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Metastable non-stoichiometric diopside and Mg-wollastonite: An occurrence in an interplanetary dust particle
Abstract
Interplanetary dust particles (IDPs) are the best samples available for the study of the dust that accreted in the early solar
system to form protoplanets at 4.56 Ga. Chondritic aggregate IDPs have a matrix of (sub)-spherical units with variable amounts
of micrometer-sized Fe,Ni-sulfides, Mg,Fe-olivines, and (Ca,Mg,Fe)-pyroxenes. The crystallographic and chemical properties
of these materials can be modified by energetic thermal processes such as irradiation by energetic atoms (space weathering)
and flash heating when an IDP decelerates in the Earth's upper atmosphere. Both thermal events have high heating and quench
rates. Thermal alteration that occurs during atmospheric entry, or dynamic pyrometamorphic alteration, could obscure many
details of earlier thermal modifications. Pure and TiO 2 - or Al 2 O 3 -bearing, non-stoichiometric diopside and Mg-wollastonite in the IDP L2011K7 are the ultimate products of these thermal modifications,
which were dominated by thermally induced loss of (Ca,Mg)O or mostly MgO in original Ca,Mg-clinopyroxene. The calculated oxygen
deficiencies, O = 22 - 24 atoms per formula unit (afu) on the basis of Si = 8.00 afu, support a sequence of "anhydrous biopyriboles":
diopside/Mg-wollastonite --> anhydrous amphibole --> (Si-rich) anhydrous smectite. This particular type of thermal modification,
which is kinetically controlled, is unique to the constituents of IDPs. The extreme environmental conditions of high temperatures
with high heating and cooling rates encountered by IDPs favor metastable equilibrium of the reaction products. That is, the
kinetic, non-equilibrium processes, do not yield random reaction products but ones with predictable chemical compositions.
The Ca,Mg-clinopyroxene compositions observed in this IDP were determined by the metastable eutectic in the enstatite-wollastonite
system. The "anhydrous biopyribole" reaction sequence breaks down at calculated oxygen deficiency (O<20 afu) in the vesicular,
amorphous, amoeboid grains that could have been melted by atmospheric-entry flash heating.
... The latter crystallochemical reason for the scarcity of natural pseudowollastonite consists more specifically in the fact that wollastonite is stabilized with respect to pseudowollastonite by preferential substitution of Mg 2 + , Fe 2 + , and Mn 2 + in low-temperature CaSiO 3 structures. The subsolidus conversion of pseudowollastonite to wollastonite solid solution is effectuated by dissolving the former in the melt (Deer et al., 1997;Longhi, 1987;Morey, 1963;Osborn and Schairer, 1941;Rietmeijer, 1999;Scott et al., 1986;Shinno, 1970). Pseudowollastonite may be expected to occur in paralava, a lowpressure melt rock of non-igneous origin, as it bears free Ca much more often than magmas. ...
... Pseudowollastonite forms binary eutectics (saddle points) with akermanite at 1402°C, gehlenite at 1310°C, and anorthite at 1292°C. Natural non-stoichiometric Ti(±Al)-Mg-wollastonites complicated by coupled substitution (CaTiAl 2 O 6 ) was identified only in rapidly heated and quenched products of dynamic pyrometamorphism, namely in interplanetary dust particles (Rietmeijer, 1999). ...
... Both crystallization and melting paths where wollastonite and pseudowollastonite are involved, in various petrologically significant systems, were detailed in Deer et al. (1997), Jung et al. (2005), Lindsley et al. (1969), Longhi (1987, Morey (1963), Osborn and Schairer (1941), Rietmeijer (1999), Scott et al. (1986), andShinno (1970). Therefore, in this discussion we limit ourselves to a few most important postulates. ...
Pseudowollastonite, an extremely rare constituent of
ultrahigh-temperature combustion metamorphic and igneous rocks, has been
found as a rock-forming mineral in Ca-rich paralava veins of Nabi Musa
fossil mud volcano (Dead Sea area). Pseudowollastonite-bearing paralavas
are the products of combustion metamorphism associated with spontaneous
burning of methane. The melt began to crystallize at 1480-1500 °C
about the ambient pressure. Pseudowollastonite enters two mineral
assemblages: (1) rankinite, larnite, nagelschmidtite, wollastonite (1T),
gehlenite-rich melilite, Ti-rich andradite, cuspidine, and fluorapatite;
(2) parawollastonite (2M), wollastonite (1T), gehlenite-rich melilite,
Ti-rich andradite, fluorellestadite. In this study we present the first
single-crystal structure determination of natural pseudowollastonite.
