ELSEVIER Chinese Astronomy and Astrophysics 38 (2014) 355–366

CHINESEASTRONOMYAND ASTROPHYSICS

The Petrology and Mineralogy Analysis ofNoble Metal Alloys in the Inclusions of a

Chondrite: An Implication on the Evolution ofthe Solar Nebula† �

WU Yun-hua1,2 XING Wei-fan1 XU WEi-biao2,3�1Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074

2Key Laboratory of Planetary Sciences, Chinese Academy of Sciences, Nanjing 2100083Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008

Abstract The calcium-aluminium-rich inclusions (CAIs) in chondrites are be-lieved to be the most early solids formed in the solar system, and retain the orig-inal information of the early solar nebula. However, in-depth researches revealedthat most inclusions had experienced a complex history of evolution, includingpartial melting and secondary alteration. Focused on the refractory and chem-ically stable noble metal particles in the CAI of a CV meteorite (NWA 2140),a study of astrochemistry is made in this paper. The petrology and mineralogyanalysis as well as the composition determination have been made on the noblemetal particles. Based on the result of composition analysis, the thermodynamicprocess experienced by the CAI is inferred, and two kinds of noble metal alloysare identified, which correspond to the early-stage condensation products andthe secondary alteration products of primordial metals, respectively.

Key words: astrochemistry—meteorites—solid state: refractory—methods:data analysis

† Supported by National Natural Science Foundation (41173076, 41273079) and Asteroid ResearchFoundation of Chinese Academy of Sciences

Received 2013–03–31; revised version 2013–10–14� A translation of Acta Astron. Sin. Vol. 55, No. 2, pp. 105–115, 2014�wbxu@pmo.ac.cn

0275-1062/01/$-see front matter c© 2014 Elsevier Science B. V. All rights reserved.PII:

0275-1062/14/$-see front matter © 2014 Elsevier B.V. All rights reserved.doi:10.1016/j.chinastron.2014.10.003

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1. INTRODUCTION

The carbonaceous chondrites are the condensates of the primary solar nebula, they arethe meteorites most close to the average composition of the solar system. The calcium-aluminium-rich inclusions (CAIs) in chondrites are common seen, they are the aggregatesto be wrapped up in the early solar system during the formation of meteorites, which arecomposed of refractory oxides and silicates, including corundum, black aluminium calciummineral, spinel, melilite, fassaite, perovskite, and other minerals, therefore they retain themost early information of the solar nebula, reflect the physico-chemical processes of distil-lation and condensation, and the environment in the early solar nebula[1]. Initially, CAIsare considered to be the direct condensation products of the primary solar nebula, but thein-depth researches have revealed that most inclusions have a complex evolutionary history,such as the partial melting and secondary alteration, etc.[2−3].

According to the relevant calculations of thermodynamic condensation, most noblemetals, such as Re, Os, Ir, Mo, Ru, Pt, Rh, and the refractory W, should be the elementscondensed most early in the solar nebula[4]. Palme et al.[4] have separated a 20μm-sizedmetal particle containing W, Re, Os Ir, Mo, Ru, Pt, and Rh from a CAI, and the analysisindicates that this refractory metal particle is the multi-element metal alloy condensed fromthe solar nebula. The later in-depth research shows that this kind of noble metal grainprobably has experienced the processes of exsolution, oxidization, and sulphurization, caus-ing the refractory metal to redistribute among the physical states of metal, sulphide, andoxide[5]. But compared with the CAI, the refractory metal is more likely to retain the orig-inal information, hence from the analysis on the noble metal grains in CAIs, we can obtainas much as possible the primary information about the formation of meteorites. The noblemetals discussed in this paper indicate mainly the 8 kinds of metal elements, including Au,the Pt-family metals (Os, Ir, Ru, Rh, Pt, and Pd), and other refractory noble metals (W,Mo, and Re).

