Multiple precursors of secondary mineralogical assemblages in CM chondrites

Isabella Pignatelli, Yves Marrocchi, Lionel. Vacher, Remi Delon, Matthieu Gounelle

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Isabella Pignatelli, Yves Marrocchi, Lionel. Vacher, Remi Delon, Matthieu Gounelle. Multiple precursors of secondary mineralogical assemblages in CM chondrites. Meteoritics and Planetary Science, 2016, 51 (4), pp.785-805. 10.1111/maps.12625. hal-01772661

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# Multiple precursors of secondary mineralogical assemblages in CM chondrites

Isabella PIGNATELLI<fnref n="1" />,<fnref n="2" />, Yves MARROCCHI<fnref n="1" />,<fnref n="2" />*, Lionel. G. VACHER<fnref n="1" />,<fnref n="2" />, Rémi DELON<fnref n="1" />,<fnref n="2" />, and Matthieu GOUNELLE<fnref n="3" />,<fnref n="4" />

**Abstract**–We report a petrographic and mineralogical survey of tochilinite/cronstedtite intergrowths (TCIs) in Paris, a new CM chondrite considered to be the least altered CM identified to date. Our results indicate that type-I TCIs consist of compact tochilinite/cronstedtite rims surrounding Fe-Ni metal beads, thus confirming kamacite as the precursor of type-I TCIs. In contrast, type-II TCIs are characterized by complex compositional zoning composed of three different Fe-bearing secondary minerals: from the outside inwards, tochilinite, cronstedtite, and amakinite. Type-II TCIs present well-developed faces that allow a detailed morphological analysis to be performed in order to identify the precursors. The results demonstrate that type-II TCIs formed by pseudomorphism of the anhydrous silicates, olivine, and pyroxene. Hence, there is no apparent genetic relationship between type-I and type-II TCIs. In addition, the complex chemical zoning observed within type-II TCIs suggests that the alteration conditions evolved dramatically over time. At least three stages of alteration can be proposed, characterized by alteration fluids with varying compositions (1) Fe- and S-rich fluids; (2) S-poor and Fe- and Si-rich fluids; and (3) S- and Si-poor, Fe-rich fluids. The presence of unaltered silicates in close association with euhedral type-II TCIs suggests the existence of microenvironments during the first alteration stages of CM chondrites. In addition, the absence of Mg-bearing secondary minerals in Paris TCIs suggests that the Mg content increases during the course of alteration.

## INTRODUCTION

Primitive meteorites known as chondrites can help us decipher the earliest stages of planetary formation, which occurred in the solar protoplanetary disk (SPD) (Davis et al. 2014; Marrocchi and Chaussidon 2015). Unfortunately, the record of the chemical conditions and physical processes in the SPD has been blurred in most chondrites by secondary parent body processes such as hydrothermal alteration and thermal metamorphism (Brearley 2006). It is therefore essential to understand secondary processing within asteroids and comets in order to access the primary properties of the SPD. Understanding hydrothermal alteration is also interesting in its own right because it is one of the most important manifestations of asteroidal and cometary

geology, resulting in wide compositional diversity (DeMeo and Carry 2014). Although much progress has been made in understanding hydrothermal alteration, fundamental first order questions remain open: What were the compositions of the fluids that affected the primitive chondrites? Under what physicochemical conditions did alteration occur (e.g., temperature, pH, $fO_2$)? What was the length of the period during which alteration took place in the early solar system?

Among the different types, CMs are the chondrites of choice for understanding hydrothermal alteration. Although they represent only 1.5% of meteorite falls (Grady 2000), CM-like material dominates the present and past micrometeorite flux (Gounelle et al. 2003; Genge et al. 2008) and could therefore represent the

<footnote n="1">¹Université de Lorraine, CRPG, UMR 7358, Vandoeuvre-lès-Nancy F-54501, France</footnote>

<footnote n="2">²CNRS, CRPG UMR 7358, Vandoeuvre-lès-Nancy F-54501, France</footnote>

<footnote n="3">³IMPMC, MNHM, UPMC, UMR CNRS 7590, 61 rue Buffon, 75005 Paris, France</footnote>

<footnote n="4">⁴Institut Universitaire de France, Maison des Universités, 103 boulevard Saint-Michel, 75005 Paris, France</footnote>

<footnote>*Corresponding author. E-mail: yvesm@crpg.cnrs-nancy.fr</footnote>

<footnote>*(Received 04 July 2014; revision accepted 22 January 2016)</footnote>
most abundant material in interplanetary space and at the surface of the Moon (Wasson et al. 1975). In addition, CM-like matter also constitutes an important fraction of xenoliths discovered in meteorites (Gounelle et al. 2003; Briani et al. 2012). It has also been suggested that CM chondrites might form part of the larger continuum between dark asteroids and comets (Gounelle 2011). Finally, studying CM chondrites is an indirect way of studying the key small bodies in the solar system that may have contributed to the terrestrial water and organic matter budget. Importantly, since the pioneering work of McSween (1979), several authors have shown that CM chondrites span a range of alteration degrees (Browning et al. 1996; Zolensky et al. 1997; Rubin et al. 2007). While some CM chondrites contain almost exclusively secondary minerals (e.g., phyllosilicates, carbonates, and iron oxides), other CMs have preserved up to 50% of their primary minerals (Howard et al. 2009). Different scales have been proposed to describe this progressive alteration. In the latest scale (Rubin et al. 2007), the alteration index ranges from 2.6 (moderately altered) to 2.0 (highly altered). This progressive alteration index makes it possible, assuming that all CMs come from the same parent body, to follow the progression of hydrothermal alteration.

In CM meteorites, phyllosilicates are the most abundant alteration products, followed by complex submicrometer intergrowths of S-rich phases and Fe-bearing serpentine, usually referred to as TCIs for tochilinite-cronstedtite intergrowths (previously reported as PCPs for poorly characterized phases; Fuchs et al. 1973). Although a number of studies have focused on TCIs since their discovery (Tomeoka and Buseck 1985; Nakamura and Nakamura 1996), the identification of their precursors and the chemical reactions leading to their formation are still a matter of debate. The difficulty in identifying TCI precursors is due to the fact that they have been almost completely replaced by secondary phases during alteration and it is quite rare to find TCIs containing remnants of primary minerals. To overcome this problem, we chose to characterize TCIs in the Paris chondrite. Although Paris is considered as the least altered CM meteorite currently known (Bourot-Denise et al. 2010; Leroux et al. 2013, 2015; Hewins et al. 2014; Marrocchi et al. 2014), it does contain abundant TCIs (Marrocchi et al. 2014). In addition, Paris, in common with many other CM chondrites, is a complex breccia that allows the nature and the evolution of TCI to be unraveled. Furthermore, the euhedral morphological and chemical features of some TCIs analyzed in the Paris meteorite have allowed us to firmly identify TCI precursors and to propose chemical reactions that lead to their formation.

STATE OF THE ART OF TCIS

Fuchs et al. (1973) first described the occurrence of complex associations of alteration minerals in CM chondrules and matrices that correspond to TCIs. However, they reported these associations as “poorly characterized phases” due to the difficulty of determining the mineralogy of the constituent minerals. Later investigations showed that TCIs are composed of complex intergrowths of tochilinite; cronstedtite; and minor amounts of magnetite, chromite, and a mineral containing Fe, Ni, Cr, and P (Tomeoka and Buseck 1982, 1983a, 1983b, 1985; Mackinnon and Zolensky 1984; Zolensky and Mackinnon 1986).

Two kinds of TCIs were distinguished on the basis of their location, texture, and composition. Sulfur-rich type-I TCIs are mainly composed of tochilinite with variable but minor amounts of cronstedtite and magnetite. They appear as massive, rounded forms occurring almost exclusively in chondrules (Nakamura and Nakamura 1996), sometimes around remnant kamacite grains (Tomeoka and Buseck 1985; Lauretta et al. 2000; Palmer and Lauretta 2011). Unlike type-I TCIs, type-II TCIs exhibit fibrous textures and are generally found within the matrix. The most abundant mineral in this type of TCI is cronstedtite, but they also contain tochilinite and/or a mixed-layer mineral made of cronstedtite and tochilinite (Tomeoka and Buseck 1985; Nakamura and Nakamura 1996). Occasionally, type-II TCIs present euhedral morphologies that suggest an origin via replacement of a precursor material without affecting the outward form of the original crystal (i.e., pseudomorphism; Tomeoka and Buseck 1985).

The formation of both types of TCI is not yet well understood and doubts remain concerning their origins and potential relationship. Tomeoka and Buseck (1985) proposed that type-I TCIs are formed first, by low-temperature interaction of S-bearing fluids with kamacite. The host chondrule is believed to be unaffected by the alteration process. As the alteration degree and fragmentation of chondrules increase, cronstedtite is preferentially produced instead of tochilinite, thereby promoting the conversion of type-I into type-II TCIs. However, TCIs with intermediate amounts of cronstedtite, in between type-I and type-II TCIs, have never been observed and this suggests the existence of distinct precursors to explain the occurrence of these two distinct types of TCIs. Nakamura and Nakamura (1996) therefore proposed kamacite as the precursor of type-I TCIs and unidentified minerals for type-II TCIs. Melilite, troilite, and Ca-carbonate have also been proposed as possible precursors of type-II TCIs (Lee and Greenwood 1994; Degawa et al. 1995; Lee et al. 2013), but this has not yet been proven experimentally.
# MATERIAL AND METHODS

Detailed observations of a section of the Paris chondrite (#2010-1) were performed at CRPG with a JEOL JSM-6510 equipped with an energy dispersive X-ray (EDX) Genesis detector, using a 3 nA primary beam accelerated at 15 kV. We also performed multi-element EDX mapping (Fe, S, Si, and Mg) of 45 selected TCIs. The SEM was calibrated using silicate, metal, and sulfide standards prior to the acquisition of each map over a 1-h period. After acquisition, some of the element maps were combined in a single image in order to highlight the mineralogical characteristics of TCIs. When possible, EDX spectra were also recorded to identify the precursor remnants.