Pseudowollastonite from Nabi Musa dome is stoichiometric
CaSiO3 and belongs to the most widespread four-layer
polytype: a = 6.83556(10) Å, b = 11.86962(18) Å, c =
19.6255(3) Å, β = 90.6805(13)°, V = 1592.21(4)
Å3, space group C2/c. We argue that pseudowollastonite
is so scarce in nature because its formation requires joint action of
several uncommon factors: availability of hot melts of T > 1200
°C that bear free calcium but are poor in Mg and Fe (mostly as
Fe3 +) and their crystallization in the shallow crust
followed by quenching.
... The latter, pre-solar, dust included silicates (Messenger 2000;Keller et al. 2000) that had formed around other, older stars by vapor-phase condensation. It is important that the common silicates [e.g., forsterite, diopside (Rietmeijer 1999)], Mg-rich ferromagnesiosilica grains (Rietmeijer et al. 1999a), GEMS (glass with embedded metals and sulÞ des ;Bradley 1994;Bradley et al. 1999), and Fe,Ni-sulÞ des in chondritic aggregate, interplanetary dust particles (IDPs) collected in the Earth's lower stratosphere (for reviews see, Rietmeijer 1998Rietmeijer , 2002, are also present around O-rich stars in different stages of stellar evolution (Bouwman et al. 2001;Molster and Waters 2003) and "pure" Mg-silicate crystals are observed in stellar outß ows (Nuth et al. 2002). ...
... The latter, pre-solar, dust included silicates (Messenger 2000;Keller et al. 2000) that had formed around other, older stars by vapor-phase condensation. It is important that the common silicates [e.g., forsterite, diopside (Rietmeijer 1999)], Mg-rich ferromagnesiosilica grains (Rietmeijer et al. 1999a), GEMS (glass with embedded metals and sulÞ des ;Bradley 1994;Bradley et al. 1999), and Fe,Ni-sulÞ des in chondritic aggregate, interplanetary dust particles (IDPs) collected in the Earth's lower stratosphere (for reviews see, Rietmeijer 1998Rietmeijer , 2002, are also present around O-rich stars in different stages of stellar evolution (Bouwman et al. 2001;Molster and Waters 2003) and "pure" Mg-silicate crystals are observed in stellar outß ows (Nuth et al. 2002). ...
... The latter, pre-solar, dust included silicates (Messenger 2000;Keller et al. 2000) that had formed around other, older stars by vapor-phase condensation. It is important that the common silicates [e.g., forsterite, diopside (Rietmeijer 1999)], Mg-rich ferromagnesiosilica grains (Rietmeijer et al. 1999a), GEMS (glass with embedded metals and sulÞ des ;Bradley 1994;Bradley et al. 1999), and Fe,Ni-sulÞ des in chondritic aggregate, interplanetary dust particles (IDPs) collected in the Earth's lower stratosphere (for reviews see, Rietmeijer 1998Rietmeijer , 2002, are also present around O-rich stars in different stages of stellar evolution (Bouwman et al. 2001;Molster and Waters 2003) and "pure" Mg-silicate crystals are observed in stellar outß ows (Nuth et al. 2002). ...
Vapors of Al-Fe-SiO-O-2-H-2 having two different compositions produced ferroaluminosilica grains as a function of agglomeration and fusion along mixing lines in the Al2O3-FeO-SiO2 System that are defined by the predictable, deep metastable eutectic (DME) compositions of the smallest condensate grains. Disorder of these amorphous grains is higher than in quenched glass of identical composition, which is the very property of dissipative structures (Prigogine 1978, 1979) that are states of organization of matter where disequilibrium becomes a source of order. Iron-oxidation states control ferrosilica condensate compositions. We present the first magnetic measurements showing a high Fell content in condensed ferrosilica grains. The Fe-cordierite grain composition is primarily the result of predictable non-equilibrium condensation, not the bulk gas phase composition. Natural terrestrial and anthropogenic (e.g., smelters, coal fly ash) Fe-corderite might well be a metastable phase due to kinetically controlled processes. Amorphous Mg,Fe-bearing aluminosilica dust in chondritic interplanetary dust aggregates and (rare) Mg,Fe-aluminosilicates in meteorites might have condensed via similar processes.