Noble metals generally have the following three geneses: (1) to be condensed directly inthe nebula’s cooling process[4,6−7]; (2) to be crystalized in the forming process of chondritesand refractory inclusions[8]; and (3) owing to the reactions of exsolution, oxidization, andsulphurization that happened in the low temperature condition when the primordial metalsin CAIs and chondrites experienced the secondary alterations[9−11]. The study on the noblemetals in the primary chondrites will help us with understanding the physical and chemicalenvironments of the early solar nebula and the formation of meteorites, as well as the alter-ation process happened in the late stage. For example, if the noble metal is of the primordialgenesis, then its forming environment may reflect some physical and chemical properties ofthe primary solar nebula, such as the nebula’s chemical composition, temperature, coolingrate, etc.

2. EXPERIMENTAL METHOD

The studies on noble metals were performed mainly by observing and measuring their in-trinsic states and compositional characteristics. Using the energy spectrometer and electronmicroscope we analyzed the sample slices and blank plates. The experiment was made inthe astrochemistry and planetary science laboratory of Purple Mountain Observatory. The

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scanning electron microscope (SEM) to be used was Hitachi S3400N-II, with the acceler-ating voltage 15 kV, and the resolution better than 5 nm. The energy spectrometer wasOxford INCA. At first, according to the result analyzed by using the SEM, in combinationwith the energy spectrometer, we made the preliminary petrological analysis and mineralidentification on the sample. Then with the energy spectrometer, we made the compositiondetermination on the tiny grains of noble metals.

3. EXPERIMENTAL RESULTS

3.1 PetrologyNWA 2140 is a carbonaceous chondrite discovered 2003 in the Northwest Africa desert,and it was named by the International Society for Meteoritics in December 2008, with thetotal weight 324 g, it has the typical CV3 petrological characteristics in many aspects. TheCV3-group of carbonaceous chondrites are produced in the relatively remote regions fromthe sun, with a relatively high oxygen fugacity, and the Mg/Si abundance ratio similarwith the solar system[12−13]. NWA 2140 has an evident chondritic structure with the mm-sized spherical grains (mainly the porphyritic texture), but for the base material (occupyingabout 60%∼70%) they are μm-sized or smaller, the extent of hydrous alteration of the wholesample is relative low.

Two slices are prepared by taking a small amount of sample of NWA 2140, they arerespectively PMO-1024 and PMO-1031. The diameters of spherical grains are in the rangeof 0.7∼3mm, most of them are spherical or oblate, and the main minerals of the grainsare olivine and pyroxene. In the sample we found a rather large refractory inclusion, whichapproximately has a round shape with the major axis to be about 8mm, and the minoraxis to be about 7mm, and a clear boundary exists between the inclusion edge and thecontact surface with the base material, in a corner of the inclusion some smashed filling canbe seen, which can be distinguished from the main body distinctly, Figs.1(a),(b) are the twoapproximately parallel cross-sections of the same inclusion.

This inclusion has a double-layer structure, its outer part is rich of melilite, its corepart is rich of fassaite, the crystal particles are relatively large, 200∼800μm for pyroxene,it is a typical B1-type refractory inclusion[14]. The crystal particles of spinel are generallysmall, about 20μm. Relatively, the spinel and anorthite are enriched in the core part. FromFig.1 we can see clearly a core-mantle structure. In the form of fine crystals the perovskiteand the oxides of iron and titanium are generally wrapped up in the main minerals such asthe melilite and fassaite.

The main minerals in the CAI are melilite, spinel, fassaite, anorthite, common pyroxene,metals, and oxides of iron (for example the chromite and ilmenite), the secondary mineralsinclude the whitlockite, and as tiny nm-sized particles, the noble metals are distributed init (Fig.2). The mineral particles in the base material are relatively tiny, in the range fromseveral nm to several ten nm.

In the CAI we can find apparently two groups of noble metals. The first group indicatesthe noble metal particles neighboring directly the main minerals pyroxene, melilite or spinel,we call them the group-I noble metals. A part of the group-I noble metals may neighborwith FeNi metal or be wrapped up in FeNi metal, as shown in Fig.3(a); the second group

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Fig. 1 (a) CAI in NWA 2140 (PMO-1024); (b) CAI in NWA 2140 (PMO-1031), another cross section of

the same CAI as shown in Fig.1(a)

Fig. 2 A close-up view of the distribution of noble metal particles in the CAI

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Fig. 3 (a) The noble metal particles of the group-I; and (b) the noble metal particles of the group-II

indicates the particles to be surrounded by or co-existent with the complex, aggregatedFeNi metal particles, sulfides or phosphates, oxides and silicates, namely the noble metalsin opaque mineral aggregates, we call them the group-II noble metals, as shown in Fig.3(b).

