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1.
《Planetary and Space Science》1999,47(6-7):781-785
Amorphous silicate dust grains have been produced in the laboratory by means of laser ablation of solid targets in different ambient atmospheres. In this work we show that, if the condensation occurs in the presence of hydrogen, the spectra of silicate grains, together with the characteristic 10 and 20 μm features, exhibit an absorption band around 4.6 μm. Such features, absent in the spectra of the same silicate grains produced in an oxygen atmosphere, may be attributed to a fundamental stretching vibration of –SiH functional groups bound into the grains or on their surface.Based on the cosmic abundance of the elements, silicates are expected to condense in the atmospheres of oxygen-rich stars where hydrogen is also abundant. This means that –SiH functional groups may be present also in the circumstellar and interstellar silicate dust grains. An absorption feature at 4.6 μm has already been observed in the absorbing dust of several protostellar embedded sources. The observation of a similar feature in comets can give important information on the origin and evolution of cometary material. © 1999 Elsevier Science Ltd. All rights reserved.  相似文献   

2.
We report the results of 3-μm spectroscopy towards the nucleus of a nearby Seyfert 2 galaxy, NGC 5506. A weak absorption feature of carbonaceous dust is detected at ∼3.4 μm. The optical depth ratio of the 3.4-μm carbonaceous dust absorption to the 9.7-μm silicate dust absorption is smaller by more than a factor of 2 than that in the interstellar medium in our Galaxy. The small ratio is consistent with the idea that the obscuration towards the nucleus of NGC 5506 is caused mainly by dust grains in its host galaxy (type S0/a) and that the contribution of carbonaceous dust grains to the visual extinction there is smaller than that in our Galaxy (type Sb/bc).  相似文献   

3.
H2O, CO and CO2 ices are condensed on carbonaceous and silicate dust grains in dense interstellar clouds and circumstellar environments. The presence of these ices is inferred by analysing their infrared (IR) spectra. The upcoming Herschel space observatory (HERSCHEL) and ground-based astronomy project (ALMA) will provide new spectral data in the unexplored far infrared (FIR) and sub-millimetre range. In our laboratory we are developing instrumentation to study ices at IR region. One of the key components of our laboratory is a silicon composite bolometer in our IFS. This detector allows us to obtain spectra with a sensitivity much greater than that obtained with a standard deuterated triglycine sulphate (DTGS) detector working at room temperature and under vacuum conditions. We plan to collect mid infrared (MIR) and FIR spectra of simple ices and their mixtures and compare these with observational data. It is also planned to do a systematic laboratory study of the effects that ultraviolet (UV) photolysis and thermal annealing have on the ice band profiles and their structure.  相似文献   

4.
For submicrometer-sized particles of the carbonaceous chondrite Nogoya and, for comparison purposes, also of terrestrial chamosite, the mass absorption coefficients have been derived from laboratory spectra for the wavenumber range 250–1300 cm–1. Using these data the expected spectrum of an infrared source has been calculated under simple model assumptions and compared with the observed flux from the infrared source OH 26.5+0.6. Although certain discrepancies are left in the 20-m region it is concluded that phyllosilicates, especially such occurring in carbonaceous chondrites, are interesting candidates for the material of interstellar grains.  相似文献   

5.
Profiles of the 4430 band are calculated for resonant absorbers distributed within graphite particles, silicate particles and solid H2 grains. The sizes of grains adopted are those which give agreement with the interstellar extinction. Only in the latter two cases can satisfactory agreement be obtained with recent observational data.  相似文献   

6.
We present narrowband spectrophotometry between 2.3 and 3.5 μm for 14 main-belt C asteroids greater than 100 km in diameter. Absorption features at 3 μm due to water of hydration are present in the spectra of 9 of the asteroids, with intensities ranging from 6 to 23%. The other 5 asteroids have no 3-μm absorption greater than 2% in intensity. The strength of the “water” feature in the spectra correlates positively with the strength of the UV absorption feature shortward of 0.4 μm, and negatively with the slope of the continuum between 1.2 and 2.2 μm. These correlations are the same as those seen in laboratory spectra of carbonaceous chondrites, whose silicate compositions range from hydrated phyllosilicates to anhydrous olivine. We find no correlation between composition and semimajor axis for C asteroids as a class. The present C-asteroid population may be fragments of larger parent bodies with anhydrous C3-like cores and hydrated C11- or C2M-like mantles.  相似文献   

