Experiments on Condensation of Calcium Sulfide Grains to Demarcate

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Experiments on Condensation of Calcium Sulfide Grains to Demarcate Environments for the Formation of Enstatite Chondrites Kaori Yokoyama, Yuki Kimura, and Chihiro Kaito ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00076 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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ACS Earth and Space Chemistry

Experiments on Condensation of Calcium Sulfide Grains to Demarcate Environments for the Formation of Enstatite Chondrites

Kaori Yokoyama†, Yuki Kimura‡,*, Chihiro Kaito†

†

Laboratory for Nano-Structure Science, Department of Physics, Ritsumeikan

University, 1-1-1 Nojihigashi, Kusatsu-shi, Shiga-ken 525-8577, Japan ‡

Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan

*Correspondence to: [email protected]

ABSTRACT To achieve a better understanding of material evolution in the early solar system, experiments have been performed to constrain the environments in which many of the dust grains formed.

Sulfur is an element whose chemical processes and mineralization

of related grains are poorly understood.

The high reactivity of sulfur makes it difficult

to perform experiments in conventional metallic chambers, as these become heavily contaminated.

Nevertheless, sulfur is expected to be a key element to understand

processes in the early solar system.

Here, we performed experiments on the

condensation of calcium sulfide (CaS) in a glass chamber in an attempt to identify constraints on the possible formation environments of components of enstatite chondrites in terms of the effects of oxygen.

Condensation experiments showed that

calcium sulfate (CaSO4) or solid-solution particles of CaS and calcium oxide (CaO), i.e., [Ca(S,O)], were formed at various partial pressures of oxygen.

Our results expand the

range of possible conditions for the condensation of meteoritic CaS (oldhamite) from a

1 ACS Paragon Plus Environment


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nebula gas and extend the range of environments for the formation of the parent bodies of enstatite chondrites to include those more-oxidizing environments in the solar nebula where the atomic ratio of oxygen to sulfur was less than 6 and where CaS could have incorporated oxygen to form Ca(S,O) without formation of CaSO4.

Keywords: calcium sulfide; enstatite chondrites; nucleation; oldhamite; solar nebula; condensation

1. INTRODUCTION The environment in which particles condense from the vapor phase and grow strongly affects their subsequent geometry, crystal structure, composition, and crystal defects.1 Therefore, grains found distributed in primitive meteorites provide a great deal of information that can assist understanding the environment present in the solar nebula before the formation of the asteroids.

Several models have been proposed for the

production of the parent bodies of enstatite chondrites; for example, Herndon and Suess2 and Sears3 suggested that the minerals in enstatite chondrites were formed by condensation at high pressures (>105 Pa).

Alternatively, Larimer and Bartholomay4

and Lodders and Fegley5 suggested that the minerals present in enstatite chondrites (see Supplementary Information, Table S1) were formed in a reducing environment with a high carbon–oxygen (C/O) ratio (>1), because it is difficult for such minerals to form in an O-rich atmosphere, and they would not have been stable in a gas of solar system composition.

Iron in enstatite chondrites is present as an iron–nickel alloy or as iron

sulfides, rather than as iron oxides.

In addition, the pyroxene in enstatite chondrites is

almost entirely in the form of enstatite (MgSiO3).

The observation of low or zero


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amounts of iron oxides supports the view that enstatite chondrites were formed in a reducing environment.

Unequilibrated enstatite chondrites (UEC) composed of EH3,

EL3 and one anomalous member (LEW 87223) show a peculiar mineralogy, which has been characterized by their unusual mineral compositions such as more sulfides than in any other chondrite group, Si-rich Fe-Ni beads and osbornite (TiN).6-8

Metal-sulfide

nodules have been found in the matrix along with pyroxene-rich porphyritic chondrules and other peculiar minerals.9

It has been considered that these peculiar minerals

condensed directly from the solar nebula gas because of their relatively higher concentration of rare-earth-elements (REE).5,10 Sulfides are useful minerals for elucidating the nature of the environment that was present during the formation of solar grains and the chondritic parent bodies in the primitive solar nebula, because sulfides are sensitive to oxidation and hydration during both their condensation and weathering stages.