... Of course, solar fare tracks could have been erased during flash heating. Radiation-induced amorphization (Flynn 1996;Rietmeijer 1999;Zolensky et al. 2003) might have produced the amorphous lamellae in pentlandite of IDP L2005E40. Whatever process was responsible to cause the brittle behavior and shattering of sulfides, it predated the time of ferromagnesiosilica melting Fig. 13. ...
... Vesicles have been reported in (unspecified) hydrocarbons (Rietmeijer 1998), olivine (Rietmeijer 1996b), diopside (Rietmeijer 1999) and sulfides (Brownlee et al. 1998;Rietmeijer 1996c) in a number of IDPs. The vesicles in volatile hydrocarbons quickly developed during exposure to the incident electron microbeam, but not in olivine and sulfides. ...
Abstract— Petrological changes in Ni-free and low-Ni pyrrhotite, and much less in pentlandite, during atmospheric entry flash-heating of the sulfide IDPs L2005E40, L2005C39, and L2006A28 support 1) ferrous sulfide oxidation with vacancy formation and Fe3+ ordering; and 2) Fe-oxide formation and sulfur vapor loss through abundant vesicles. Melting of metastable chondritic aggregate materials at the IDP surface has occurred. All changes, e.g., formation of a continuous maghémite rim, proceeded as solid-state reactions at a peak heating temperature of ˜700 °C. This temperature in combination with particle size and density suggest a ˜10 km/s−1 entry velocity. The IDPs probably belonged to cluster IDPs that entered the atmosphere with near-Earth or Earth-crossing asteroid velocities. They could be debris from extinct or dormant comet nuclei, which is consistent with shock comminution of pyrrhotite in these IDPs.
... (1) a ferromagnesiosilica reservoir of mostly amorphous "silicate" materials and silicates (Rietmeijer 1996a(Rietmeijer , 1999(Rietmeijer , 2002 and (2) a Fe-Ni-S reservoir of Fe,Ni-sulfide and Fe,Nimetal grains (Rietmeijer 2002(Rietmeijer , 2004. This binary mixture repeats itself during hierarchical, fractal dust accretion in the <500 nm-sized Principal Components (PCs) in the matrix of aggregate IDPs, in 10-15 µm chondritic aggregate IDPs, in cluster IDPs, and probably at larger scales also in comets (Rietmeijer 1998;Rietmeijer and Nuth 2004). ...
... The "silicate line" (Fig. 8) identifies the Si (el%) range for olivine, Ca-free and low-Ca pyroxenes and Ca-rich clinopyroxenes in IDPs represented by chondritic aggregate IDP L2011A9. The silicates in this IDP are common to many other aggregate IDPs (Rietmeijer 1998(Rietmeijer , 1999Zolensky and Barrett 1994). These silicates with ~12 to 35 Si (el%) contribute Mg and silica to the aerogel melt wherein they are completely assimilated. ...
Abstract— Flight aerogel in Stardust allocation C2092,2,80,47,6 contains percent level concentrations of Na, Mg, Al, S, Cl, K, Ca, Cr, Mn, Fe, and Ni that have a distinctive Fe- and CI-normalized distribution pattern, which is similar to this pattern for ppb level chemical impurities in pristine aerogel. The elements in this aerogel background were assimilated in non-vesicular and vesicular glass with the numerous nanometer Fe-Ni-S compound inclusions. After correction for the background values, the chemical data show that this piece of comet Wild 2 dust was probably an aggregate of small (<500 nm) amorphous ferromagnesiosilica grains with many tiny Fe,Ni-sulfide inclusions plus small Ca-poor pyroxene grains. This distinctive Fe- and CI-normalized element distribution pattern is found in several Stardust allocations. It appears to be a common feature in glasses of quenched aerogel melts but its exact nature is yet to be established.