3.2 Mineral Chemistry3.2.1 Noble metalsMost noble metal particles coexist with the FeNi metal, their composition and size haveno significant correlation. If the condensation of noble metals is realized in the form ofalloys, then their theoretical condensation curves are shown as Fig.4. This figure displaysthe composition information of the produced alloys under the different temperatures, theabscissa represents the condensation temperature, the ordinate indicates the weight fractionof alloy component. The different curves represent the condensations of different elements,namely the contents of different elements in the alloy under the different temperatures. Thearrow indicates the condensation temperature of a single-component noble metal.

This work has made the composition analysis on the 51 noble metal particles, includingthe above-mentioned two groups, the compositional characteristics for a part of typical noblemetals are listed in Table 1 and Table 2, respectively.

Table 1 The composition for the group-I noble metal alloys

Element Mass fraction/(%) AverageAlloy 1 Alloy 2 Alloy 3 Alloy 4 Alloy 5

Fe 3.9 2.5 2.0 10.9 9.1 5.7Ni 2.9 2.6 1.6 27.0 5.5 8.0Mo 18.7 2.1 15.2 - 19. 6 11.1Ru 22.4 24.6 30.4 30.8 27.9 27.2Os 13.1 47.9 42.6 27.3 16.6 29.5Ir 16.0 20.4 5.5 4.1 21.3 13.5Pt 18.0 - - - - 3.6W - - 2.7 - - 0.5Rh 5.1 - - - - 1.0

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Fig. 4 The theoretical condensation curves for the noble metal elements Os, Mo, Ru, Pt, Ni, Fe, and Co in

an alloy at equilibrium state[4]

Table 2 The composition for the group-II noble metal alloys

Element Mass fraction/(%) AverageAlloy 1 Alloy 2 Alloy 3 Alloy 4 Alloy 5

Fe 2.7 3.0 5.7 7.4 2.4 4.2Ni 4.7 11.9 4.5 16.3 3.2 8.1Mo 6.5 18.1 3.4 3.7 6.3 7.6Ru 45.2 31.7 46.1 29.1 24.9 35.4Os 26.9 12.2 26.3 12.6 50.8 25.8Ir 14.0 23.1 11.7 8.3 12.4 13.9Pt - - - 19.6 - 19.6W - - - - - -Rh - - 2.4 - - 2.4

Here we assume that the genesis for noble metals is condensation, using the researchfindings of predecessors about the theoretical relationship between the condensation temper-ature and the fractional content of noble metal, we have made the comparison and analysiswith the measurement. According to the study of Palme et al.[4] on the compositions ofnoble metal particles, the content of the condensed element E and the temperature have thefollowing relation:

αE =

⎡⎢⎢⎣1 +

CH2 × γE

Ptot ×13∑

i=1

αiCi

× 10(−AT +B)

⎤⎥⎥⎦

−1

.

In the refractory metal alloy the atomic fraction of an element can be expressed as

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XE = αECE13∑

i=1

αiCi

, in which, Ci is the cosmic abundance of the element; γE represents the

activity coefficient of the element; Ptot indicates the pressure of the solar nebula, here itadopts 10Pa[7]; T is the condensation temperature of the noble metal; A and B are thecoefficients used for the derivation of the element’s fractional pressure. The known quantitiesin this formula are taken from Campbell et al.[15]. The calculated result of this equationcan be obtained by iterations.

For an alloy particle, we have to obtain a suitable value of temperature at first, it canbe obtained by the least square method, namely to obtain the temperature value that makesthe summation of squared residuals between the actually measured values and the values onthe theoretical curve become minimum. Both the solution of equation and the derivationof the suitable temperature can be realized by Matlab. Adopting different parameters mayinfluence the result, but the error is not large. Taking the calculated temperature values asthe abscissa, the contents of the every element in different particles as the ordinate, thus wemake the spot diagrams. The different particles may correspond to different temperaturevalues, but the temperature values of the same particle are unified, the obtained resultsare shown as Fig.5. The data obtained by other authors are also displayed in Fig.5 forcomparisons.