Magnesium, manganese, and calcium

sulfides are present in enstatite chondrites, which were formed in a reducing environment; in contrast, these sulfides are absent from carbonaceous chondrites and ordinary chondrites.11,12

Among the sulfides present in enstatite chondrites, CaS is the

most easily oxidized, and will react even at oxygen partial pressures of 10–10 to 10–5 Pa at 800–1000°C.13

The weathering processes of CaS are caused by reactions with

oxygen to form CaSO4.14

In addition. CaS can also react with water to form calcium

hydroxide [Ca(OH)2] and CaSO4 hydrates.15

For CaS particles to have formed from

the nebula gas and be incorporated into the enstatite chondrites without displaying weathering changes in their oxidation status, a reducing environment with a high C/O ratio must have existed. Several possible origins of CaS have been proposed.


These include the


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formation of CaS by vapor–solid reactions in the nebular gas from precursors that include calcium,2,13 and the formation of CaS by solid-state reactions between calcium-containing dust and sulfur-containing dust particles.15

CaS is also present in

chondrules in enstatite chondrites and it has been suggested that these might have formed by crystallization from sulfur dissolved Fe-poor chondrule melts.17-23 Aubrites, which are enstatite achondrites, are breccias and have similarities in mineralogy to enstatite chondrites6,24 suggesting formation in highly reducing environments. CaS has also been found in aubrites and is a major REE carrier.

These results suggest that at

least a part of CaS grains originated as nebular condensates, though the varieties of REE patterns suggest a complex process such as partial melting, fractional crystallization and subsolidus annealing.25-26

According to the equilibrium calculations of Lodders and

Fegley,5 CaS does not condense at the C/O ratio present in the solar gas (C/O = 0.42) at a total pressure of 102 Pa.

In contrast, when C/O = 1.2 at a total pressure of 102 Pa,

CaS condenses at a temperature of 1379 K.

In addition to the C/O ratio, the fugacities

of oxygen and sulfur are important parameters pertaining to the environment for the formation of CaS.16

In the solar nebula, dust particles might have experienced many

processes, including evaporation, condensation, and melting by the radiant heat from the Protosun and by shock-wave heating.27–34 In contrast with reaction experiments performed under equilibrium conditions, little was known concerning the effects of sulfur and oxygen on the nucleation of grains in nonequilibrium gas atmospheres.

Laboratory smoke experiments are capable of

duplicating processes of condensation from the vapor phase in natural environments.35,36 In this study, we attempted to identify conditions for the formation of CaS particles in terms of the atomic ratio of sulfur and oxygen in the vapor phase.


As a result, we

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identified limits on the ratio of sulfur and oxygen partial pressures in environments where solar CaS particles can form.

The formation process of CaS in enstatite

chondrites provides some clues regarding the sulfur and oxygen ratio in the environment where the parent bodies of the chondrites were formed.

2. EXPERIMENTAL PROCEDURES CaS grain analogues were produced by the gas-evaporation method.

Calcium (99.5%

purity; Dowa Mining Co., Ltd., Tokyo) and sulfur (99.9% purity; Newmet Koch Co., Waltham Abbey, UK) were simultaneously evaporated into a He buffer gas.

The

buffer gas decreases the mean free paths of atoms and increases the chances of collisions between atoms and/or embryonic particles before these atoms collide with and stick to the chamber wall.

As a result, the experimental system and reaction time

can be scaled down to permit conditions for natural formation of the condensates to be duplicated in laboratory studies.37 The evaporation sources of sulfur and calcium were arranged in parallel, and a glass cone with a hole at one end was used to mix the evaporated sulfur and calcium. The resulting particles were collected on a glass plate after passing through a hole in the end of the cone (Figure 1).