... Magnesite-siderite [MgCO 3 -FeCO 3 ] carbonates could be among the Comet Tempel 1 dust (Lisse et mary condensates (Greshake et al., 1998). These CAI minerals are also found in rare refractory IDPs (Zolensky, 1987) while diopside and amorphous CaMg-bearing aluminosilica grains occur in anhydrous aggregate IDPs (Rietmeijer, 1998(Rietmeijer, , 1999. ...
... Diopside, MgCaSi 2 O 6 (a pyroxene mineral) is observed around protostars and in O-rich planetary nebulae (Molster and Waters, 2003;Chiavassa et al., 2005), in Comets 81P/Wild 2 (Zolensky et al., 2006) and 9P/Tempel 1 (Lisse et al., 2006), and in aggregate IDPs (Rietmeijer, 1999). Its complex chemical composition could be the result of thermal annealing (aging) of mixed amorphous grains that consisted of DME smectite dehydroxylate and amorphous CaSiO 3 condensates, viz. ...
Condensates produced in a laboratory condensation experiment of a refractory Ca–SiO–H2–O2 vapor define four specific and predictable deep metastable eutectic calciosilica compositions. The condensed nanograins are amorphous solids, including those with the stoichiometric CaSiO3 pyroxene composition. In evolving dust-condensing astronomical environments they will be highly suitable precursors for thermally supported, dust-aging reactions whereby the condensates form more complex refractory silicates, e.g., diopside and wollastonite, and calcite and dolomite carbonates. This kinetically controlled condensation experiment shows how the aging of amorphous refractory condensates could produce the same minerals that are thought to require high-temperature equilibrium condensation. We submit that evidence for this thermal annealing of dust will be the astronomical detection of silica (amorphous or crystalline) that is the common, predicted, by-product of most of these reactions.
... Wollastonite does not therefore correspond to a stoichiometric phase as might initially be expected. Mg-rich wollastonite has been found in interplanetary dust particles which suffer a kind of pyrometamophism with rapidly heated and quenched products (Rietmeijer, 1999), and also in ultra-high-temperature paralavas (T > 1150°C) in which a pyrometamorphic event occurred by combustion (Seryotkin et al., 2012). The incorporation of Mg, produced by the decomposition of the dolomite, provides stability to the pure wollastonite, which at atmospheric pressure is only stable up to temperatures of < 1125 ± 10°C (Osborn and Schairer, 1941). ...
This paper studies the mineralogical and textural changes that take place during the firing in an electric kiln at 800, 950 and 1100 °C of brick samples made with or without additives. Samples were made with a clayey raw material which was mixed with either halite or calcined diatomite sludge and then fired. These samples were then compared with control samples made without additives. Different analytical techniques (X-ray fluorescence, thermogravimetric and differential scanning calorimetric analyses, X-ray diffraction, polarized optical microscopy and scanning electron microscopy) were used to reconstruct the changes that took place inside the bricks from a mineralogical and textural point of view, changes that are similar to those that take place in nature during pyrometamorphism. The carbonates decomposed and reacted with silicates to form gehlenite, diopside and wollastonite; the plagioclase was enriched in calcium and the quartz concentration fell; the clay minerals favoured the melting of the matrix and the appearance of mullite, and K-feldspar changed from microcline to sanidine. The extent of vitrification increased in line with the increase in the firing temperature. When halite was added, new silicates appeared earlier at lower firing temperatures and molysite was formed, while the most important mineralogical difference in the bricks made with added calcined diatomite sludge was the presence of cristobalite, a component of the sludge. It is interesting to observe that the newly-formed phases contain certain chemical elements that are not normally found in their standard chemical composition.
... The Ca and Mg end-members have the composition of the wollastonite (CaSiO 3 ) and enstatite (MgSiO 3 ) minerals and CaMgSi 2 O 6 corresponds to the diopside composition. Mg-rich silicate phases are believed to be the primary constituents of the earth and lunar mantle (McDonough and Sun, 1995;Helffrich and Wood, 2001;Thompson et al., 2005;Hazen et al., 2008) and of interstellar materials (Dorschner et al., 1995;Molster et al., 1999;Rietmeijer, 1999;Fabian et al., 2001). Wollastonite occurs in metamorphosed limestones and constitute important ceramic and cement substances (Swamy and Dubrovinsky, 1997). ...