Quantitative determination of the chemical composition of TCIs was performed using a CAMECA SX-50 microprobe (EMPA) at the University of Paris VI. The analyses were recorded from the rim toward the core of TCIs with an accelerating voltage of 15 kV, a beam current of 10 nA, and a 15-s counting time per element. The microprobe was calibrated using the following natural and synthetic standards: albite for Na, forsterite for Mg and Si, orthoclase for Al and K, diopside for Ca, pyrite for S, MnTiO₃ for Ti and Mn, magnetite for Fe, Cr₂O₃ for Cr, and NiO for Ni. The detection limits were estimated to be 0.06 wt% for Na₂O; 0.05 wt% for MgO, SiO₂, Al₂O₃; 0.03 wt% for K₂O; 0.04 wt% for CaO; 0.10 wt% for S, Cr₂O₃, MnO, and NiO; and 0.16 wt% for FeO.

The hydrated and porous nature of TCIs, as well as the occurrence of submicrometer tochilinite/cronstedtite intergrowths, means that reliable identification of mineralogical phases in TCIs is difficult using the EMPA technique. We therefore excluded any analysis with a total wt% lower than 73, while analyses with less than 10 wt% SiO₂ were retained due to the possible occurrence of iron oxides/hydroxides in TCIs. In addition, in accordance with Palmer and Lauretta (2011), we regarded analyses with S >10 wt% and Si <5 wt% as corresponding to tochilinite, whereas analyses with Si >7 wt% and S <5 wt% were assigned to cronstedtite.

For the morphological analysis, interfacial angles of the most frequently observed minerals in CM meteorites were calculated using Shape 7.3.0 software (Dowty 1980, 1987) and then compared to the angles of euhedral TCIs.

# RESULTS

Thin section 2010-1 of the Paris meteorite presents a brecciated texture, with relatively fresh, metal-rich zones that are in direct contact with moderately altered,

metal-poor zones (Fig. 1; Leroux et al. 2013; Hewins et al. 2014). Slightly altered kamacite beads surrounded by type-I TCIs and type-II TCIs are present in both zones (Fig. 1). Type-II TCIs present rounded or anhedral shapes but can also be euhedral in good agreement with the descriptions of Tomeoka and Buseck (1985). Both types of TCIs were analyzed and are described in detail below.

## Type-I TCIs

In the metal-rich zones, kamacite beads within chondrules show clear signs of alteration such as surface pitting at the peripheries of metallic beads (Fig. 2) (Morlok and Libourel 2013; Marrocchi et al. 2014). Iron-Ni metal beads rarely occur in the metal-poor zones (Fig. 3a) but they appear similar to those observed in the metal-rich zone of Paris (Fig. 2a). In both cases, Fe-Ni metal grains are commonly surrounded by rims of variable thickness that correspond to type-I TCIs. Energy dispersive X-ray mapping and EMPA data for the TCIs show that these alteration rims are iron-rich, with iron contents ranging from 31.6 to 66.9 wt% (Figs. 2b, 4a, and 5; Table 1). This TCI presents well-defined zoning. An S-bearing phase, close to tochilinite in composition, is located in the inner part of the rim (Figs. 2c and 2f), while the outer part is mainly composed of a mixture of tochilinite and a Fe-silicate that is chemically close to cronstedtite (Figs. 2d and 2e). The composition of these complex intergrowths evolves from the inside to the outside of the rims with (1) a progressive increase in Si content from 5.6 to 18.5 wt% (Fig. 5; Table 1), and (2) a decrease in the sulfur concentration from 27.9 to 9.4 S wt% (Fig. 5; Table 1). These chemical variations highlight the change in the relative proportions of tochilinite and cronstedtite within type-I TCIs. The proportion of cronstedtite, calculated using the Si-Mg-Fe ternary diagram of McSween (1987), varies from 30% in the vicinity of kamacite to up to 75% at the edge of the rims (Fig. 4a). The analytical point recorded near the border of the rim falls within the serpentine region of the Si-Mg-Fe diagram (Fig. 4a) due to its high MgO content (11.3 wt%). Given that MgO is nearly constant in the other analyses, it is not possible to say whether Mg has effectively been incorporated into the serpentine structure or whether the high MgO is due the electron beam excitation of the adjacent matrix during microprobe analysis.

## Type-II TCIs

Type-II TCIs are dispersed throughout the matrix, regardless of the degree of alteration, and show
<figure><img src="image_2.png" /><figcaption>Fig. 1. Composite backscattered electron image of Paris 2010-1 that reveals the brecciated nature of Paris with a relatively fresh metal-rich zone in direct contact with more altered zones devoid of Fe-Ni metal grains (metal-poor area, dotted red line). a–g) Backscattered electron images of representative examples of TCIs in metal-rich and metal-poor zones.</figcaption></figure>

rounded, anhedral, or euhedral shapes (Fig. 1). Whatever the shape of the TCIs, two concentric zones can be distinguished in each of the type-II TCIs in the Paris meteorite, because of the sharp interfaces between the different alteration phases (Figs. 6 and 9). The outer zone is characterized by (1) a compact texture of varying thickness (Figs. 6 and 7) and (2) a high concentration of sulfur, ranging from 5.2 to 23.5 wt% (Fig. 5; Table 1). Lath-shaped crystals observed in this zone emphasize the morphology of the TCIs and have a chemical composition close to tochilinite (Fig. 5; Table 1). The outer zone of type-II TCIs contains small areas in which both Si and S are present in addition to Fe (e.g., Figs. 6g, 6h, 7g, and 7h). This suggests the coexistence of two secondary minerals, probably tochilinite and cronstedtite. The relative proportions of these two minerals are highly variable from one TCI to another and also within each individual TCI, with cronstedtite proportions ranging from 10 to 60 vol% (Fig. 3). The inner zones of type-II TCIs are characterized by a fibrous and porous texture (Figs. 6 and 7) and a chemical composition dominated by iron (55 < FeO < 73.6 wt%), sometimes in combination with small concentrations of Si and S (<9.2 and <6.6 wt%, respectively). These compositional variations indicate the presence of three main phases: cronstedtite and tochilinite in the areas where Si and S are detected and amakinite (Fig. 8), an iron hydroxide belonging to the

brucite group (Sun et al. 1995), in the zones composed mostly of iron. Euhedral TCIs are characterized by fairly well-developed faces, which have preserved the original morphologies of their precursors (Figs. 6 and 7). Morphological analysis of many of these TCIs was possible by measuring the interfacial angles and comparing them to the theoretical values (Table 2). The angles determined for three well-preserved TCIs are close to the theoretical angles of orthorhombic pyroxenes. For example, the values of 115–127° measured for the TCI shown in Fig. 6 correspond to the angles between the {100} and {110} faces. Similarly, angles of 146–150° are found between {010} and {110}, and these differ from the theoretical angle (154°) by only a few degrees (4–8°). Angles of 119–120° were measured between {110} and {110} instead of 121°. In addition to pyroxene, seven well-preserved TCIs show specific angles characteristic of olivine. For the TCI shown in Fig. 7, the angles range from 115° to 126° between the {010} and {110} faces and from 125° to 130° between the {110} and {110} faces (Table 2). In one case, we observed the presence of remnant precursors in the center of the TCI, corresponding to a crystal of forsterite (Fo₉₈) (Fig. 7). Interestingly, the sharp interfaces between the outer zone with a high concentration of S and the Fe-rich zone are parallel to the faces of remnant forsterite (Fig. 9). This suggests that secondary minerals have progressively replaced the
<figure><img src="image_3.png" /><figcaption>Fig. 2. a) Backscattered electron image of a Fe-Ni metal bead located within a chondrule from the metal-rich zone of the Paris meteorite (section 2010-1). b–d) EDX elemental maps of Fe, S, and Si distribution in the Fe-Ni metal bead and surrounding TCI and silicate grains. e, f) Compiled EDX maps of Si-S and Si-Fe revealing the mineralogical zoning of this type-I TCI.</figcaption></figure>

forsterite crystal during the course of alteration. Moreover, one side of the forsterite crystal shows a curved surface that is typical of etching, confirming the dissolution of Mg-rich olivine to form the present TCIs (Fig. 9).

## Euhedral Unaltered Silicates

The mineralogical and petrographic survey of Paris' matrix reveals the presence of abundant, euhedral unaltered silicates (Fig. 3). The Mg-rich olivine

(Fig. 3b), low-Ca pyroxene (Fig. 3c), and iron-rich olivine (Fig. 3d) observed within the matrix present sizes in the range 5–50 µm that are comparable to euhedral TCIs (Figs. 1, 6, and 7).