The evaporation sources were set in a glass chamber with

an inner diameter of 170 mm and a height of 330 mm, covered with a stainless-steel plate with electrodes on its top, connected to a high-vacuum exhaust through a valve at its bottom. The chamber was evacuated to 2 × 10–5 Torr (2.6 × 10–3 Pa) by using a combination of a mechanical pump and a diffusion pump. He and O2 gases were introduced into the chamber.

After the valve was closed,

Six partial pressure ratios of He

and O2 gases were used (He:O2 = 80:0, 79:1, 77:3, 76:4, 75:5, 70:10) at a total pressure



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of 80 Torr (1.1 × 104 Pa).

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Sulfur powder (75 mg), placed on the upper V-shaped Ta

boat, was evaporated in the mixed gas by gradual heating through thermal convection generated by heating a second Ta boat set beneath the sulfur-evaporation source (Figure 1).

This indirect heating technique is useful in controlling the evaporation of materials

such as sulfur that have high vapor pressures.38 The temperature of the upper boat as a sulfur-evaporation source was controlled at ~560 K to maintain a sulfur partial pressure of ~1 Torr (1.3 × 102 Pa). K.

The temperature of the lower Ta boat at that time was ~1273

The evaporated vapor subsequently cooled and condensed to form smoke particles

in the gaseous atmosphere; i.e., solid grains were obtained directly from the gas cloud. As soon as a sulfur smoke was observed in the chamber, a calcium slug was evaporated by resistance heating in a Ta conical basket. Convection was driven by the heater below the sulfur source and therefore mixed the He/O2 background gas into the slowly diffusing sulfur vapor.

For this reason, the O to S ratio did not increase significantly.

The sulfur vapor and particles diffused to the calcium evaporation source.

There, the

particles were revaporized by the higher temperature of the calcium evaporation source (1273 K).

As a result, grain analogues were produced by homogeneous nucleation

from the mixture of oxygen with calcium and sulfur vapors. The resulting particles were collected on a glass plate set above the glass cone. Radiation from the hot evaporation sources and thermal convection can elevate the temperature of a sample collector.

To avoid increasing the temperature of collected

particles, heating of the evaporation source was terminated within ten seconds.

The

location of the sample collector, behind a glass cone, is also helpful to decrease radiation from the evaporation source. collection of the sample are negligible.

As a result, temperature increases after The samples were then dispersed on a thin


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film of amorphous carbon supported on a standard copper transmission electron microscope (TEM) grid, and subsequently examined by using a Hitachi H-7100R TEM operated at an accelerating voltage of 100 keV, and equipped with an energy-dispersive X-ray (EDX) analyzer.

3. PRODUCTION OF CALCIUM OXIDE AND CALCIUM SULFIDE PARTICLES Figure 2(a) shows a TEM image and the corresponding electron-diffraction (ED) pattern of particles produced by the evaporation of calcium in the absence of sulfur in a mixture of He [70 Torr (9.3 × 103 Pa)] and O2 [10 Torr (1.3 × 103 Pa)]. The particles were 20–30 nm in diameter and were connected to each other by a neck. They consisted of CaO (cubic, a = 4.81059) fine crystals, as shown by the broad ED rings and the strong contrast in the bright-field image. Figure 2(b) shows a TEM image and the corresponding ED pattern of particles produced from calcium and sulfur gases in an atmosphere of He (80 Torr).

The TEM

image shows that the particles had a cubic shape, characterized by a NaCl-type structure, and their size was approximately 50 nm.