7.
Interstellar extinction curves obtained from the ‘extinction without standard’ method were used to constrain the dust characteristics in the mean ISM (R V = 3.1), along the lines of sight through a high latitude diffuse molecular cloud towards HD 210121 (R V = 2.1) and in a dense interstellar environment towards the cluster NGC 1977 (R V = 6.42). We have used three-component dust models comprising silicate, graphite and very small carbonaceous grains (polycyclic aromatic hydrocarbons) following the grain size distributions introduced by Li & Draine in 2001. It is shown that oxygen, carbon and silicon abundances derived from our models are closer with the available elemental abundances for the dust grains in the ISM if F & G type stars atmospheric abundances are taken for the ISM than the solar. The importance of very small grains in modelling the variation of interstellar extinction curves has been investigated. Grain size distributions and elemental abundances locked up in dust are studied and compared at different interstellar environments using these three extinction curves. We present the albedo and the scattering asymmetry parameter evaluated from optical to extreme-UV wavelengths for the proposed dust models.  相似文献   

8.
Comets and the chondritic porous interplanetary dust particles (CP IDPs) that they shed in their comae are reservoirs of primitive solar nebula materials. The high porosity and fragility of cometary grains and CP IDPs, and anomalously high deuterium contents of highly fragile, pyroxene-rich Cluster IDPs imply these aggregate particles contain significant abundances of grains from the interstellar medium (ISM). IR spectra of comets (3–40 μm) reveal the presence of a warm (near-IR) featureless emission modeled by amorphous carbon grains. Broad andnarrow resonances near 10 and 20 microns are modeled by warm chondritic (50% Feand 50% Mg) amorphous silicates and cooler Mg-rich crystalline silicate minerals, respectively. Cometary amorphous silicates resonances are well matched by IRspectra of CP IDPs dominated by GEMS (0.1 μm silicate spherules) that are thought to be the interstellar Fe-bearing amorphous silicates produced in AGB stars. Acid-etched ultramicrotomed CP IDP samples, however, show that both the carbon phase (amorphous and aliphatic) and the Mg-rich amorphous silicate phase in GEMS are not optically absorbing. Rather, it is Fe and FeS nanoparticles embedded in the GEMS that makes the CP IDPs dark. Therefore, CP IDPs suggest significant processing has occurred in the ISM. ISM processing probably includes in He+ ion bombardment in supernovae shocks. Laboratory experiments show He+ ion bombardment amorphizes crystalline silicates, increases porosity, and reduces Fe into nanoparticles. Cometary crystalline silicate resonances are well matched by IR spectra of laboratory submicron Mg-rich olivine crystals and pyroxene crystals. Discovery of a Mg-pure olivine crystal in a Cluster IDP with isotopically anomalous oxygen indicates that a small fraction of crystalline silicates may have survived their journey from AGB stars through the ISM to the early solar nebula. The ISM does not have enough crystalline silicates (<5%), however, to account for the deduced abundance of crystalline silicates in comet dust. An insufficient source of ISMMg-rich crystals leads to the inference that most Mg-rich crystals in comets are primitive grains processed in the early solar nebula prior to their incorporation into comets. Mg-rich crystals may condense in the hot (～1450 K), inner zones of the early solar nebula and then travel large radial distances out to the comet-forming zone. On the other hand, Mg-rich silicate crystals may be ISM amorphous silicates annealed at ～1000 K and radially distributed out to the comet-forming zone or annealed in nebular shocks at ～5-10 AU. Determining the relative abundance of amorphous and crystalline silicatesin comets probes the relative contributions of ISM grains and primitive grains to small, icy bodies in the solar system. The life cycle of dust from its stardust origins through the ISM to its incorporation into comets is discussed.  相似文献   