The structure of glasses along the
MgSiO3–CaSiO3 join has been investigated by
X-ray and neutron diffraction measurements. Structure models were
constructed by fitting the experimental data using the Reverse Monte
Carlo method (RMC). The structural data indicate a random mixing between
MgSiO3 and CaSiO3 glasses, in accordance with
their melt properties. Though important disordering is observed, the
structure evolves continuously along the join. The Ca environment is
essentially similar for all compositions, with an average of 6 to
7-coordinated sites. The Mg environment tends to have higher coordinated
sites as the MgO content decreases. There is a continuous mixing of
Ca–Mg pairs with a non-random distribution emphasized by the
distinct cation–cation distances. Changes were observed in the
topology of the silicate network. The proportion of non-bridging oxygens
decreases, the number of free-oxygens increases and the ring size
distribution is shifted to high-membered rings in the Mg-rich glasses.
These structural investigations indicate important differences with the
crystalline pyroxene structures.
... In both cases these processes will affect the most volatile element abundances. They will probably not significantly modify the silicate and sulfide compositions found in aggregate IDPs and cluster IDPs (Rietmeijer 2000(Rietmeijer , 2005 but partial or complete amorphization by energetic solar particles as was documented in diopside and Mg-wollastonite (Rietmeijer 1999a) might have occurred. ...
Abstract— Many of the nanometer-scale grains from comet 81P/Wild 2 did not survive hypervelocity capture. Instead, they melted and interacted with silica melt derived from the aerogel used by the Stardust mission. Their petrological properties were completely modified, but their bulk chemistry was preserved in the chemical signatures of mostly vesicular Si-rich glass with its typical Fe-Ni-S compound inclusions. Chondritic aggregate IDP L2011A9 that experienced atmospheric pre-entry thermal modification was selected as an analog to investigate these Wild 2 chemical signatures. The chemical, petrologic, and mineralogical properties of the individual constituents in this aggregate IDP are presented and used to match the chemical signatures of these Wild 2 grains. Mixing of comet material and pure silica, which is used in a diagram that recognizes this mixing behavior, is used to constrain the probable petrologic and minerals that caused the Wild 2 signatures. The Wild 2 nanometer-scale grain signatures in Si-rich glass allocations from three different deceleration tracks resembled mixtures of ultrafine-grained principal components and dense agglomerate-like material, Mg-rich silicates (<500 nm) and Fe,Ni-sulfides (<100 nm), and Si-rich amorphous material. Dust resembling the mixed matrix of common chondritic aggregate IDPs was present in Jupiter-family comet Wild 2.
... Schaal (1982) demonstrated experimentally that shockmelting mixtures of silica glass and olivine powder does not induce full mixing of both melt products. Kinetic factors probably played a role, particularly during quenching of silicate melts, and perhaps even vapors, that would cause the formation of intermediate, deep metastable eutectic solid compositions that are both well-defined and different from stoichiometric silicate mineral compositions (Nuth et al. 2002; Rietmeijer 1999 Rietmeijer , 2002). Such non-stoichiometric solids will always be amorphous (Rietmeijer et al. 1999). ...
We report the results of high-resolution, analytical and scanning transmission electron microscopy (STEM), including intensive element mapping, of severely thermally modified dust from comet 81P/Wild 2 caught in the silica aerogel capture cells of the Stardust mission. Thermal interactions during capture caused widespread melting of cometary silicates, Fe-Ni-S phases, and the aerogel. The characteristic assemblage of thermally modified material consists of a vesicular, silica-rich glass matrix with abundant Fe-Ni-S droplets, the latter of which exhibit a distinct core-mantle structure with a metallic Fe,Ni core and a iron-sulfide rim. Within the glassy matrix, the elemental distribution is highly heterogeneous. Localized amorphous "dust-rich" patches contain Mg, Al, and Ca in higher abundances and suggest incomplete mixing of silicate progenitors with molten aerogel. In some cases, the element distribution within these patches seems to depict the outlines of ghost mineral assemblages, allowing the reconstruction of the original mineralogy. A few crystalline silicates survived with alteration limited to the grain rims. The Fe- and CI-normalized bulk composition derived from several sections show CI-chondrite relative abundances for Mg, Al, S, Ca, Cr, Mn, Fe, and Ni. The data indicate a 5 to 15% admixture of fine-grained chondritic comet dust with the silica glass matrix. These strongly thermally modified samples could have originated from a finegrained primitive material, loosely bound Wild 2 dust aggregates, which were heated and melted more efficiently than the relatively coarse-grained material of the crystalline particles found elsewhere in many of the same Stardust aerogel tracks (Zolensky et al. 2006).