The ED pattern [Figure 2(b)] showed that

CaS (cubic, a = 0.56978 nm) particles were present. These results confirmed that CaS did not oxidize during transportation from the chamber to the TEM. Each CaS particle was a single crystal with a cubic shape, whereas CaO particles are polycrystalline with indeterminate shapes, although both crystal structures are of the NaCl type. The reason for this difference in morphology between CaS and CaO might lie in a difference in surface tension (σ), which strongly (σ3) affects the nucleation rate during the condensation processes of CaO and CaS. The surface tension of CaO [1.032



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N m–1 for {100}] is much larger than that of CaS [0.356 N m–1 for {100}].39 Our recent studies have shown that nucleated particles initially are in a liquid phase, due to the size effects of nanoparticles.40,41 As the nucleated particle grows, the degree of supercooling becomes larger as a result of the elevation of the melting point. Crystals then form from the supercooled liquid by secondary nucleation. Because CaO particles have a larger surface tension, CaO nucleation requires a larger supersaturation than does CaS. CaO particles nucleate and grow under highly nonequilibrium conditions.

Subsequent crystallization then occurs from a highly cooled liquid and,

consequently, the growth rate of CaO particles is reduced by the higher viscosity; furthermore, several crystalline nuclei can form simultaneously due to the greater rate of nucleation at such high levels of supersaturation.

In the case of CaS, which has a

smaller surface tension, nucleation of liquid particles occurs at a relatively low supersaturation and, consequently, CaS particles can grow as single crystals. Another possible reason for the difference in morphology is the plane dependence of the surface tension in each crystal. The surface tension of the {100} plane of CaS is very low compared with that of the other planes and, consequently, the appearance of other planes might be strongly suppressed. As an example, the ratio of the surface tensions of the {100} and {110} planes is 2.76 in the case of CaO and 4.02 in the case of CaS.

4. THE EFFECTS OF OXYGEN ON CALCIUM SULFIDE FORMATION 4.1. Lower Oxygen Partial Pressures: Formation of Solid-Solution Crystalline


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Particles. In the case of particles formed in an atmosphere of 79 Torr (1.1 × 104 Pa) of He and 1 Torr (1.3 × 102 Pa) of O2, particles with a cubic-based shape were formed [Figure 3(a)] in addition to CaS and CaO particles.

The corresponding ED pattern [Figure 3(a)]

showed the presence of a {111} ring with a significantly lower intensity than that observed for pure CaS.

Figure 3(c) shows the ED patterns of particles produced in

pure He at 80 Torr (1.1 × 104 Pa) (left) and particles formed in a mixture of He (79 Torr, 1.1 × 104 Pa) and O2 (1 Torr, 1.3 × 102 Pa) (right). It is quite obvious that the diffraction ring attributed to {111} seen in the left-hand ED pattern is almost absent in the ED pattern of the oxygen-containing samples, suggesting the formation of solid-solution crystalline particles, as well as a solid solution of ionic crystals.42

The shape of the

particles can also be explained in terms of the formation of a solid solution.

The

particles are surrounded by {100} faces and truncated at the {110} and {111} faces, as shown in the magnified image [Figure 3(b)].

In the cases of particles formed in mixed

atmospheres of 77 Torr (1.0 × 104 Pa) of He and 3 Torr (4.0 × 102 Pa) of O2 or of 76 Torr (1.0 × 104 Pa) of He and 4 Torr (5.3 × 102 Pa) of O2, solid-solution phases were similarly observed at somewhat larger abundances compared with that in the 79:1 He–O mixture (see Supplementary Information; Figure S1).

4.2. Higher Oxygen Partial Pressures: Formation of CaSO4 Particles. Figure 4 shows TEM images and the corresponding ED patterns for particles produced in a gas atmosphere with an oxygen partial pressure of 5 or 10 Torr (6.7 × 102 or 1.3 × 103 Pa, respectively).

In the case of the formation of particles in 75 Torr

(1.0 × 104 Pa) of He and 5 Torr (6.7 × 102 Pa) of O2, as shown in Figure 4(a), almost all



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of the particles consisted of CaSO4, except for a small number of solid-solution crystals of Ca(S,O).