## DISCUSSION

### Identification of TCI Precursors

SEM observations show that kamacite is the precursor of type-I TCIs (Fig. 1), as previously
<figure><img src="image_4.png" /><figcaption>Fig. 3. a) Backscattered electron image of a Fe-Ni metal bead located within a chondrule from the metal-poor zone of the Paris meteorite (section 2010-1). b) Backscattered electron image of a euhedral forsterite in the matrix of Paris. c) Backscattered electron image of a euhedral low-Ca pyroxene in the matrix of Paris. d) Backscattered electron image of a euhedral iron-rich olivine in the matrix of Paris.</figcaption></figure>

proposed in several studies (e.g., Brearley 2006; Palmer and Lauretta 2011). In contrast, the morphologies of euhedral type-II TCIs consist of at least two perpendicular mirror planes as the symmetry elements of the TCI precursors. This implies that the precursors cannot belong to the triclinic, monoclinic, and trigonal system, thereby restricting the number of possible precursor minerals (among those most commonly taken into account for TCI identification in CM meteorites) that the TCIs could have been derived from. By comparing the interfacial angles measured in the TCIs to theoretical values, it is possible to identify at least two anhydrous silicates as potential precursors: olivine and pyroxene (Table 2). The determination of euhedral angles characteristic of orthorhombic Mg-rich pyroxenes is surprising according to the rarity of this mineral in meteorites. However, the occurrence of orthopyroxenes has been described in several chondrules from the Antarctic CM chondrite ALH 81002 (Hanowski and Brearley 2001). Moreover, their rare occurrences are confirmed by the fact that only three TCIs among a total of 45 TCIs have morphologies compatible with orthopyroxenes.

Although Mg- and Fe endmembers were taken into account for olivines in the morphological comparison, it

is not possible to distinguish between Fe-rich or Mg- rich precursors. This is due to fact that differences in the theoretical angles of Mg- and Fe endmembers are negligible and are lower than the measurement errors. The presence of forsterite (Fo₉₈) in the center of one TCI (Fig. 9) suggests that TCI could have been formed from the alteration of Mg-rich precursors. However, we cannot exclude Fe-rich olivines as plausible precursors of type-II TCIs although type-II chondrules are not abundant in Paris (Fig. 4d; Hewins et al. 2014). Of note, interfacial angles characteristic of olivine are more common than those of pyroxene in type-II TCIs (n = 13). This can be explained by the lower resistance to aqueous alteration of pyroxenes compared to Mg- rich olivine, as indicated by the preferential replacement of enstatite by phyllosilicates in CM and CV chondrites (Hanowski and Brearley 2001; Ohnishi and Tomeoka 2007). Hence, an increase in the intensity of alteration would tend to smooth the angles of TCIs formed from pyroxene, while TCIs resulting from alteration of olivines would retain the euhedral shape of the primary minerals. In addition, the higher abundance of Mg- and Fe-rich olivines in the matrix of Paris relative to pyroxenes could also contribute to the higher occurrence of olivine-derived type-II TCIs.
<figure><img src="image_5.png" /><figcaption>Fig. 4. Chemical compositions of TCI minerals presented in the Si-Mg-Fe ternary diagram of McSween (1987) for hydrated minerals in CM meteorites (Cro = cronstedtite; Toch = tochilinite). The line separating cronstedtite from tochilinite represents mixtures of these two phases. a) TCI in Fig. 2. b) TCI in Fig. 4. c) TCI in Fig. 5. d) TCI in Fig. 6. e) TCI in Fig. 7.</figcaption></figure>

In both lithologies of Paris (metal-rich and metal-poor zones), some TCIs are rounded or irregular, making reconstruction of the morphology and the identification of precursors difficult, if not impossible. In such cases, only the presence of the primary mineral in the center of TCI enables the precursor to be identified (Fig. 9). This type of partial replacement texture is termed “centripetal replacement” and has been described in terrestrial rock samples (Velbel 1984, 1993). However, most of the CM chondrites so far known have been too heavily altered to allow this texture, as well as the euhedral morphology of the TCIs, to be preserved.

Hence, rounded or irregular TCIs could be the result of (1) the complete alteration of Fe-Ni metal beads, (2) protracted alteration of pseudomorphosed silicates that lost their euhedral shapes, or (3) complex alteration, and brecciation processes induced by collisions on the CM parent body (Rubin 2012). The three possibilities are obviously not exclusive. However, it appears that the least altered zone of Paris contains unaltered Fe-Ni metal beads while type-II TCIs are observed either in the metal-rich and metal-poor zones. Hence, the ratio of type-I/type-II TCIs likely decreases with increasing degree of alteration.
<figure><img src="image_6.png" /><figcaption>Fig. 5. Variations in SiO₂, FeO, MgO, NiO, and SO₂ concentrations from the rim to the core in (a) the type-I TCI presented in Fig. 1; and (b–e) the type-II TCIs presented in Figs. 6, 7, and 9.</figcaption></figure>

Some TCIs show re-entrant corners (Fig. 1d) indicating that the precursors could also be an association of crystals, likely a twin or a parallel growth of many individuals (Sunagawa 2005). In this case, the identification of precursors is prevented by the presence of at least two individuals, the orientation of crystals with respect to the cut of the thin section and, in the specific case of twin, the occurrence of several twin laws for the same mineral (e.g., olivine). In addition, we cannot exclude the possibility that re-entrant corners

might be linked to the presence of hopper growth single olivine crystals.