Allocation FC6,0,10,0,26 from Stardust track 10 shows a slightly wavy silica glass/compressed silica aerogel interface exposing a patchwork of compressed silica aerogel domains and domains of silica glass with embedded Wild 2 materials in ultra-thin TEM sections. This interface is where molten silica encountered compressed silica aerogel at temperatures <100 °C, and probably near room temperature, causing steep thermal gradients. An Mg, Fe-olivine grain, and a plagioclase-leucite intergrowth survived without melting in silica glass. A Mg-, Al-, Ca-, K-bearing silica globule moved independently as a single object. Two clusters of pure iron, low-Ni iron, and low-Ni, low-sulfur Fe-Ni-S grains also survived intact and came to rest right at the interface between silica glass/compressed silica aerogel. There are numerous Fe-Ni-S nanograins scattered throughout MgO-rich magnesiosilica glass, but compositionally similar Fe-Ni-S are also found in the compressed silica aerogel, where they are not supposed to be. This work could not establish how deep they had penetrated the aerogel. Iron nanograins in this allocation form core-ring grains with a gap between the iron core and a surrounding ring of thermally modified aerogel. This structure was caused when rapid, thermal expansion of the core heated the surrounding compressed aerogel that upon rapid cooling remained fixed in place while the iron core shrank back to its original size. The well-known volume expansion of pure iron allowed reconstruction of the quench temperature for individual core-ring grains. These temperatures showed the small scale of thermal energy loss at the silica glass/compressed silica aerogel interface. The data support fragmentation of olivine, plagioclase, and iron and Fe ± low-Ni grains from comet 81P/Wild 2 during hypervelocity capture.
The bulbous Stardust track #80 (C2092,3,80,0,0) is a huge cavity. Allocations C2092,2,80,46,1 nearest the entry hole and C2092,2,80,47,6 about 0.8 mm beneath the entry hole provide evidence of highly chaotic conditions during capture. They are dominated by nonvesicular low-Mg silica glass instead of highly vesicular glass found deeper into this track which is consistent with the escape of magnesiosilica vapors generated from the smallest comet grains. The survival of delicate (Mg,Al,Ca)-bearing silica glass structures is unique to the entry hole. Both allocations show a dearth of surviving comet dust except for a small enstatite, a low-Ca hypersthene grain, and a Ti-oxide fragment. Finding scattered TiO2 fragments in the silica glass could support, but not prove, TiO2 grain fragmentation during hypervelocity capture. The here reported dearth in mineral species is in marked contrast to the wealth of surviving silicate and oxide minerals deeper into the bulb. Both allocations show Fe-Ni-S nanograins dispersed throughout the low-Mg silica glass matrix. It is noted that neither comet Halley nor Wild 2 had a CI bulk composition for the smallest grains. Using the analogs of interplanetary dust particles (IDPs) and cluster IDPs it is argued that a CI chondritic composition requires the mixing of nonchondritic components in the appropriate proportions. So far, the fine-grained Wild 2 dust is biased toward nonchondritic ferromagnesiosilica materials and lacking contributions of nonchondritic components with Mg-Fe-Ni-S[Si-O] compositions. To be specific, “Where are the GEMS”? The GEMS look-alike found in this study suggests that evidence of GEMS in comet Wild 2 may still be found in the Stardust glass.
This paper discusses measured textures, porosity, chemical compositions and the minerals of interplanetary dust and meteorites from primitive planetesimals and how this information can be used to explain some of the observed physical and chemical properties of meteor phenomena.