Because of the coexistence of the solid-solution crystals, the particles

produced under an oxygen partial pressure of 5 Torr (6.7 × 102 Pa) had a variety of external shapes. Figure 4(b) shows particles produced at a He partial pressure of 70 Torr (9.3 × 103 Pa) and an O2 partial pressure of 10 Torr (1.3 × 103 Pa); all of the particles consisted of CaSO4 and had oblong shapes.

Since the particles had a crystal habit and

the ED pattern consisted of intense spots, we concluded that the CaSO4 particles were formed from a vapor-phase mixture of calcium, sulfur, and oxygen, rather than by a solid–solid phase transition from an oxygen-dominant part of the solid-solution phase. The atomic ratios of calcium and sulfur in the particles, as measured by the EDX analyzer attached to the TEM, are summarized in Table 1 and plotted in Figure 5.

In

the case of particles produced in gas mixtures with oxygen partial pressures of less than 3 Torr (4.0 × 102 Pa), the sulfur/calcium atomic ratio in the particles was larger than unity.

In contrast, the sulfur/calcium atomic ratio became less than unity at higher

oxygen partial pressures, because oxygen atoms were incorporated into the CaS crystals instead of sulfur.

This result is coincident with the formation of solid-solution crystals

of CaS and CaO.

Figure 5 shows a plot of the oxygen partial pressure versus the

atomic ratio of calcium to sulfur in the produced particles.

It is obvious that the

calcium-to-sulfur ratio in the particles increases with increasing oxygen partial pressure. These results show that a low oxygen partial pressure is essential for the production of calcium sulfides. It might, therefore, be possible to estimate the ratio of the partial pressures of oxygen and sulfur in the formation environment for calcium sulfide in the solar nebula from the atomic ratio in the calcium sulfide found in enstatite chondrites.


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From the EDX measurements summarized in Table 1, it is apparent that the abundance of sulfur in pure CaS with a cubic shape produced in an oxygen-free atmosphere is greater than that of calcium. All of the cubic CaS particles showed this tendency for sulfur to be present in an atomic ratio greater than the stoichiometric ratio for CaS (Ca/S = 1).

It is unlikely that sulfide ions intrude into the lattice as interstitial

ions, because the radius of the sulfide ion (0.184 nm) is considerably larger than that of the Ca2+ ion (0.099 nm).

It is therefore more likely that the particles contain Ca2+ ion

vacancies in their lattices.

As a result, there is a deficiency in calcium compared with

sulfur. Another possibility might be that the extra sulfur adheres to the surfaces of the particles because the amount of sulfur evaporated was larger than the amount of calcium present, and the condensation temperature of sulfur is lower.

If sulfur adheres only to

the topmost layer of calcium in a CaS particle with a cubic shape and a size of 50 nm, the abundance of sulfur atoms in the particle will be 17% greater than that of calcium atoms. The atomic ratio of calcium to sulfur then becomes 43:57 a figure that is close to the result obtained by EDX.

5. COMPARISON WITH METEORITIC MINERALS The CaS particles in matrices of enstatite chondrites occur in three different forms, which are isolated along with other sulfides in metal-sulfide spherules, and in chondrules.7,43

However, there are no reports that CaS was formed along with CaSO4

nor that CaSO4 is present in enstatite chondrites that have not been subjected to weathering on the Earth.

Therefore, the oxygen/sulfur ratio in the environment where

CaS was formed without the production of CaSO4 can be estimated from the results of our experiment.

If we assume that CaS in enstatite chondrites is produced by



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condensation in the solar nebula, the present experiments suggest that the CaS present in the chondrites was produced in an environment where the atomic ratio of reactive oxygen to sulfur was 6 or less.