9.
Abstract— The CH carbonaceous chondrites contain a population of ferrous (Fe/(Fe + Mg) ? 0.1‐0.4) silicate spherules (chondrules), about 15–30 μm in apparent diameter, composed of cryptocrystalline olivinepyroxene normative material, ±SiO2‐rich glass, and rounded‐to‐euhedral Fe, Ni metal grains. The silicate portions of the spherules are highly depleted in refractory lithophile elements (CaO, Al2O3, and TiO2 <0.04 wt%) and enriched in FeO, MnO, Cr2O3, and Na2O relative to the dominant, volatile‐poor, magnesian chondrules from CH chondrites. The Fe/(Fe + Mg) ratio in the silicate portions of the spherules is positively correlated with Fe concentration in metal grains, which suggests that this correlation is not due to oxidation, reduction, or both of iron (FeOsil ? Femet) during melting of metal‐silicate solid precursors. Rather, we suggest that this is a condensation signature of the precursors formed under oxidizing conditions. Each metal grain is compositionally uniform, but there are significant intergrain compositional variations: about 8–18 wt% Ni, <0.09 wt% Cr, and a sub‐solar Co/Ni ratio. The precursor materials of these spherules were thus characterized by extreme elemental fractionations, which have not been observed in chondritic materials before. Particularly striking is the fractionation of Ni and Co in the rounded‐to‐euhedral metal grains, which has resulted in a Co/Ni ratio significantly below solar. The liquidus temperatures of the euhedral Fe, Ni metal grains are lower than those of the coexisting ferrous silicates, and we infer that the former crystallized in supercooled silicate melts. The metal grains are compositionally metastable; they are not decomposed into taenite and kamacite, which suggests fast postcrystallization cooling at temperatures below 970 K and lack of subsequent prolonged thermal metamorphism at temperatures above 400–500 K.  相似文献   

10.
Abstract— Scanning electron microscopy and secondary ion mass spectrometry of the unequilibrated CH chondrite Pecora Escarpment (PCA) 91467 revealed four carriers of isotopically heavy N: (1) aggregates of carbonaceous material and silicates, (2) iron‐nickel metal grains with fine Fe‐Cr sulfide inclusions, (3) Si‐rich Fe‐Ni metal associated with Fe‐sulfide and (4) hydrated areas in the matrix. N in carbon‐silicate aggregates is isotopically heavy (δ15N is as high as 2500%0), whereas the other elements are isotopically normal, suggesting interstellar origin of carbonaceous material in the aggregates. Based on isotopic and textural evidence, we suggest that the carriers (2) and (3) were formed by brief heating in the solar nebula, whereas the carrier (4) was formed in a parent‐body asteroid. The carbon‐silicate aggregates are likely to be related to interstellar graphite found in Murchison and may also be the source of heavy N in bencubbinites.  相似文献   

11.
Abstract— We present the results of irradiation experiments aimed at understanding the structural and chemical evolution of silicate grains in the interstellar medium. A series of He+ irradiation experiments have been performed on ultra‐thin olivine, (Mg,Fe)2SiO4, samples having a high surface/volume (S/V) ratio, comparable to the expected S/V ratio of interstellar dust. The energies and fluences of the helium ions used in this study have been chosen to simulate the irradiation of interstellar dust grains in supernovae shock waves. The samples were mainly studied using analytical transmission electron microscopy. Our results show that olivine is amorphized by low‐energy ion irradiation. Changes in composition are also observed. In particular, irradiation leads to a decrease of the atomic ratios O/Si and Mg/Si as determined by x‐ray photoelectron spectroscopy and by x‐ray energy dispersive spectroscopy. This chemical evolution is due to the differential sputtering of atoms near the surfaces. We also observe a reduction process resulting in the formation of metallic iron. The use of very thin samples emphasizes the role of surface/volume ratio and thus the importance of the particle size in the irradiation‐induced effects. These results allow us to account qualitatively for the observed properties of interstellar grains in different environments, that is, at different stages of their evolution: chemical and structural evolution in the interstellar medium, from olivine to pyroxene‐type and from crystalline to amorphous silicates, porosity of cometary grains as well as the formation of metallic inclusions in silicates.  相似文献   

12.
Interpretation of reflectance spectra indicates that most belt asteroids are composed of materials similar to carbonaceous chondrites. Also, there is considerable evidence to support the origin of many, if not most, lunar and meteoritic chondrules by impact processes. The accretional histories of the carbonaceous asteroids must have influenced greatly their internal structures and textures. A model for this accretional history can be divided conveniently into three temporal stages that produce distinctly different lithologies: (1) low-velocity accretion of fine silicate and carbonaceous grains producing chondrule-free petrologic type 1 lithology; (2) continued accretion of low-velocity fine silicate and carbonaceous grains, but with a few larger, higher-velocity bodies also impacting the surface thereby producing both fluid drop and lithic chondrules (the resultant lithology would be that of petrologic type 2 and 3 carbonaceous chondrites); and (3) dominance of high-velocity low-mass meteoroid impacts, producing a sparse, thin, erosive lunar-like regolith. The lithologic product of stage 3 is not ideally represented among the presently described carbonaceous chondrites, but texturally analogous samples are known from the achondrites. The greater proportion of chondrules in the CV group meteorites, in contrast to the CM2 and CO3 groups, may be due to the origin of the CV chondrites on larger asteroid parent bodies that could withstand more numerous and higher-energy chondrule-producing impacts prior to fragmentation.  相似文献   

