993; Webster and Piccoli, 2015). Therefore, the low F concentrations in apatites from OCs, combined with analytical challenges of measuring F via EMPA, could also contribute to the apparent Cl# heterogeneities. In general, F
concentrations in both bulk rock compositions and apatites from OCs vary more than the Cl concentrations (see Fig. 15.1 from Brearley and Jones, 2018). Here, the homogeneity of Cl contents in apatites from both L and LL chondrites supports the idea that these apatites formed from comparable Cl-rich hydrothermal fluids, with only minor chemical variations in the fluid composition accounting for the observed heterogeneity within and across samples. Additionally, the dispersion of Cl values (wt.%) in apatites, reflected in standard deviations per meteorite (Table 4), shows a decrease with petrologic type in LL chondrites; however, no correlation is observed in L chondrites (Fig. 8d and Table 4). This observation could indicate that the apatites in LL chondrites show an effect of metamorphic equilibration with the increased thermal metamorphism that is not observed in L chondrites.

Fluorine-rich apatites are rare in OCs. They have been identified in regolith breccias, such as Zag (H) and Kendleton (L), with F contents reaching up to 1.4 wt% F (Lewis and Jones, 2016; Jones et al., 2016). More recently, even higher F contents have been measured in apatites from impact melt veins in the L4 melt breccia NWA 7251 (Wu and Hsu, 2019) and in the L-melt rock NWA 11042 (Li and Hsu, 2018), with values reaching up to 5.30 wt% and 3.28 wt%, respectively. Most of these meteorites are breccias, and in all of them, the lithologies hosting F-rich apatites exhibit evidence of partial or complete melting due to impact processes (Jones et al., 2016; Lewis and Jones, 2016; Li and Hsu, 2018; Wu and Hsu, 2019). Therefore, the formation of F-rich apatite in chondrites has been attributed to impact-induced processes. Such processes could include degassing of impact melts or interaction with a volatile-rich impactor (Lewis and Jones, 2016; Jones et al., 2014, 2016), or, as it has been recently suggested, the direct role of impact-induced partial melting in forming neocrystallized phosphate grains with distinct Cl/F ratios (Li and Hsu, 2018; McCubbin et al., 2023; Wu and Hsu, 2019). Recent studies have shown that F is much more compatible in apatite compared to OH and Cl, especially in OH-poor apatite, suggesting that in rocks undergoing partial melting due to impacts, the residual apatite would evolve to more F-rich compositions, even without degassing (McCubbin and Ustunisik, 2018; McCubbin et al., 2023). Our recent work also suggests that although halogen elements in apatite (Cl and F) can be mobilized during shock metamorphism, they are not necessarily lost from the system and may instead be only locally rediscovered (Dobrică et al., 2025).

This study reports additional F-rich apatite grains in Dhurmsala (LL6, S3) and, to a smaller extent, in Bald Mountain (L5, S4) and Hamlet (LL3.5–3.6, S3). In particular, Dhurmsala, which is a fragmental breccia without regolith features (i.e., no solar wind-implanted gases or tracks; Bischoff et al., 2006), exhibits the highest F-rich apatite composition among the meteorites analyzed here (mean: 2.94 ± 0.08 wt% F vs. 1.31 ± 0.19 wt% Cl; Table 4). At the micron scale, some of these F-rich apatite grains show localized Cl depletion in regions with a higher density of dislocations and planar deformation features (Dobrică et al., 2025). These observations provided evidence of shock-induced volatile mobilization in Dhurmsala, supporting the impact-related origin of F-rich apatites (Dobrică et al., 2025; Jones et al., 2014, 2016; Lewis and Jones, 2016; Li and Hsu, 2018; McCubbin et al., 2023; Wu and Hsu, 2019). However, the brittle deformation of the F-rich apatites in Dhurmsala suggests that they survived impact events rather than neocrystallizing, indicating that local solid-state degassing better explains their high F enrichment. Notably, these levels (2.94 wt% F) exceed those of previously reported impact-formed F-rich apatites produced by degassing only (up to ~ 1.4 wt% F; Jones et al., 2014, 2016; Lewis and Jones, 2016). Overall, our results suggest impact processes significantly influenced halogen distribution in OC apatites, though the rarity of F-rich apatites highlights the need for further systematic study.

## 5. Summary

By extending the study of phosphate minerals to 15 different meteorites from the L, LL, and L/LL groups, covering a total area of about

2730 mm², and including quantitative data on phosphate abundances and grain sizes, this study provides a more refined perspective on their formation processes in OCs. Additionally, we emphasize the importance of assessing the degree of thermal metamorphism of each individual fragments, following the methodology proposed by Grossman (2011), since most meteorites are brecciated.

The abundance of phosphate minerals (i) is slightly higher in L than in LL chondrites, (ii) increases with petrologic type, and (iii) anti-correlates with the Fe,Ni metal abundance. Additionally, phosphate minerals undergo grain coarsening with petrologic type. These results support a metamorphic production of phosphates, and of merrillite in particular. In LL chondrites, apatite-merrillite intergrowths, interpreted as resulting from fluid-rock reactions, suggest that apatite forms from pre-existing merrillite. This is supported by the similar grain size distributions of both phosphates in LL chondrites. In contrast, no apatite-merrillite intergrowths are observed in L chondrites, in which the size distributions of apatites and merrillite point toward two distinct histories.

Although the Cl content in apatites does not differ significantly between L and LL chondrites, a few numbers of F-rich apatites strongly influence Cl#, leading to large apparent Cl# heterogeneities. We suggest that, since Cl is the major halogen in these apatites and F concentrations are low and particularly challenging to measure, comparisons between different OC groups should primarily be based on Cl concentrations. The small dispersion of Cl concentration in apatites and its decrease with petrologic type in LL chondrites indicate that apatites in these chondrites undergo equilibration with increasing thermal metamorphism, a pattern not observed in L chondrites.

## Data availability

Data are available through Zenodo at https://doi.org/10.5281/zenodo.15298672.

## CRediT authorship contribution statement

**Valentine Megevand:** Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Conceptualization. **Alexander N. Krot:** Writing – review & editing, Validation, Methodology, Investigation, Conceptualization. **Adrian J. Brearley:** Writing – review & editing, Validation, Methodology, Investigation, Conceptualization. **François Guyot:** Writing – review & editing, Validation, Methodology, Investigation. **Sylvain Bernard:** Writing – review & editing, Validation, Methodology, Investigation. **Elena Dobrică:** Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

## 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

We thank G. Huss for providing the Bjurbôle and Paragould samples from the American Meteorite Laboratory collection for this study. We are thankful for the constructive reviews provided by F. M. McCubbin and an anonymous reviewer, and for the efficient editorial handling of our manuscript. This work was funded by NASA grant 80NSSC22K1310 to E. Dobrică (PI).

## Appendix A. Supplementary material

The supplementary material contains a first section with a variance
comparison matrix of Fs values in meteorite sections using Fisher's F-test. Displayed as a heatmap, it highlights statistically significant differences in variance between meteorites and supports the re-evaluation of petrologic types based on Fs variability. Key groupings and exceptions are discussed to refine the classification of certain meteorites. The second section of the supplementary material contains two tables listing individual EPMA measurements of merrillite and apatite. Supplementary material to this article can be found online at https://doi.org/10.1016/j.gca.2025.08.023.

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