## Location of TCI Formation

The preservation of euhedral type-II TCIs in Paris confirms that Paris should be considered as the least altered CM chondrite known so far (Hewins et al. 2014; Marrocchi et al. 2014; Rubin 2015). In addition, the occurrence of type-II TCIs in the metal-poor area
<table><caption>Table 1. EMPA data of TCIs</caption><thead><tr><th></th><th>Na<sub>2</sub>O</th><th>MgO</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>K<sub>2</sub>O</th><th>CaO</th><th>S</th><th>TiO<sub>2</sub></th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>MnO</th><th>FeO</th><th>NiO</th><th>Total</th><th>S/SiO<sub>2</sub></th><th>FeO/SiO<sub>2</sub></th><th>MgO/SiO<sub>2</sub></th><th>NiO/SiO<sub>2</sub></th></tr></thead><tbody><tr><td>1/1</td><td>0.48</td><td>11.26</td><td>18.47</td><td>0.87</td><td>bdl</td><td>0.85</td><td>9.35</td><td>bdl</td><td>1.61</td><td>0.13</td><td>31.55</td><td>2.95</td><td>77.53</td><td>0.51</td><td>1.71</td><td>0.61</td><td>0.16</td></tr><tr><td>1/2</td><td>0.26</td><td>3.65</td><td>14.02</td><td>1.25</td><td>bdl</td><td>0.05</td><td>11.30</td><td>bdl</td><td>0.65</td><td>0.11</td><td>45.13</td><td>6.11</td><td>82.54</td><td>0.81</td><td>3.22</td><td>0.26</td><td>0.44</td></tr><tr><td>1/3</td><td>0.38</td><td>2.18</td><td>6.17</td><td>0.40</td><td>bdl</td><td>0.14</td><td>12.88</td><td>bdl</td><td>0.42</td><td>0.12</td><td>66.93</td><td>5.33</td><td>94.95</td><td>2.09</td><td>10.84</td><td>0.35</td><td>0.86</td></tr><tr><td>1/4</td><td>0.07</td><td>3.07</td><td>5.62</td><td>0.42</td><td>bdl</td><td>0.07</td><td>27.91</td><td>bdl</td><td>0.76</td><td>0.09</td><td>54.10</td><td>6.25</td><td>98.25</td><td>4.97</td><td>9.63</td><td>0.55</td><td>1.11</td></tr><tr><td>Mean</td><td>0.30</td><td>5.04</td><td>11.07</td><td>0.74</td><td>bdl</td><td>0.28</td><td>15.36</td><td>bdl</td><td>0.86</td><td>0.09</td><td>49.43</td><td>5.16</td><td>88.32</td><td>2.09</td><td>6.35</td><td>0.44</td><td>0.64</td></tr><tr><td>2/1</td><td>0.15</td><td>5.50</td><td>7.80</td><td>1.45</td><td>bdl</td><td>0.10</td><td>13.72</td><td>bdl</td><td>0.15</td><td>61.12</td><td>0.19</td><td>90.18</td><td>1.76</td><td>7.84</td><td>0.70</td><td>0.02</td><td>0.02</td></tr><tr><td>2/2</td><td>0.15</td><td>2.95</td><td>1.26</td><td>1.62</td><td>bdl</td><td>0.09</td><td>8.13</td><td>bdl</td><td>0.12</td><td>0.11</td><td>71.44</td><td>0.43</td><td>86.30</td><td>6.48</td><td>56.92</td><td>2.35</td><td>0.35</td></tr><tr><td>2/3</td><td>0.21</td><td>2.58</td><td>1.31</td><td>1.75</td><td>bdl</td><td>0.12</td><td>5.69</td><td>bdl</td><td>0.19</td><td>72.86</td><td>0.46</td><td>85.17</td><td>4.33</td><td>55.44</td><td>1.96</td><td>0.35</td><td>0.21</td></tr><tr><td>2/4</td><td>0.22</td><td>2.76</td><td>2.92</td><td>1.77</td><td>bdl</td><td>0.10</td><td>12.63</td><td>bdl</td><td>0.13</td><td>71.00</td><td>0.59</td><td>92.20</td><td>4.33</td><td>24.35</td><td>0.95</td><td>0.20</td><td>0.20</td></tr><tr><td>2/5</td><td>0.14</td><td>3.35</td><td>4.64</td><td>1.63</td><td>bdl</td><td>0.06</td><td>19.55</td><td>bdl</td><td>0.11</td><td>67.66</td><td>0.40</td><td>97.43</td><td>4.21</td><td>14.58</td><td>0.72</td><td>0.09</td><td>0.09</td></tr><tr><td>2/6</td><td>0.18</td><td>3.57</td><td>3.76</td><td>1.48</td><td>bdl</td><td>0.12</td><td>15.00</td><td>bdl</td><td>0.11</td><td>69.04</td><td>0.32</td><td>93.57</td><td>3.99</td><td>18.37</td><td>0.95</td><td>0.08</td><td>0.08</td></tr><tr><td>2/7</td><td>0.18</td><td>2.69</td><td>5.32</td><td>1.62</td><td>bdl</td><td>0.06</td><td>1.95</td><td>bdl</td><td>0.13</td><td>62.75</td><td>0.96</td><td>75.53</td><td>0.37</td><td>11.79</td><td>0.50</td><td>0.18</td><td>0.18</td></tr><tr><td>2/8</td><td>0.16</td><td>3.19</td><td>9.17</td><td>1.64</td><td>bdl</td><td>0.05</td><td>1.69</td><td>bdl</td><td>0.13</td><td>63.15</td><td>0.53</td><td>79.72</td><td>0.18</td><td>6.88</td><td>0.35</td><td>0.06</td><td>0.06</td></tr><tr><td>2/9</td><td>0.28</td><td>2.81</td><td>4.34</td><td>1.98</td><td>bdl</td><td>0.08</td><td>2.48</td><td>bdl</td><td>0.10</td><td>60.60</td><td>0.92</td><td>73.50</td><td>0.57</td><td>13.96</td><td>0.65</td><td>0.21</td><td>0.21</td></tr><tr><td>2/10</td><td>0.40</td><td>1.91</td><td>2.09</td><td>1.67</td><td>0.12</td><td>5.38</td><td>5.83</td><td>bdl</td><td>0.11</td><td>55.09</td><td>0.53</td><td>73.24</td><td>2.79</td><td>26.37</td><td>0.92</td><td>0.25</td><td>0.25</td></tr><tr><td>2/11</td><td>0.36</td><td>1.61</td><td>1.90</td><td>1.86</td><td>0.10</td><td>7.46</td><td>6.63</td><td>bdl</td><td>0.11</td><td>54.95</td><td>0.35</td><td>75.22</td><td>3.49</td><td>28.90</td><td>0.85</td><td>0.18</td><td>0.18</td></tr><tr><td>2/12</td><td>0.27</td><td>1.51</td><td>1.61</td><td>1.66</td><td>bdl</td><td>0.08</td><td>2.14</td><td>bdl</td><td>0.11</td><td>63.31</td><td>0.60</td><td>71.18</td><td>1.33</td><td>39.26</td><td>0.94</td><td>0.37</td><td>0.37</td></tr><tr><td>2/13</td><td>0.23</td><td>1.64</td><td>2.87</td><td>1.92</td><td>bdl</td><td>0.07</td><td>2.12</td><td>bdl</td><td>0.02</td><td>66.88</td><td>0.13</td><td>75.87</td><td>0.74</td><td>23.29</td><td>0.57</td><td>0.05</td><td>0.05</td></tr><tr><td>Mean</td><td>0.23</td><td>2.77</td><td>3.77</td><td>1.70</td><td>0.02</td><td>1.06</td><td>7.51</td><td>bdl</td><td>0.07</td><td>64.60</td><td>0.49</td><td>82.23</td><td>2.66</td><td>25.23</td><td>0.95</td><td>0.18</td><td>0.18</td></tr><tr><td>3/1</td><td>0.16</td><td>3.57</td><td>1.19</td><td>1.95</td><td>bdl</td><td>0.18</td><td>18.93</td><td>bdl</td><td>0.11</td><td>65.39</td><td>1.60</td><td>93.08</td><td>15.93</td><td>55.05</td><td>3.00</td><td>1.35</td><td>1.35</td></tr><tr><td>3/2</td><td>0.22</td><td>4.56</td><td>7.96</td><td>2.29</td><td>bdl</td><td>0.09</td><td>12.84</td><td>bdl</td><td>0.20</td><td>59.48</td><td>1.05</td><td>88.67</td><td>1.61</td><td>7.47</td><td>0.57</td><td>0.13</td><td>0.13</td></tr><tr><td>3/3</td><td>0.16</td><td>6.14</td><td>11.93</td><td>2.44</td><td>bdl</td><td>0.09</td><td>7.41</td><td>bdl</td><td>0.11</td><td>53.99</td><td>0.81</td><td>83.08</td><td>0.62</td><td>4.52</td><td>0.51</td><td>0.07</td><td>0.07</td></tr><tr><td>3/4</td><td>0.16</td><td>6.08</td><td>11.08</td><td>2.40</td><td>bdl</td><td>0.04</td><td>13.12</td><td>bdl</td><td>0.11</td><td>53.84</td><td>1.04</td><td>87.87</td><td>1.18</td><td>4.86</td><td>0.55</td><td>0.09</td><td>0.09</td></tr><tr><td>3/5</td><td>0.16</td><td>4.35</td><td>6.23</td><td>1.87</td><td>bdl</td><td>0.06</td><td>16.44</td><td>bdl</td><td>0.11</td><td>63.59</td><td>1.11</td><td>93.93</td><td>2.64</td><td>10.20</td><td>0.70</td><td>0.18</td><td>0.18</td></tr><tr><td>3/6</td><td>0.12</td><td>4.11</td><td>1.69</td><td>1.81</td><td>bdl</td><td>0.07</td><td>23.49</td><td>bdl</td><td>0.11</td><td>66.29</td><td>1.14</td><td>98.72</td><td>13.89</td><td>39.22</td><td>2.43</td><td>0.67</td><td>0.67</td></tr><tr><td>3/7</td><td>0.17</td><td>3.50</td><td>0.92</td><td>1.89</td><td>bdl</td><td>0.11</td><td>22.09</td><td>bdl</td><td>0.11</td><td>68.18</td><td>1.03</td><td>97.89</td><td>23.97</td><td>73.98</td><td>3.80</td><td>1.12</td><td>0.56</td></tr><tr><td>3/8</td><td>0.17</td><td>2.89</td><td>1.77</td><td>1.70</td><td>bdl</td><td>0.16</td><td>6.23</td><td>bdl</td><td>0.11</td><td>69.69</td><td>0.64</td><td>83.25</td><td>3.51</td><td>39.30</td><td>1.63</td><td>0.36</td><td>0.47</td></tr><tr><td>3/9</td><td>0.12</td><td>2.73</td><td>1.64</td><td>1.91</td><td>bdl</td><td>0.19</td><td>5.21</td><td>bdl</td><td>0.11</td><td>70.61</td><td>0.77</td><td>83.28</td><td>3.17</td><td>43.01</td><td>1.66</td><td>0.86</td><td>0.86</td></tr><tr><td>3/10</td><td>0.20</td><td>2.31</td><td>1.13</td><td>2.24</td><td>bdl</td><td>0.25</td><td>2.78</td><td>bdl</td><td>0.11</td><td>70.72</td><td>0.97</td><td>80.60</td><td>2.47</td><td>62.66</td><td>2.05</td><td>0.86</td><td>0.86</td></tr><tr><td>3/11</td><td>0.15</td><td>2.20</td><td>1.13</td><td>2.21</td><td>bdl</td><td>0.15</td><td>2.38</td><td>bdl</td><td>0.11</td><td>70.43</td><td>0.93</td><td>79.58</td><td>2.10</td><td>62.25</td><td>1.95</td><td>0.82</td><td