13.
We investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains of fine‐grained chondrule rims in six Mighei‐type (CM) carbonaceous chondrites (Banten, Jbilet Winselwan, Maribo, Murchison, Murray and Yamato 791198), and the CM‐related carbonaceous chondrite Sutter's Mill. Sixteen O‐anomalous grains (nine silicates, six oxides) were detected, corresponding to a combined matrix‐normalized abundance of ~18 ppm, together with 21 presolar SiC grains (~42 ppm). Twelve of the O‐rich grains are enriched in 17O, and could originate from low‐mass asymptotic giant branch stars. One grain is enriched in 17O and significantly depleted in 18O, indicative of additional cool bottom processing or hot bottom burning in its stellar parent, and three grains are of likely core‐collapse supernova origin showing enhanced 18O/16O ratios relative to the solar system ratio. We find a presolar silicate/oxide ratio of 1.5, significantly lower than the ratios typically observed for chondritic meteorites. This may indicate a higher degree of aqueous alteration in the studied meteorites, or hint at a heterogeneous distribution of presolar silicates and oxides in the solar nebula. Nevertheless, the low O‐anomalous grain abundance is consistent with aqueous alteration occurring in the protosolar nebula and/or on the respective parent bodies. Six O‐rich presolar grains were studied by Auger Electron Spectroscopy, revealing two Fe‐rich silicates, one forsterite‐like Mg‐rich silicate, two Al‐oxides with spinel‐like compositions, and one Fe‐(Mg‐)oxide. Scanning electron and transmission electron microscopic investigation of a relatively large silicate grain (490 nm × 735 nm) revealed that it was crystalline åkermanite (Ca2Mg[Si2O7]) or a an åkermanite‐diopside (MgCaSi2O6) intergrowth.  相似文献   

14.
The observed high abundance of boron in type I carbonaceous chondrites may be due to the presence in the primitive solar nebula of graphite grains which have been irradiated by high energy nucleons at some stage of their history. The boron atoms thus produced by spallation reactions are stably locked within interstellar graphite grains and could make a significant contribution to the boron abundance of C1 chondrites.  相似文献   

15.
Abstract— We report the discovery of presolar silicate, oxide (hibonite), and (possibly) SiC grains in four Antarctic micrometeorites (AMMs). The oxygen isotopic compositions of the eighteen presolar silicate (and one oxide) grains found are similar those observed previously in primitive meteorites and interplanetary dust particles, and indicate origins in oxygen‐rich red giant or asymptotic giant branch stars, or in supernovae. Four grains with anomalous C isotopic compositions were also detected. 12C/13C as well as Si ratios are similar to those of mainstream SiC grains; the N isotopic composition of one grain is also consistent with a mainstream SiC classification. Presolar silicate grains were found in three of the seven AMMs studied, and are heterogeneously distributed within these micrometeorites. Fourteen of the 18 presolar silicate grains and 3 of the 4 C‐anomalous grains were found within one AMM, T98G8. Presolar silicate‐bearing micrometeorites contain crystalline silicates that give sharp X‐ray diffractions and do not contain magnesiowüstite, which forms mainly through the decomposition of phyllosilicates and carbonates. The occurrence of this mineral in AMMs without presolar silicates suggests that secondary parent body processes probably determine the presence or absence of presolar silicates in Antarctic micrometeorites.  相似文献   