The typical pressure in the solar nebula was much

lower than that in the present experiment; however, higher-pressure environments (~102 Pa) could have been produced locally by vaporization of dust particles due to stochastic heating, for example by shock-wave heating.2,33,44

Actually, Sears suggested

that the minerals in enstatite chondrites were formed by condensation at a high pressure (>105 Pa),3 in which case our experimental results might be directly applicable. Table 1 shows a reverse in the abundance of calcium and sulfur atoms between 3 and 5 Torr of oxygen partial pressure.

It can be estimated that if the atomic ratio of

calcium to sulfur in enstatite chondrites and achondrites is 1:1, the CaS grains possibly consist of solid solutions of crystals of CaS and CaO.

The CaSO4 in carbonaceous

chondrite found in a matrix of CO3 chondrites (ALH 77307)45 might have condensed directly from the nebula in addition to the possibility of formation by the weathering of CaS.

6. ENVIRONMENTS FOR THE FORMATION OF CALCIUM SULFIDE AND THE PARENT BODIES OF ENSTATITE CHONDRITES IN THE SOLAR NEBULA Below we discuss the environment in which the parent bodies of enstatite chondrites may have formed, based on our experimental results.

The sources of calcium gas for

the formation of CaS are refractory minerals in the solar nebula. Refractory materials containing calcium are evaporated in the hottest region (>2000 K) near the Protosun. However, it is unlikely that CaS would have condensed in this region because it is a


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relatively oxygen-rich environment (C/O < 1), due to the evaporation of oxides, such as calcium–aluminum-rich inclusions or silicates. We therefore suggest that CaS was formed at about 1–3 AU from the central star, where the environment contained a relatively higher C/O ratio as a result of the evaporation of organic compounds.46

This

would have been the environment with the highest C/O gas ratio in the nebula, sandwiched between environments with low C/O ratios, resulting from evaporation of water ice in the outer part and the evaporation of oxides in the inner part.

Calcium gas

was produced by the evaporation of calcium-containing dust at the inside of the disk and blown off by the X-wind from the central star to a distance of several AU.

It was

exposed to x-rays and UV radiation from the central star, without interception by gas or dust, and was heated to more than 2000 K.47

Although a part of calcium forms

refractory minerals such as calcium-aluminum-rich inclusions, some amount of calcium gas may be able to remain with the help of irradiation from the central star.

On the

other hand, H2S gas, which was the main source of sulfur, could have been dissociated to H and SH by near-UV photons (λ ≤ 270 nm) at the surface of the nebula. These species then formed H2 and S by a second dissociation. The Ca then reacted with S to form CaS. Another possible process for the formation of CaS is condensation following chondrule formation after shock-wave heating.29–34

In this case, calcium- and

sulfur-containing minerals might have been vaporized simultaneously within a very short time after flash heating, and a relatively high-pressure environment might have been produced.33,44

As a result, CaS particles could have formed by direct

condensation from calcium and sulfur gases in the 1–3 AU region with a higher C/O ratio. In terms of the ratio of oxygen to sulfur, our experimental results suggest that the



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site of formation of CaS and solid-solution crystals of Ca(S,O) was about 1–3 AU from the central star in an environment with an O/S ratio of ≤6 during the condensation of sulfur and calcium.

If O/S ≥ 10, the region would not have been suitable for the

formation of CaS, because CaS would have been produced together with CaSO4 in such an environment. We therefore propose that the components in the parent bodies of enstatite chondrites were mainly formed in a specifically reduced environment with an O/S ratio of ≤6, which might be present for a limited time and in specific locations at about 1–3 AU from the central star.

In particular, the surface of the nebular disk would

be the most plausible site for formation of CaS.

7. CONCLUSION The experiments described above were performed to identify new constraints on the possible environment for the formation of components of enstatite chondrites by examining the effects of oxygen gas during the vapor-phase condensation of CaS, a mineral present in enstatite chondrites. A series of condensation experiments in various gas mixtures suggests that the ratio of the partial pressures of oxygen and sulfur affects the formation of CaS particles. An O/S atomic ratio of

Experiments on Condensation of Calcium Sulfide Grains to Demarcate

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