>0.24</td></tr><tr><td>3/12</td><td>0.11</td><td>2.39</td><td>1.07</td><td>2.30</td><td>bdl</td><td>0.11</td><td>2.02</td><td>bdl</td><td>0.11</td><td>71.19</td><td>0.66</td><td>79.86</td><td>1.90</td><td>66.72</td><td>2.24</td><td>0.62</td><td>0.62</td></tr><tr><td>3/13</td><td>0.11</td><td>2.57</td><td>1.49</td><td>2.44</td><td>bdl</td><td>0.12</td><td>2.10</td><td>bdl</td><td>0.11</td><td>69.96</td><td>0.83</td><td>79.60</td><td>1.41</td><td>47.03</td><td>1.73</td><td>1.06</td><td>1.06</td></tr><tr><td>3/14</td><td>0.12</td><td>2.53</td><td>0.89</td><td>2.27</td><td>bdl</td><td>0.09</td><td>2.00</td><td>bdl</td><td>0.11</td><td>71.32</td><td>0.94</td><td>80.17</td><td>2.24</td><td>80.05</td><td>2.84</td><td>0.60</td><td>0.60</td></tr><tr><td>Mean</td><td>0.15</td><td>3.57</td><td>5.58</td><td>2.12</td><td>bdl</td><td>0.12</td><td>9.79</td><td>bdl</td><td>0.04</td><td>66.05</td><td>0.97</td><td>86.40</td><td>5.48</td><td>42.60</td><td>1.83</td><td>0.12</td><td>0.12</td></tr><tr><td>4/1</td><td>0.17</td><td>5.39</td><td>11.88</td><td>2.98</td><td>bdl</td><td>0.07</td><td>18.12</td><td>bdl</td><td>0.12</td><td>57.96</td><td>1.43</td><td>98.12</td><td>1.52</td><td>4.88</td><td>0.45</td><td>0.10</td><td>0.20</td></tr><tr><td>4/2</td><td>0.17</td><td>4.51</td><td>6.49</td><td>2.01</td><td>bdl</td><td>0.07</td><td>17.00</td><td>bdl</td><td>0.12</td><td>62.45</td><td>1.33</td><td>94.03</td><td>2.62</td><td>9.63</td><td>0.69</td><td>0.20</td><td>0.20</td></tr><tr><td>4/3</td><td>0.15</td><td>3.47</td><td>4.67</td><td>2.03</td><td>bdl</td><td>0.09</td><td>9.16</td><td>bdl</td><td>0.12</td><td>68.21</td><td>1.10</td><td>88.99</td><td>1.96</td><td>14.62</td><td>0.74</td><td>0.24</td><td>0.43</td></tr><tr><td>4/4</td><td>0.21</td><td>1.87</td><td>1.24</td><td>2.00</td><td>bdl</td><td>0.06</td><td>3.16</td><td>bdl</td><td>0.11</td><td>70.95</td><td>0.54</td><td>80.02</td><td>2.56</td><td>57.40</td><td>1.51</td><td>0.43</td><td>0.55</td></tr><tr><td>4/5</td><td>0.18</td><td>1.54</td><td>0.95</td><td>2.00</td><td>bdl</td><td>0.07</td><td>2.84</td><td>bdl</td><td>0.11</td><td>71.09</td><td>0.52</td><td>79.18</td><td>3.00</td><td>75.12</td><td>1.63</td><td>0.55</td><td>0.55</td></tr><tr><td>4/6</td><td>0.20</td><td>1.28</td><td>1.27</td><td>1.82</td><td>bdl</td><td>0.05</td><td>2.16</td><td>bdl</td><td>0.11</td><td>70.28</td><td>0.54</td><td>77.60</td><td>1.70</td><td>55.35</td><td>1.01</td><td>0.43</td><td>0.34</td></tr><tr><td>4/7</td><td>0.26</td><td>1.34</td><td>1.44</td><td>1.75</td><td>bdl</td><td>0.05</td><td>2.18</td><td>bdl</td><td>0.12</td><td>70.91</td><td>0.50</td><td>78.55</td><td>1.51</td><td>49.14</td><td>0.93</td><td>0.34</td><td>0.34</td></tr></tbody></table>
<table><caption>Table 1. *Continued*. EMPA data of TCIs.</caption><thead><tr><th></th><th>Na<sub>2</sub>O</th><th>MgO</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>K<sub>2</sub>O</th><th>CaO</th><th>S</th><th>TiO<sub>2</sub></th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>MnO</th><th>FeO</th><th>NiO</th><th>Total</th><th>S/SiO<sub>2</sub></th><th>FeO/SiO<sub>2</sub></th><th>MgO/SiO<sub>2</sub></th><th>NiO/SiO<sub>2</sub></th></tr></thead><tbody><tr><td>4/8</td><td>0.30</td><td>0.97</td><td>1.03</td><td>1.74</td><td>bdl</td><td>1.04</td><td>2.57</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>67.83</td><td>0.62</td><td>76.09</td><td>2.50</td><td>66.10</td><td>0.95</td><td>0.60</td></tr><tr><td>4/9</td><td>0.27</td><td>1.16</td><td>0.97</td><td>1.24</td><td>bdl</td><td>0.13</td><td>2.07</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>68.15</td><td>0.58</td><td>74.57</td><td>2.14</td><td>70.45</td><td>1.20</td><td>0.60</td></tr><tr><td>4/10</td><td>0.29</td><td>1.20</td><td>1.34</td><td>2.04</td><td>bdl</td><td>0.15</td><td>2.74</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>69.18</td><td>0.81</td><td>77.74</td><td>2.05</td><td>51.68</td><td>0.90</td><td>0.60</td></tr><tr><td>4/11</td><td>0.27</td><td>1.08</td><td>0.99</td><td>2.03</td><td>bdl</td><td>0.05</td><td>2.18</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>71.25</td><td>0.61</td><td>78.46</td><td>2.20</td><td>71.94</td><td>1.09</td><td>0.62</td></tr><tr><td>4/12</td><td>0.35</td><td>1.01</td><td>0.82</td><td>1.60</td><td>bdl</td><td>0.06</td><td>1.85</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>68.52</td><td>0.49</td><td>74.69</td><td>2.25</td><td>83.61</td><td>1.23</td><td>0.60</td></tr><tr><td>Mean</td><td>0.23</td><td>2.07</td><td>2.76</td><td>1.94</td><td>bdl</td><td>0.16</td><td>5.50</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>68.06</td><td>0.75</td><td>81.50</td><td>2.17</td><td>50.83</td><td>1.03</td><td>0.44</td></tr><tr><td>5/1</td><td>0.19</td><td>5.58</td><td>10.74</td><td>2.41</td><td>bdl</td><td>0.09</td><td>18.79</td><td>bdl</td><td>0.14</td><td>bdl</td><td>bdl</td><td>59.04</td><td>0.89</td><td>97.88</td><td>1.75</td><td>5.50</td><td>0.52</td><td>0.08</td></tr><tr><td>5/2</td><td>0.15</td><td>4.45</td><td>6.03</td><td>1.61</td><td>bdl</td><td>0.08</td><td>23.39</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>64.29</td><td>0.68</td><td>100.68</td><td>3.88</td><td>10.66</td><td>0.74</td><td>0.11</td></tr><tr><td>5/3</td><td>0.23</td><td>4.00</td><td>5.77</td><td>2.07</td><td>bdl</td><td>0.07</td><td>14.82</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>65.66</td><td>0.52</td><td>93.28</td><td>2.57</td><td>11.39</td><td>0.69</td><td>0.09</td></tr><tr><td>5/4</td><td>0.26</td><td>2.22</td><td>0.56</td><td>1.75</td><td>bdl</td><td>0.08</td><td>4.76</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>71.25</td><td>0.28</td><td>81.15</td><td>8.54</td><td>127.87</td><td>3.99</td><td>0.49</td></tr><tr><td>5/5</td><td>0.36</td><td>1.58</td><td>0.75</td><td>1.97</td><td>bdl</td><td>0.04</td><td>0.06</td><td>4.63</td><td>bdl</td><td>bdl</td><td>bdl</td><td>72.07</td><td>0.22</td><td>81.69</td><td>6.16</td><td>95.91</td><td>2.10</td><td>0.29</td></tr><tr><td>5/6</td><td>0.40</td><td>2.09</td><td>0.64</td><td>1.69</td><td>bdl</td><td>0.07</td><td>0.06</td><td>6.48</td><td>bdl</td><td>bdl</td><td>bdl</td><td>70.24</td><td>0.18</td><td>81.79</td><td>10.13</td><td>109.70</td><td>3.27</td><td>0.28</td></tr><tr><td>5/7</td><td>0.26</td><td>1.88</td><td>0.83</td><td>2.07</td><td>bdl</td><td>0.04</td><td>0.06</td><td>3.29</td><td>bdl</td><td>bdl</td><td>bdl</td><td>73.58</td><td>0.20</td><td>82.35</td><td>3.96</td><td>88.46</td><td>2.26</td><td>0.24</td></tr><tr><td>5/8</td><td>0.30</td><td>2.28</td><td>0.78</td><td>2.03</td><td>bdl</td><td>0.11</td><td>2.42</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>70.25</td><td>0.26</td><td>78.43</td><td>3.13</td><td>90.59</td><td>2.94</td><td>0.34</td></tr><tr><td>5/9</td><td>0.28</td><td>2.12</td><td>1.11</td><td>1.83</td><td>bdl</td><td>0.09</td><td>2.79</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>0.18</td><td>68.77</td><td>0.24</td><td>77.41</td><td>2.52</td><td>61.99</td><td>1.91</td><td>0.21</td></tr><tr><td>5/10</td><td>0.19</td><td>17.97</td><td>18.39</td><td>2.25</td><td>bdl</td><td>0.14</td><td>1.73</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>0.15</td><td>46.72</td><td>0.56</td><td>88.10</td><td>0.09</td><td>2.54</td><td>0.98</td><td>0.03</td></tr><tr><td>5/11</td><td>bdl</td><td>55.15</td><td>42.93</td><td>bdl</td><td>bdl</td><td>0.10</td><td>0.11</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>0.18</td><td>2.23</td><td>bdl</td><td>100.78</td><td>0</td><td>0.05</td><td>1.28</td><td>0</td></tr><tr><td>5/12</td><td>bdl</td><td>56.21</td><td>43.46</td><td>bdl</td><td>bdl</td><td>0.09</td><td>0</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>0.15</td><td>2.07</td><td>bdl</td><td>102.00</td><td>0</td><td>0.05</td><td>1.29</td><td>0</td></tr><tr><td>Mean</td><td>0.22</td><td>12.96</td><td>11.00</td><td>1.64</td><td>0.01</td><td>0.09</td><td>6.94</td><td>bdl</td><td>0.01</td><td>bdl</td><td>0.08</td><td>55.51</td><td>0.34</td><td>88.80</td><td>3.55</td><td>50.39</td><td>1.83</td><td>0.18</td></tr></tbody></table>