Fig. 5 The comparison between the theoretically calculated and actually measured compositions of noble

metal particles (for main elements)

From the obtained diagrams in combination with a composition analysis, we can findthat the data of the group-I noble metals have the following characteristics: (1) Com-pared with the fitting results of predecessors, these particles basically have the features ofcondensation genesis. (2) According to the temperature distribution of noble metals, thecondensation temperature is basically in a range of 1450∼1600K. (3) The Mo content exists

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a certain deficit. (4) The Ir content in a part of particles exists an obvious deficit, but inother part of particles it exhibits a significant enrichment.

The characteristics of the group-II noble metal data differ from those of the group-I noble metals: (1) The chemical compositions of a part of noble metal particles fit thetheoretical values quite well, exhibiting the features of condensation genesis. (2) In otherpart of noble metal particles, the deficit of Mo, the excess and deficit of Ir, as well as theenrichment of Ru, are very apparent. (3) According to the distribution of noble metalcontents on the theoretical curve, it is inferred that the formation temperature is in a rangeof 1420∼1580K.

In the mean while, by the analysis of mineral chemistry on the noble metal particles,we find that the element W has commonly a deficit; in a part of the group-I noble metalsand the neighboring FeNi metal, the content of Ni is higher than the content of Fe; in a partof the group-II noble metals the content of Ni is also higher than the content of Fe, but thedifference between the two becomes more significant, the Ni/Fe abundance ratio can be aslarge as 4:1, and this phenomenon is rather common.3.2.2 Whitlockites

As shown by Fig.6, the whitlockite has not exhibited a significant crystal form, butthe edge exhibits corner angles. The forming environments of whitlockites are quite special,one inference is that this mineral is produced by the hydrous alteration in the environmentof late-period low temperature[16−17]. This process might be happened in the solar nebulabefore or after the parent meteorite formed by accretion[9−10].

Fig. 6 A whitlockite adjacent to the noble metal particles (the dark grey area in the center of the light

grey region)

4. DISCUSSIONS

4.1 Genesis Analysis of Noble Metal ComponentsFrom the above experimental data we know that the formation temperature of the group-Inoble metals is about 1450∼1600K, and that they basically form from the condensationgenesis. However, different from the group-I noble metals, in a part of the group-II noblemetals, some elements exhibit a rather large deficit or excess, this rejects that they areproducts of direct condensation. The temperature range of particle formation is about

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1420∼1580K, because the group-II noble metals are not of condensation genesis, so theirformation temperatures are only the estimated values.4.1.1 The genesis of the group-I noble metals

In the group-I noble metals, we have found the metal particle containing the elementsW, Mo, Ir, Ru, Rh, Pt, Ni, Fe, Co, Pd, etc., but Os and Ru are hexagonal metals, Fe,Ni, and so on are face-centered cubic metals, and Mo is body-centered cubic metal, theseelements coexist in one and the same particle of condensation genesis. The coexistence ofthe metal elements with different structures indicates that after formation the particle hasnot experienced any violent melting event[7]. Fig.5 shows that the contents of Os and Ruare relatively consistent with the theoretical curves.

The highest condensation temperature of the group-I noble metals is consistent with theequilibrium condensation temperature of melilite, and the lowest condensation temperatureis consistent with the equilibrium condensation temperatures of olivine and pyroxene, hencewe believe that the group-I noble metals may be captured in the early stage of silicateformation, and wrapped up in silicates in the process of silicate crystallization[18].