This thermal annealing experiment at 1000 K for up to 167 h used a physical mixture of vapor phase-condensed magnesiosilica grains and metallic iron nanograins to test the hypothesis that a mixture of magnesiosilica grains and an Fe-source would lead to the formation of ferromagnesiosilica grains. This exploratory study found that coagulation and thermal annealing of amorphous magnesiosilica and metallic grains yielded ferromagnesiosilica grains with the Fe/(Fe + Mg) ratios in interplanetary dust particles. Furthermore, decomposition of brucite present in the condensed magnesiosilica grains was the source for water and the cause of different iron oxidation states, and the formation of amorphous Fe3+-ferrosilica, amorphous Fe3+-Mg, Fe-silicates, and magnesioferrite during thermal annealing. Fayalite and ferrosilite that formed from silica/FeO melts reacted with forsterite and enstatite to form Mg, Fe-silicates. The presence of iron in different oxidation states in extraterrestrial materials almost certainly requires active asteroid-like parent bodies. If so, the possible presence of trivalent Fe compounds in comet P/Halley suggests that Halley-type comets are a mixture of preserved presolar and processed solar nebula dust. The results from this thermal annealing experiment further suggest that the Fe-silicates detected in the impact-induced ejecta from comet 9P/Temple 1 might be of secondary origin and related to the impact experiment or to processing in a regolith.
The structural evolution of sol-gel-produced amorphous
Mg(x)Ca(1-x)SiO3 silicates is
investigated. Mid- IR Fourier transform infrared
spectroscopy and synchrotron X-ray diffraction are used to confirm the
amorphous nature of the as-prepared silicates, while subsequent in situ
synchrotron X-ray powder diffraction measurements are used to study the
evolution of crystalline mineral phases as a function of annealing
temperature. Multiple silicate phases, including diopside, enstatite,
forsterite, and SiO2, are identified, while Rietveld (i.e.,
structure) refinement of the diffraction data is used to quantify phase
change relationships. Investigated as possible analogs for the
refractory dust grain materials likely to have been present in the early
solar nebula, the likely relevance of these investigations to the
observed silicate compositions of chondritic meteorites and cometary
bodies and the processing of their precursor materials is discussed.
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 mum.
By means of transmission electron microscopy (TEM), the study on hypersthene megacryst and uralitized hypersthene from anorthosite
complex of Chengde, Hebei Province of China shows that the layer silicate minerals such as talc and smectite would grow on
the (100) under the condition of supergenesis. This kind of silicate is the product of the polysomatic reaction, and their
evolution of growth from small to big was observed by TEM. In most cases, the layer silicate nucleus in the form of rhomb
block firstly grow out of the interfaces of the augite exsolution lamellae in the hypersthene matrix. The boundary faces of
the rhomb blocks are parallel to 210 of hypersthene. The small blocks could link each other in the form of veins, and further
in the form of flakes. It is proposed that the growth of layer silicates is constrained by the nucleation mechanism of polysomatic
reaction. The characteristics of the super-microstructure for the uralitized hypersthenes show that the polysomatic reaction
from pyroxene to amphiboles is caused by the bulk reaction mechanism.
Amorphization of crystalline olivine to glass with a pyroxene composition is well known from high-energy irradiation experiments. This report is on the first natural occurrence of this process preserved in a chondritic aggregate interplanetary dust particle. The Fe-rich olivine grain textures and compositions and the glass grain compositions delineate this transformation that yielded glass with Fe-rich pyroxene compositions. The average glass composition, (Mg, Fe)3Si2O7, is a serpentine-dehydroxylate with O/Si = 3.56 ± 0.25, (Mg+Fe)/Si = 1.53 ± 0.24, and Mg/(Mg+Fe) = 0.74 ± 0.1. These measured atomic ratios match the ratios that have been proposed for amorphous interstellar silicate grains very well, albeit the measured Mg/(Mg+Fe) ratio is lower than was proposed for amorphous interstellar silicate grains, Mg/(Mg+Fe) > 0.9.
This paper discusses measured textures, porosity, chemical compositions and the minerals of interplanetary dust and meteorites
from primitive planetesimals and how this information can be used to explain some of the observed physical and chemical properties
of meteor phenomena.