bdl = below detection limit.
<figure><img src="image_7.png" /><figcaption>Fig. 6. a) Backscattered electron image of a type-II TCI located in the matrix. This TCI presents well-developed interfacial angles that correspond to pyroxene. b) Schematic representation of the TCI, with the angles determined between each face labeled. c–e) EDX elemental maps of S, Si, and Fe distribution in the type-II TCI. f, h) Compiled EDX maps of Mg-Si, S-Fe, and Si-Fe revealing the mineralogical zoning of this type-II TCI.</figcaption></figure>

<footnote>1945100, 2016, 4. Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/maps.12625 by Cochrane France, Wiley Online Library on 20/08/2025. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use. OA articles are governed by the applicable Creative Commons License</footnote>
<figure><img src="image_8.png" /><figcaption>Fig. 7. a) Backscattered electron image of a type-II TCI located in the matrix. This TCI presents well-developed interfacial angles that correspond to olivine. b) Schematic representation of the TCI, with the angles determined between each face reported. c–e) EDX elemental maps of S, Si, and Fe distribution in the type-II TCI. f, h) Compiled EDX maps of Mg-Si, S-Fe, and Si-Fe revealing the mineralogical zoning of this type-II TCI. A significant enrichment of Mg is visible around this type II TCI, suggesting that Mg could have been extracted from the Mg-rich precursor by the fluid.</figcaption></figure>

suggests that even the most extreme alteration conditions experienced by Paris are still less intense than the other CMs where euhedral TCIs are rare. Regarding these features, an important question arises: in what

kind of environment did the alteration take place? Several settings are usually proposed for the formation of chondritic phyllosilicates, either in the solar protoplanetary disk or within asteroids during their
<figure><img src="image_9.png" /><figcaption>Fig. 8. Chemical composition of the iron-rich zone of type-II TCIs presented in an Mg-Fe-(O+H) ternary diagram (element wt%). Our results show that the mean values for iron hydroxides in each TCI fall close to amakinite composition (Mag = magnetite; Fer = ferrihydrite, Goe = goethite, Amak = amakinite).</figcaption></figure>

<table><caption>Table 2. Comparison between theoretical and measured interfacial angles for pyroxene and olivine. Theoretical interfacial angles were calculated using the Shape 7.3.0 software (Dowty 1980, 1987) for enstatite (Pbca, a = 18.2429, b = 8.8171, c = 5.1894 Å; Gatta et al. 2007), ferrosilite (Pbca, a = 18.417, b = 9.073, c = 5.239 Å; Hugh-Jones et al. 1997), forsterite (Pbnn, a = 4.756, b = 10.195, c = 5.981 Å; Palache et al. 1952), and fayalite (a = 4.818, b = 10.470, c = 6.086 Å; Della Giusta et al., 1990). Measured angles referred to TCIs in Figs. 4 and 5.</caption><caption>Interfacial Angles (°)</caption><thead><tr><th rowspan="2">Faces</th><th colspan="2">Theoretical values</th><th rowspan="2">Measured values</th></tr><tr><th>Enstatite</th><th>Ferrosilite</th></tr></thead><tbody><tr><td><strong>Pyroxene</strong></td><td></td><td></td><td></td></tr><tr><td>(100)* (110)</td><td>115.80</td><td>116.23</td><td>118</td></tr><tr><td>(100)* (1110)</td><td>115.80</td><td>116.23</td><td>127</td></tr><tr><td>(110)* (1110)</td><td>115.80</td><td>116.23</td><td>123</td></tr><tr><td>(110)* (1110)</td><td>115.80</td><td>116.23</td><td>115</td></tr><tr><td>(010)* (110)</td><td>154.20</td><td>153.67</td><td>149</td></tr><tr><td>(010)* (1110)</td><td>154.20</td><td>153.67</td><td>146</td></tr><tr><td>(010)* (1110)</td><td>154.20</td><td>153.67</td><td>150</td></tr><tr><td>(010)* (1110)</td><td>154.20</td><td>153.67</td><td>150</td></tr><tr><td>(110)* (1110)</td><td>128.41</td><td>127.55</td><td>120</td></tr><tr><td>(110)* (1110)</td><td>128.41</td><td>127.55</td><td>119</td></tr></tbody></table>

<table><thead><tr><th rowspan="2">Faces</th><th colspan="2">Theoretical values</th><th rowspan="2">Experimental values</th></tr><tr><th>Forsterite</th><th>Fayalite</th></tr></thead><tbody><tr><td><strong>Olivine</strong></td><td></td><td></td><td></td></tr><tr><td>(010)* (110)</td><td>115</td><td>117</td><td>115</td></tr><tr><td>(010)* (1110)</td><td>115</td><td>117</td><td>115</td></tr><tr><td>(010)* (1110)</td><td>115</td><td>117</td><td>117</td></tr><tr><td>(010)* (1110)</td><td>115</td><td>117</td><td>126</td></tr><tr><td>(110)* (1110)</td><td>129.98</td><td>130.58</td><td>130</td></tr><tr><td>(110)* (1110)</td><td>129.98</td><td>130.58</td><td>125</td></tr></tbody></table>

geological evolution. For instance, thermodynamic calculations suggest that gas-solid reactions during shock waves in icy regions of the solar nebula could produce conditions that allow anhydrous minerals to be rapidly hydrated (Ciesla et al. 2003). Another scenario states that the alteration processes took place within small, ephemeral, water-bearing protoplanetary bodies that were disrupted before their accretion with unaltered material into final asteroid parent bodies (Metzler et al. 1992). These two scenarios appear difficult to support according to the subsequent intense collisions and brecciation experienced by CM chondrites (Rubin 2012) that should have destroyed the euhedral textures of type-II TCIs. However, euhedral type-II TCIs located in the metal-rich zones are surrounded by fine-grained matrix that is mainly composed of unaltered GEMS-like amorphous silicates (Leroux et al. 2013). In addition, these TCIs are frequently in direct contact with unaltered Mg- or Fe-rich olivines (Figs. 5 and 6). Although such features are more compatible with nebular or pre-accretionary environments, we however, favor an asteroidal origin of type-II TCIs. Heterogeneous and variable alteration degrees have indeed been extensively reported at the micron scale within chondrites (Bland et al. 2009), demonstrating that complex and changing geochemical microenvironments have played a significant role in alteration processes. Furthermore, the high Fe³⁺/ΣFe ratio observed in amorphous silicates in weakly altered CRs, CO, and CMs show that anhydrous amorphous silicates could have been significantly hydrated and oxidized without being altered into phyllosilicates (Le Guillou and Brearley 2014; Le Guillou et al. 2015). In conclusion, although nebular or pre-accretionary alteration may have played a role in generating the characteristics of CM chondrites, its effects have certainly been overprinted by later alteration within the final CM parent body. Hence, the pseudomorphism of anhydrous silicates reported in this study likely took place in the final stage of alteration of CM chondrites.

Although an asteroidal setting appears as the most probable location for the formation of TCIs, our observations also show surprising occurrences of unaltered silicates sitting over few tenths of microns from type-II TCIs (Fig. 6). This suggests that alteration was heterogeneous and that silicates in the same local region of the meteorite could be altered at completely different rates. Such a view is quite different from the homogeneous alteration features reported for chondrules of ALH A81002, which has been used as a main argument to support parent body alteration (Hanowski and Brearley 2001). However, recent re-estimation of the porosity and permeability of
<figure><img src="image_10.png" /><figcaption>Fig. 9. a) Backscattered electron image of a type-II TCI located in the matrix. This TCI presents an anhedral morphology that prevents the precursor from being determined from measurements of interfacial angles. However, the presence of an Mg-rich olivine crystal in the center confirms forsterite as the precursors of type-II TCIs. b) Schematic representation of the anhedral TCI. c–e) EDX elemental maps of S, Si, and Fe distribution in the type-II TCI. f, h) Compiled EDX maps of Mg-Si, S-Fe, and Si-Fe revealing the mineralogical zoning of this type-II TCI.</figcaption></figure>

chondrites suggests that the latter is likely 4–8 orders of magnitude lower than previously estimated (Grimm and McSween 1989; Bland et al. 2009). Based on this

revaluation, the numerical simulations show that liquid water transport within CM chondrites occurs in the range of 10's–100's µm (Bland et al. 2009), in good
agreement with the small-scale alteration and heterogeneous alteration we observed in Paris. Understanding the origin of these micro-environments and how their physicochemical conditions evolved over time are challenging questions and are extremely important to understand in the future.

## Pathways of TCI Formation

The presence of remnant kamacite surrounded by type-I TCIs clearly attests to the fact that these TCIs were formed through the alteration of Fe-Ni metal beads by S-bearing water (Tomeoka and Buseck 1985; Lauretta et al. 2000; Palmer and Lauretta 2011). Thus, tochilinite is formed according to the simplified reaction:

$$ \text{kamacite} + \text{H}_2\text{O} + \text{S}_{\text{(aq)}} \rightarrow \text{tochilinite.} \quad (1) $$

This reaction explains the S-enrichment observed at the boundary between the TCI and the remnant kamacite beads (Fig. 2c). The presence of variable concentrations of Si in type-I TCIs ($5.62 < \text{SiO}_2 < 18.47$ wt%; Table 1) suggests that a cronstedtite-type Fe-silicate is also formed during the alteration of Fe-Ni metal beads. Unlike tochilinite, cronstedtite formation requires alteration fluids depleted in S but enriched in Si.

Their different morphological and mineralogical features suggest that formation of type-II TCIs is completely different from that of type-I TCIs. The euhedral morphology of type-II TCIs indicates pseudomorphic replacement of olivine and pyroxene precursors by secondary minerals. The replacement occurs first along fractures traversing the precursors (Fig. 7) and can be complete or partial according to the presence of remnant in TCI (Fig. 9). Partial replacement may be due to an aqueous alteration process that is locally reactant-limited and/or to transient alteration conditions (Velbel et al. 2012). Pseudomorphic replacement of olivine and pyroxene is typical of the serpentinization process in terrestrial ultramafic rocks (Wicks and Whittaker 1977; Dungan 1979a, 1979b; Klein et al. 2009).