16.
Abstract— We have investigated the mineralogy, petrography, bulk chemistry, and light element isotope composition of the ungrouped chondrites North West Africa (NWA) 1152 and Sahara 00182. NWA 1152 contains predominantly type 1 porphyritic olivine (PO) and porphyritic olivinepyroxene (POP) chondrules. Chondrule silicates are magnesium‐rich (Fo98.8 ± 1.2, n = 36; Fs2.3 ± 2.1 Wo1.2 ± 0.3, n = 23). Matrix comprises ?40 vol% of the sample and is composed of a micron sized silicate groundmass with larger silicate, sulfide, magnetite, and Fe‐Ni metal (Ni ?50 wt%) grains. Phyllosilicates were not observed in the matrix. Refractory inclusions are rare (0.3 vol%) and are spinel pyroxene aggregates or amoeboid olivine aggregates; melilite is absent from the refractory inclusions. Sahara 00182 contains predominantly type 1 PO chondrules, POP chondrules are less common. Most chondrules contain blebs of, and are often rimmed with, Fe‐Ni metal and sulfide. Chondrule phenocrysts are magnesium‐rich (Fo92.2 ± 0.6, n = 129; Fs4.4 ± 1.8 Wo1.3 ± 1.1, n = 16). Matrix comprises ?30 vol% of the meteorite and is predominantly sub‐micron silicates, with rare larger silicate gains. Matrix Fe‐Ni metal (mean Ni = 5.8 wt%) and sulfide grains are up to mm scale. No phyllosilicates were observed in the matrix. Refractory inclusions are rare (1.1 vol%) and melilite is absent. The oxygen isotope composition of NWA 1152 falls within the range of the CV chondrites with δ17O = ?3.43%0 δ18O = 0.70%0 and is similar to Sahara 00182, δ17O = ?3.89%0, δ18O = ?0.19%0 (Grossman and Zipfel 2001). Based on mineralogical and petrographic characteristics, we suggest NWA 1152 and Sahara 00182 show many similarities with the CR chondrites, however, oxygen isotopes suggest affinity with the CVs. Thus, neither sample can be assigned to any of the currently known carbonaceous chondrite groups based on traditionally recognized characteristics. Both samples demonstrate the complexity of inter‐ and intra‐group relationships of the carbonaceous chondrites. Whatever their classification, N WA 1152 and Sahara 00182 represent a source of relatively pristine solar system material.  相似文献   

17.
Past studies of the various separable carbonaceous fractions have been unable to account for all of C in primitive chondrites. In particular, up to 20–50% of the C is lost during acid leaching of bulk samples even after the C in carbonates and soluble organic matter is accounted for. To try to better characterize the nature of this “missing C,” we have compared the bulk infrared (IR) absorption spectra of a number of primitive chondrites with those of their previously reported insoluble organic matter (IOM). The aliphatic C–H stretching bands, in particular, allow us to compare the molecular structures of bulk C with that of IOM. The spectral differences between bulk C and IOM reflect “missing C” phases that were lost during acid leaching, although we cannot completely exclude the possibility that the OM was modified after demineralization. Comparing IR spectra of bulk meteorite powder and IOM suggests that the missing C varies in its molecular structure, and that mildly thermally metamorphosed type 3 chondrites tend to be richer in an aliphatic fraction with lower CH2/CH3 ratios, relative to IOM, compared to aqueously altered carbonaceous chondrites (CI/CM/CR). The missing C is most likely released from acid‐labile functional groups, such as esters, acetals, and amides, during demineralization, although it cannot be ruled out that some fraction of the missing C is in small grains that are difficult to recover from suspension, or in water‐soluble compounds trapped in phyllosilicates.  相似文献   

18.
A pulsed laser has been used to vaporize olivine, pyroxene, nickel-iron alloy, Al2O3, carbon, calcium carbonate, and silicon carbide, as well as mixtures of immiscible phases (Au–Al2O3 and Au-olivine) in oxidizing, reducing, and inert atmospheres. The collected condensates usually consist of strings of grains which have a median diameter of 20–30 nm, which is comparable to the calculated sizes of some interstellar and circumstellar dust grains. The silicate minerals vaporized in O2 as well as calcium carbonate and carbon vaporized in Ar or H2, are collected as glassy grains while the other materials produced crystalline grains. The systems of immiscible phases when vaporized produced condensates consisting of intermixed 2–50 nm grains of both components. The type of size distribution, crystal structures, and qualitiative elemental analyses of the condensates are given. Possible similarities between the mechanism of grain growth, structure, morphology, and chemistry of laboratory grains compared to interstellar and circumstellar grains, phases in meteorites and extraterrestrial dust collected in the stratosphere are examined. Applications of the experimental technique include the production of grain systems to serve as laboratory analogues for spectral studies of grain materials believed to exist in astronomical environments, and studies of the structure of grains condensed from complex gas mixtures.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.  相似文献   