In the group-I noble metals we have found that a very small part of particles exhibitthe deficit of Ir, however the Ir element is refractory and erosion resisting, the Ir-deficit canhappen only in the melted potassium cyanide and sodium cyanide, or under the action ofhalide or oxygen in the condition of high temperature[19−20]. Hence, in the condition ofsolar nebula, the deficit of Ir may imply that the particle has experienced the environmentof high-temperature oxidization. Similarly, the element Ru can be deficit in the environmentof high-temperature oxidization, it is found that for a part of particles the Ru content isslightly lower than the theoretical value, but not significant. The overall composition analysison these particles finds that in a small number of particles the contents of Ni and Fe arerelatively high, for example the rather high Ni-content of the alloy 4 as listed in Table 1, incombination with the relationship of the corresponding particle with its adjacent particles,it is found that the particle’s composition may be influenced by its adjacent FeNi metal, asshown by Fig.3(a). The rather high contents of Ni and Fe make the overall content of otherrefractory metal elements reduce. Exist also some particles with relatively high contents ofOs and Ru. The chemical properties of Ir, Ru, and Os are very similar, they often coexistin one and the same particle. But relatively, Os is more refractory, hence the enrichmentsof Os and Ru may result in a small deficit of Ir. But this case is rare, most particles exhibita small enrichment of Ir. It is noteworthy that the calculation of theoretical curves is basedon the assumption that the various elements can be ideally condensed in the form of alloys,so when the local reaction is incomplete, the excess of Ir may be resulted. Since the group-I noble metals are captured in the early formation of silicates, when they left the nebulaenvironment, a certain influence on the condensation of a part of refractory metal elementsmay be produced.

In noble metal particles the deficits of W and Mo are rather common. Both W and Moare relatively instable, and easy to be oxidized. Generally, in the rising process of oxygenfugacity, a noble metal particle will lose W at first, then Mo, then again Re, Os, Ru, Ir, andPt, both the loss of W and the deficit of Mo can represent the raise of oxygen fugacity[21−22].But because that Mo has not produced a large-scale and high-degree deficit, so the value ofoxygen fugacity has not raised to very large, and in the whole the environment remains tobe reductive.

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Comparing the research data of Murchison obtained by Berg et al.[7] with this paper(Fig.5), the compositional characteristics of Os, Ru, and Fe in the noble metal alloys aresimilar to each other, the Ir element exhibits equally an enrichment, the group-I noble metalsand the metals in Murchison all have the features of condensation genesis. However, thenoble metals in Murchison have not exhibited the deficit of Ir, and the contents of Mo and Whave not exhibited any significant deficits, based on the analysis on the chemical propertiesof deficient elements, it is possible that the oxygen fugacity in the forming environmentsof the group-I noble metals is higher than that of Murchison. At the same time, the noblemetals in Murchison are in lack of secondary alteration, its compositional information ismore simple, therefore the noble metals of other geneses have not been found.4.1.2 The genesis of the group-II noble metals

In the group-II noble metals, the deficit of Mo, the excess and deficit of Ir, and theenrichment of Ru are more significant, obviously deviating from the theoretical compositionof a particle of condensation genesis, hence we neglect the possibility that they are conden-sation products. These noble metals may be produced by the exsolution, oxidization, andsulfidation reactions happened in the primordial metal of a CAI under the late-stage low-temperature condition. After the late-stage alteration happened in the primordial metal,some elements will be further enriched or deficient.

Most of the observed opaque mineral aggregates exhibit a polygonal shape, hence itis inferred that they have not experienced liquid state, namely in the inclusion large-scalemelting has not happened, happened only some local exsolution events.

Since the group-II noble metals are affected by the late-period reactions, the analysison the contents of this group of noble metal elements becomes more difficult. The modesof element enrichment and deficit are more complicated, and deviate even large from thetheoretical values, this may be determined by the different reaction conditions in the processof alteration, and related with the element redistribution in the process of reactions[9,11]. Atthe same time, the composition of the primordial metal from which forms this kind of noblemetal influences also the composition of this metal. The degrees of deficit of W and Mo canstill represent the raise and decline of oxygen fugacity, because W and Mo are very easy tobe lost in the environment of rising oxygen fugacity, but Mo has not been lost completely,this implies that the oxygen fugacity has not raised too much.

According to the studies made by Palme et al.[10] and Fuchs et al.[23] on the noblemetals in the opaque mineral aggregates in the CAIs of the Allende meteorite, the deficit ofW exists as well, but the sulfuration of Mo is very common, a large quantity of MoS2 exists,and a large quantity of refractory elements are redistributed in the later alteration process,Pt and Rh enter into the Ni-Fe phase, Os and Ru enter into other phases. In the differentopaque mineral aggregates the distributions of refractory metal elements differ greatly, butthe group-II noble metals do not have this characteristic. It is possible that in the lateralteration process, the differences of temperature, oxygen fugacity, and sulfur fugacity leadto the happenings of the different reactions and the rebalancing process. At the same time,the difference of primordial mineral will also lead to the differences of element distributionand of mineral formation in different inclusions.