It should be noted that, contrary to matrix euhedral silicates, chondrule silicates are mainly unaltered in Paris. This dichotomy could be due to the higher surface/volume ratio of matrix silicates, which induces a better efficiency of fluid alteration. In addition, isolated silicate grains dispersed throughout the matrix are in direct contact with the alteration fluid while chondrule silicates are partially shielded due to their compact nature. Furthermore, glassy mesostases and Fe-Ni metal are the most prone to be altered within chondrules and can protect in some way the chondrule silicates from fluid alteration. This observation is coherent with the

fact that pseudomorphic replacements of chondrule silicates have been only observed in heavily altered CMs (Zolensky et al. 1997; Brearley 2006; Rubin et al. 2007), suggesting that protracted alteration processes are required to alter chondrule silicates contrary to isolated silicate grains in the matrix.

Regarding the type-II TCIs of the Paris meteorite, we observed no significant textural and/or chemical differences between those formed from olivine precursors and those formed from pyroxene precursors. Nevertheless, one can remark that in some TCIs with olivine precursors the zoning appears thicker parallel to the {001} faces in comparison to the others (Fig. 10). These observations are consistent with the findings of Hanowski and Brearley (2001), which showed (1) a similar zoning for olivine precursors and (2) that the near-identical compositions of secondary minerals were due to fluid control and not to the type of primary mineral. These authors also reported complex chemical zoning in the replacement products of olivine and pyroxene, but a striking difference between their results and those of this study concerns the S content, which in Paris chondrite clearly decreases from the edges of type-II TCIs toward the center. Like pseudomorphism, the replacement reactions proceed from the surface of the parent mineral and move through it (Putnis 2002). The S-enrichment in the outer zone of pseudomorphs indicates that the first mineral formed is tochilinite. This is consistent with the observations of type-I TCIs, in which tochilinite is still the first product of aqueous alteration. As the composition of secondary minerals evolves progressively in type-II TCIs, the conditions (e.g., T, fluid chemistry, $fH_2$, $fO_2$) must also change as olivine and pyroxene replacement continues. From the

<figure><img src="image_11.png" /><figcaption>Fig. 10. Example of zoning in TCI with olivine precursor. The outer zoning is thicker parallel to the short {001} faces in comparison to the other faces.</figcaption></figure>
outer to the inner zone of the pseudomorphs, tochilinite, cronstedtite, and then amakinite are formed. Consequently, assuming that Mg- and Fe-rich olivines and enstatite are the precursors of type-II TCIs, three steps of alteration can be defined, each of them characterized by elevated Fe availability:

1. **Interaction with S-rich fluid.** Alteration of Mg-rich olivine and pyroxene precursors to tochilinite, following the reactions (see supporting information for equations related to the alteration of Fe-rich olivines):

$$ 
\begin{aligned}
2.6(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{SiO}_4[\text{Forsterite}] + 6\text{S}^{2-}(\text{aq}) & \\
& + 5.21\text{Fe}^{2+}(\text{aq}) + 5\text{H}_2\text{O} \\
& \rightarrow 6\text{Fe}_{0.9}\text{S} \cdot 5(\text{Mg}_{0.96}\text{Fe}_{0.04})(\text{OH})_2[\text{Tochilinite}] \\
& + 2.6\text{Si}^{4+} + 2.7\text{O}_2
\end{aligned}
\quad (2) $$

$$ 
\begin{aligned}
2.6(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{Si}_2\text{O}_6[\text{Pyroxene}] + 6\text{S}^{2-}(\text{aq}) & \\
& + 5.21\text{Fe}^{2+}(\text{aq}) + 5\text{H}_2\text{O} \\
& \rightarrow 6\text{Fe}_{0.9}\text{S} \cdot 5(\text{Mg}_{0.96}\text{Fe}_{0.04})(\text{OH})_2[\text{Tochilinite}] \\
& + 5.2\text{Si}^{4+} + 5.3\text{O}_2
\end{aligned}
\quad (3) $$

2. **Interaction with low-S, high-Si fluid.** The enrichment in Si in the fluid is due to the alteration of anhydrous silicates to tochilinite (Equations 2 to 3). Consequently, the formation of cronstedtite crystals occurs according to the following reactions:

$$ 
\begin{aligned}
(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{SiO}_4[\text{Forsterite}] + 3.85\text{Fe}^{2+}(\text{aq}) & \\
& + 5\text{H}_2\text{O} \rightarrow \text{Fe}_3(\text{SiFe})\text{O}_5(\text{OH})_4[\text{Cronstedtite}] \\
& + 1.85\text{Mg}^{2+} + 3\text{H}_2
\end{aligned}
\quad (4) $$

$$ 
\begin{aligned}
(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{Si}_2\text{O}_6[\text{Pyroxene}] + 3.85\text{Fe}^{2+}(\text{aq}) & \\
& + 3\text{H}_2\text{O} \rightarrow \text{Fe}_3(\text{SiFe})\text{O}_5(\text{OH})_4[\text{Cronstedtite}] \\
& + \text{Si}^{4+} + 1.85\text{Mg}^{2+} + \text{H}_2
\end{aligned}
\quad (5) $$

3. **Interaction with a low-S, low-Si fluid.** The formation of cronstedtite reduces the quantity of Si present in the fluid and a secondary mineral depleted in Si, that is, amakinite, can form according to:

$$ 
\begin{aligned}
(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{SiO}_4[\text{Forsterite}] + 0.65\text{Fe}^{2+}(\text{aq}) & \\
& + \text{H}_2\text{O} \rightarrow (\text{Fe}_{0.80}\text{Mg}_{0.20})(\text{OH})_2[\text{Amakinite}] \\
& + \text{Si}^{4+} + 1.65\text{Mg}^{2+} + 1.5\text{O}_2
\end{aligned}
\quad (6) $$

$$ 
\begin{aligned}
(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{Si}_2\text{O}_6[\text{Pyroxene}] + 0.65\text{Fe}^{2+}(\text{aq}) & \\
& + \text{H}_2\text{O} \rightarrow (\text{Fe}_{0.80}\text{Mg}_{0.20})(\text{OH})_2[\text{Amakinite}] \\
& + 2\text{Si}^{4+} + 1.65\text{Mg}^{2+} + 2.5\text{O}_2
\end{aligned}
\quad (7) $$

Because pseudomorphic replacement generally occurs at constant volume (Merino et al. 1994; Merino and Dewers 1998; Hanowski and Brearley 2001), the replacement Reactions 2 and 3 can be rewritten to be balanced on volume. Assuming molar volumes of ~44 cm³.mol⁻¹ for olivine, ~271 cm³.mol⁻¹ for tochilinite, ~113 cm³.mol⁻¹ for cronstedtite, and ~30 cm³.mol⁻¹ for amakinite, Reactions 2, 4, and 6 can be balanced as follows:

$$ 
\begin{aligned}
& 6.2(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{SiO}_4[\text{Forsterite}] + 6\text{S}^{2-}(\text{aq}) \\
& + 4.67\text{Fe}^{2+}(\text{aq}) + 5\text{H}_2\text{O} \\
& \rightarrow 6\text{Fe}_{0.9}\text{S5}(\text{Mg}_{0.96}\text{Fe}_{0.04})(\text{OH})_2[\text{Tochilinite}] \\
& + 6.67\text{Mg}^{2+} + 6.2\text{Si}^{4+} + 9.9\text{O}_2
\end{aligned}
\quad (8) $$

$$ 
\begin{aligned}
& 2.6(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{SiO}_4[\text{Forsterite}] + 3.61\text{Fe}^{2+}(\text{aq}) \\
& + 2\text{H}_2\text{O} \rightarrow \text{Fe}_3(\text{SiFe})\text{O}_5(\text{OH}_4)[\text{Cronstedtite}] \\
& + 4.81\text{Mg}^{2+} + 1.6\text{Si}^{4+} + 1.7\text{O}_2
\end{aligned}
\quad (9) $$

$$ 
\begin{aligned}
& 0.7(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{SiO}_4[\text{Forsterite}] + 0.7\text{Fe}^{2+}(\text{aq}) \\
& + \text{H}_2\text{O} \rightarrow (\text{Fe}_{0.80}\text{Mg}_{0.20})(\text{OH}_2)[\text{Amakinite}] \\
& + 1.1\text{Mg}^{2+} + 0.7\text{Si}^{4+} + 0.9\text{O}_2.
\end{aligned}
\quad (10) $$