19.
Frans J.M. Rietmeijer 《Icarus》2011,211(2):948-959
Chondrite aggregate interplanetary dust particle IDP L2011K7, collected in the Earth’s lower stratosphere, is an agglomerate of diopside, Mg,Fe-olivine, rare Fe-sulfide and abundant amorphous Mg,Fe-silicates. The overwhelming majority of amorphous silicates have a serpentine-dehydroxylate [(Mg,Fe)3Si2O7] composition; a few have a smectite-dehydroxylate [(Mg,Fe)6Si8O22] composition. The cation ratios of the amorphous silicates are notably identical to those of serpentine and smectite phyllosilicates. This paper follows the chronological changes in the amorphous silicates that include (1) formation of nanometer scale crystalline silicates (Mg,Fe-olivine and pyroxene), (2) partial hydration and formation of antigorite-serpentine proto-phyllosilicates, (3) partial dehydration of these proto-phyllosilicates, and finally oxidation and Fe-oxide formation by flash heating during atmospheric entry. Environmental conditions capable of driving these changes in the diffuse interstellar medium or solar nebula, in a comet nucleus, or in circumsolar orbit as a cometary meteoroid were marginal at best. These changes could only proceed because of the unique amorphous silicate compositions. While this study cannot make a firm statement about an interstellar or solar nebula origin for its amorphous silicates that are irradiation-induced olivine, this study does find that amorphous silicates with serpentine and (rare) smectite compositions are an important fraction of the amorphous silicates in comets in addition to amorphous olivine and pyroxene. It is noted that an ice and water-free, millimeter-scale, structurally coherent crumb would be an ample ‘microenvironment’ to evolve micrometer-scale dust. After all IDP L2011K7 only measures 22 × 17 μm.  相似文献   

20.
Abstract— We have studied an Allende dark inclusion by optical microscopy, scanning electron microscopy, electron microprobe analysis and transmission electron microscopy. The inclusion consists of chondrules, isolated olivines and matrix, which, as in the Allende host, is mainly composed of 5–20 μm long lath-shaped fayalitic grains with a narrow compositional range (Fa42 ± 2) and nepheline. Olivine phenocrysts in chondrules and isolated olivine grains show various degrees of replacement by 5–10 μm wide fayalitic rims (Fa39 ± 2) and 100–1000 μm wide translucent zones, which consist of 5–20 μm long lath-shaped fayalitic grains (Fa41 ± 1) intergrown with nepheline. These fayalitic olivines, like those in the matrix of the dark inclusion, contain 10–20 nm sized inclusions of chromite, hercynite, and Fe-Ni sulfides. The fayalitic rims around remnant olivines are texturally and compositionally identical to those in Allende host, suggesting that they have similar origins. Chondrules are surrounded by opaque rims consisting of tiny lath-shaped fayalitic olivines (<1–3 μm long) intergrown with nepheline. As in the Allende host, fayalitic olivine veins may crosscut altered chondrules, fine-grained chondrule rims and extend into the matrix, indicating that alteration occurred after accretion. We infer that fayalitic olivine rims and lath-shaped fayalites in Allende and its dark inclusions formed from phyllosilicate intermediate phases. This explanation accounts for (1) the similarity of the replacement textures observed in the dark inclusion and Allende host to aqueous alteration textures in CM chondrites; (2) the anomalously high abundances of Al and Cr and the presence of tiny inclusions of spinels and sulfides in fayalitic olivines in Allende and Allende dark inclusions; (3) abundant voids and defects in lath-shaped fayalites in the Allende dark inclusion, which may be analogous to those in partly dehydrated phyllosilicates in metamorphosed CM/CI chondrites. We conclude that the matrix and chondrule rims in Allende were largely converted to phyllosilicates and then completely dehydrated. The Allende dark inclusions experienced diverse degrees of aqueous/hydrothermal alteration prior to complete dehydration. The absence of low-Ca pyroxene in the dark inclusion and its significant replacement by fayalitic olivine in Allende is consistent with the lower resistance of low-Ca pyroxene to aqueous alteration relative to forsteritic olivine. Hydro-thermal processing of Allende probably also accounts for the low abundance of planetary noble gases and interstellar grains, and the formation of nepheline, sodalite, salite-hedenbergite pyroxenes, wollastonite, kirschsteinite and andradite in chondrules and Ca,Al-rich inclusions.  相似文献   

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