4.2 The Genesis and Environment Analysis of Other MineralsThe formations of sulfides and oxides can reflect the rather high oxygen fugacity and sul-

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fur fugacity, but they ought to be the products in different periods, namely the overallenvironment is not simple.

Lauretta et al.[24] suggested that in sulfides the contents of Ni and Fe are concernedwith the supply of sulfur in the sulfuration process. In the nebula environment, the supplyof sulfur is sufficient, the rate of sulfuration is dominated by the rate that metallic ions passthrough the sulfide layer by diffusion, because the diffusion of Ni in the sulfide is faster thanFe, a layer of Ni-rich sulfide forms very easy in the outside of a metal particle to preventthe enrichment of Ni in the metal, so in the metal alloy the content of Ni is relatively small;when the supply of sulfur is limited, the factor that dominates the reaction becomes thedegree of easiness of the combination of the Fe-S and Ni-S ionic links. If the Fe-S link ismore easy to generate than the Ni-S link, then as the sulfuration reaction proceeds, sulfurprefers to link Fe first, thus Ni will be enriched in the alloy.

In a part of the group-I noble metals and their adjacent FeNi metal, the content of Ni isalways slightly higher than the content of Fe. This phenomenon widely exists in the group-IInoble metals, and the content of Ni is further increased. The group-I noble metal particle ofhigh Ni-content may be wrapped up and constrained in the inclusion by the melilite whenthe particle forms, and it further deviates from the nebula environment as the melilite iscrystalized. In the enclosed environment the supply of sulfur changes from infinite to limited,hence in a part of particles and their adjacent FeNi metal the content of Ni is relativelyincreased. However, most group-II noble metals form in the environment of deficient sulfursupply. Since in meteorites exist troilite, FeNi sulfurite, and other sulphur minerals, itindicates that meteorites did not form in the environments of deficient sulfur supply. Thusit is further verified that between the group-II noble metals and parent meteorites exist onlya weak correlation in environments, and that when the group-II noble metal forms, it isconstrained in the inclusion, and isolated from the outside environment, therefore in lack ofsulfur supply.

The ilmenite forms in a rather low temperature, so it is the product in the late periodof the meteorite formation, in the mean while the oxygen fugacity is relatively raised, butbecause that this mineral is not rich, and that as the product under the reductive conditionthe troilite exists, hence the oxygen fugacity is still on the low side.

5. CONCLUSIONS

Most group-I noble metals are characterized by the condensation genesis, they form directlyby condensation, the formation environment tends to be reductive, but still there are partof them experienced the later alteration. Probably in the late period the meteorite hasexperienced a part of melt events, or in the early condensation of the nebula exist high-temperature melt events.

In the group-II noble metals, the deficit of Mo, the excess and deficit of Ir, as well asthe enrichment of Ru appear to be more significant, but not irregular in comparison withthe theoretical curves. Probably they are produced by the exsolution, oxidization, and sul-fidation reactions happened in the primordial metal of a CAI under the low-temperaturecondition, and in the process of formation the different elements happened further enrich-ment or deficit, the processes of these reactions are quite complicated.

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In summary, the parent meteorite of NWA 2140 may form in a rather reductive nebulaenvironment, during the crystallization of the main silicate minerals, the group-I noblemetals are wrapped up in the silicates, later, the oxygen fugacity raises gradually, the degreeof reduction decreases, making the deficit of W appear in the group-I noble metals. Then,the oxygen fugacity raises continuously, but still in a not very high level, the whole bodyis still situated in a relatively reductive environment. In this process, a part of primordialmetal happen the exsolution, oxidization, and sulfidation reactions to form some group-II noble metals. In this period, the group-I noble metals may be affected to have weakalterations. In the late stage, as the temperature declines, the meteorite experiences thelate-period alteration under the low-temperature environment to form whitlockites, but theextent of alteration is not high.

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