The isovolumetric equations for pyroxene with a molar volume of ~63 cm³ mol⁻¹ are instead:

$$ 
\begin{aligned}
& 4.3(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{Si}_2\text{O}_6[\text{Pyroxene}] \\
& + 6\text{S}_2(\text{aq}) + 4.95\text{Fe}^{2+}(\text{aq}) + 5\text{H}_2\text{O} \\
& \rightarrow 6\text{Fe}_{0.9}\text{S} \cdot 5(\text{Mg}_{0.96}\text{Fe}_{0.04})(\text{OH}_2)[\text{Tochilinite}] \\
& + 3.15\text{Mg}^{2+} + 8.6\text{Si}^{4+} + 10.4\text{O}_2
\end{aligned}
\quad (11) $$

$$ 
\begin{aligned}
& 1.8(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{Si}_2\text{O}_6[\text{Pyroxene}] + 3.73\text{Fe}^{2+}(\text{aq}) \\
& + 2\text{H}_2\text{O} \rightarrow \text{Fe}_3(\text{SiFe})\text{O}_5(\text{OH}_4) \\
& + 3.33\text{Mg}^{2+}[\text{Cronstedtite}] + 2.6\text{Si}^{4+} + 1.9\text{O}_2
\end{aligned}
\quad (12) $$

$$ 
\begin{aligned}
& 0.5(\text{Mg}_{1.85}\text{Fe}_{0.15})\text{Si}_2\text{O}_6[\text{Pyroxene}] + 0.72\text{Fe}^{2+}(\text{aq}) \\
& + \text{H}_2\text{O} \rightarrow \text{Fe}_{0.80}(\text{Mg}_{0.20})(\text{OH}_2)[\text{Amakinite}] \\
& + 0.73\text{Mg}^{2+} + \text{Si}^{4+} + \text{O}_2.
\end{aligned}
\quad (13) $$

The reactions 2 to 5 clearly show that at the beginning of the alteration process both olivine and pyroxene pseudomorphic replacements cannot take place if the molar volume is to be preserved. This is because formation of tochilinite and cronstedtite requires a considerable volume expansion (e.g., ~6 moles of forsterite are needed to form one mole of tochilinite in
Reaction 11). As volume changes are required to form tochilinite and cronstedtite, the pseudomorphic replacement would be limited if the relative solubilities of primary and secondary minerals do not also vary (Putnis and Putnis 2007; Putnis et al. 2007; Putnis 2009). In contrast, the molar volume of amakinite is lower than those of olivine and pyroxene, which might explain the porosity of the inner zones of type-II TCIs (Figs. 6 and 7). The presence of porosity, the structural differences, as well as the sharp reaction front, all suggest that replacement occurs by dissolution-reprecipitation rather than by a solid-state process as proposed for leucite-analcime pseudomorphism (Putnis et al. 2007).

It is also worth noting, as far as physical properties are concerned, that olivine crystals are compact and strong. However, these minerals are composed of independent ($SiO_4$) tetrahedra linked together by readily soluble divalent ions ($Mg^{2+}$) or oxidizable divalent ions ($Fe^{2+}$). These observations are in good agreement with the Mg enrichment observed in the adjacent matrix of type-II TCIs (Figs. 6 and 7) and suggest that the Mg content of secondary alteration products increases during the course of alteration as previously proposed (Tomeoka and Buseck 1982, 1985). Considering the silicon, most of the Si atoms are not removed from the system but are incorporated into tochilinite/cronstedtite intergrowths and then by cronstedtite (Table 1).

The formation of type-II TCIs via the pseudomorphism of silicates raises many questions relative to the conditions of alteration. One of the most intriguing is the formation of S-rich, Mg-poor tochilinite instead of Mg-rich serpentine as we could have expected from the alteration of Mg-rich precursors. However, the chemical composition of the alteration products depends on several parameters, especially the composition of the fluid and the physicochemical conditions during the alteration. Hence, a possible explanation may lie in the fact that the physicochemical conditions during the first stage of alteration were out of range of the stability field of Mg-rich serpentines. Because the first fluids were S- and Fe-rich (due to the high sensitivity of FeS and Fe-Ni metal beads to alteration), Mg-rich silicates will be the first altered to tochilinite instead of Mg-rich serpentines. In addition, due to the high solubility of Mg in aqueous fluid and the important difference of the Mg content between TCI precursors (≈ 57 wt% Mg for forsterite) and the GEMS-like material that composed the unaltered surrounding matrix (≈ 10 wt%; Leroux et al. 2013), the Mg could have been removed from the system by the fluid itself. Laboratory experiments are, however, required to better understand the kinetic and

the conditions of Mg-rich silicate alteration by S- and Fe-rich fluids.

## Formation Conditions of Secondary Minerals

The formation of tochilinite by alteration of kamacite blebs and via olivine/pyroxene pseudomorphsim indicates extremely reducing conditions (−91 < log $PO_2$ < −85; Browning and Bourcier 1996) during the first step of alteration. The limited information available on tochilinite stability suggests that the growth medium was probably characterized by low $fO_2$, high $fS_2$, and $fH_2$ (Zolensky and Mackinnon 1986; Browning and Bourcier 1996). Laboratory syntheses show that tochilinite can form in neutral or alkaline medium (7.8 < pH < 11.5) at temperatures ranging from 80 to 320 °C (Kozerenko et al. 1996, 2001). Although tochilinite stability depends on temperature and sulfur concentration, it is also strongly affected by chemical composition. Fe-rich tochilinite is, in fact, metastable and can form at temperatures up to 130 °C, while ferromagnesian tochilinite is stable and its upper temperature limit reaches 320 °C (Kozerenko et al. 2001).

Compared to tochilinite, cronstedtite formation requires less-reducing conditions due to the presence of both $Fe^{2+}$ and $Fe^{3+}$. In contrast, these conditions are more-reducing than those required for formation of magnetite and goethite (Zolotov 2014). Neutral-alkaline conditions were reported for cronstedtite formation, but the pH range (7–8) is narrower than that of tochilinite (Pignatelli et al. 2013). Thermodynamic calculations and experimental observations indicate that the stability field of cronstedtite decreases with increasing temperature and cronstedtite is unstable at $T > 100$ °C (Schulte and Shock 2004; McAlister and Kettler 2008; Pignatelli et al. 2014; Zolotov 2014). However, a lower limit of 50 °C has recently been determined experimentally (Pignatelli et al. 2013).

The different conditions required for formation of cronstedtite and tochilinite imply that certain changes occurred in the growth medium during the alteration process. Moreover, the tochilinite/cronstedtite mixtures frequently observed in TCIs suggests that, at least for a limited period of time, the two minerals formed under common conditions before cronstedtite became progressively more stable.

Little information is available in the literature regarding amakinite stability (Moody 1976; Zolensky 1984). However, it appears that amakinite forms at slightly lower temperature than brucite (Moody 1976). By structural analogy, its formation conditions are presumably similar to brucite. Assuming that, like brucite, amakinite forms at pH >10 and with very low
dissolved SiO₂ activity (Zolotov [2014] and reference therein), a further change in alteration conditions would be required to move from cronstedtite to amakinite formation. An increase in the iron concentration of the fluid, probably due to concomitant alteration of kamacite and/or a depletion of Si and S in fluids after the precipitation of secondary minerals, would favor the formation of this iron hydroxide. The occurrence of a hydroxide in olivine/pyroxene pseudomorphs is not surprising considering that brucite is often formed during serpentinization in terrestrial rocks.

## CONCLUSIONS

Chemical and morphological analyses of type-I and type-II TCIs in the Paris meteorite have enabled identification of their precursors. The results confirm that kamacite is the precursor of type-I TCIs. Type-II TCIs are not formed from type-I TCIs, as was proposed by Tomeoka and Buseck (1985), but instead by pseudomorphism of anhydrous silicates (olivine, pyroxene).

Type-II TCIs are characterized by complex chemical zoning, reflecting the formation of different secondary minerals throughout the course of alteration. Hence, the mean values of FeO/SiO₂, S/SiO₂, MgO/SiO₂, and NiO/SiO₂ proposed by Rubin et al. (2007) to describe the compositional changes as a function of the degree of alteration cannot be used for these types of TCIs. A more detailed analysis is required because oxide ratios vary greatly within each TCI from the outside inwards due to progressive changes in the alteration conditions.

On the basis of zoning in type-II TCIs, at least three steps of aqueous alteration can be proposed for the Paris meteorite. These reflect the compositional evolution of the fluid. Always present in the secondary minerals (tochilinite, cronstedtite, amakinite), the availability of iron appears to be a common feature of each of these steps. The source of the iron is probably linked to alteration of Fe-Ni metal and FeS beads and Fe-rich amorphous silicates that are dispersed throughout the matrix. The first step of alteration is characterized by interaction with an S-rich fluid, as suggested by the formation of tochilinite in both type-I and type-II TCIs. The sources of S in the fluid have not yet been identified but several hypotheses have been put forward (1) H₂S, S₂, and other S-bearing gases in the early solar nebula; (2) H₂S contained in water-ice grains (Lauretta et al. 1996, 1998); or (3) troilite alteration contemporaneous with Fe-Ni metal bead alteration (Zolensky et al. 1993; Hanowski and Brearley 2001; Marrocchi and Libourel 2013). In the second step, an alteration fluid also enriched in Si is required to form

cronstedtite. Silicon is released by the dissolution of anhydrous silicates such as olivine and pyroxene during the pseudomorphic replacement. A further source of Si might be the glassy mesostasis of chondrules that usually alter into phyllosilicates (Hewins et al. 2014; Marrocchi et al. 2014). In the final step of the alteration process, the composition of the fluid changes to allow amakinite to be precipitated from a Si- and S-depleted fluid.

*Acknowledgments*—Laurent Tissandier, Michel Fialin, and Nicolas Rividi are thanked for assistance during sample preparation and analysis. Laurent Truche and Raphaël Pik are thanked for helpful discussion. This work was funded by l'Agence Nationale de la Recherche grant ANR-14-CE33-0002-01 SAPINS (PI Yves Marrocchi), Alan Rubin, Tomoki Nakamura, and Mike Zolensky are thanked for constructive reviews. This is CRPG-CNRS contribution #2328 and SAPINS contribution #04.

*Editorial Handling*—Dr. Adrian Brearley

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