We will focus on Mg-rich ferromagnesiosilica principal components (PCs) in the matrix of collected interplanetary dust particles to address the question: “Are these components the original presolar and nebular dust or are they made from precursor dust(s)?” Non-equilibrium condensation experiments show they are fused agglomerates of amorphous MgSiO and FeSiO silicate condensates with unique metastable eutectic compositions. While the dust condensates were not preserved, the PCs survived secondary processing comet nuclei and primitive asteroids wherein they were stored for the last 4.5 billion years.
AbstractThe astrophysical aspects of the evolutionary path from dust to planets have been known for a long time but it was only recently that it could be followed by astronomical observations (Hubble Space Telescope). It was also recently that the mineral and chemical properties of dust around young starsqu in different stage of stellar evolution could be determined (the Infrared Space Observatory). Dust around these stars, including Vega-type stars that serve as an analog for the solar nebula, was identified as pure Mg-silicates (forsteriteenstatite), Fe-bearing and pure-Fe amorphous ‘silicates’, silica, Fe-metal, Fe-oxides (possibly Fe-sulfides) amorphous carbon and polycyclic aromatic hydrocarbons. They are the same phases that make up the aggregate and cluster interplanetary dust particles (IDPs) collected in the lower stratosphere. Recent vapor phase condensation experiments showed that the original condensates were mostly amorphous, chemically ordered, metastable eutectic ‘FeSiO’, ‘MgSiO’and ferromagnesiosilica ‘silicate’dust from which the observed non-carbon mineralogy could have evolved during hierarchical dust accretion in the solar nebula. The hypothesis of hierarchical dust accretion uses the size distributions for the surprisingly limited number of non-chondritic dust types in aggregate and cluster IDPs as a measure of relative time. It predicts the accretion of gradually larger, relatively younger, dust aggregates with increasingly diverse chemical and mineral properties of increasingly larger crystalline grains that evolved from initially mostly amorphous dust. This early chemical and mineral dust evolution can be traced in the collected aggregate and in larger cluster IDPs and in even larger aggregate meteoroids that burn up during atmospheric entry flash-heating but whereof the resulting meteors contain information on the chemistry, grain size and texture of the original dust. These aggregate particles were protected against post-accretion, thermal or aqueous dust modification of the original presolar dust and the evolved mineralogy and chemistry during cold-storage inside icy protoplanets such as comet nuclei. The interplanetary dust particles provide ground truth to the properties and modification of the presolar dust in dense molecular clouds wherein stars, such as our sun, were born.
The mineralogical properties of aggregate interplanetary dust particles (IDPs) and matrix of CI and CM carbonaceous chondrites support a sequence of increasingly intense aqueous alteration that could have modified pristine organics in aggregate IDPs to more complex organic compounds in chondrite matrix. Water for hydration was initially derived from sulfidation of condensed Fe-oxides whereby pyrrhotite produced contributed to the formation of increasingly Fe-rich serpentine layer silicates. This sequence was possible as consequence of the unique, metastable eutectic, compositions of the amorphous silicates.
In situ-heating transmission electron microscopy and high-resolution transmission electron microscopy studies of the thermal decomposition of tremolite shhow that transformed pyroxenes have C2/c and P21c structures and retain axial orientation of the tremolite structure with I2/m orientation. Amorphous silica forms only on the surface of the heated crystals. Thermal decomposition of tremolite, which takes place at 740°C, is characterized by the formation of (010) clinopyroxene slabs with thicknesses of 18 and 36 Å along the b axis. The (010) slabs with C2/c symmetry are coherent with the tremolite matrix. Heated tremolite with (010) slabs has the same composition as 'anhydrous tremolite.' Clinopyroxene domains formed at 780°C are composed of C2/c and P21/c clinopyroxenes with an exsolution lamella-like texture. The products characterized by clinopyroxene domains are isolated by tremolite with (010) clinopyroxene slabs.
It is a fundamental goal of interplanetary dust particle (IDP) research
to determine the sources and histories of these primitive
extraterrestrial materials. Chondritic IDPs have been divided into
anhydrous and hydrous varieties. The presumption is that hydrated IDPs
experienced aqueous alteration on parent bodies (hydrous asteroids or
possibly comets). We wish to discover whether the anhydrous IDPs were
the initial raw materials for these reactions. We report here analyses
of olivines and pyroxenes from 22 large (>15 micrometers) chondritic
IDPs: 11 anhydrous and 11 hydrous. We find there exists no significant
difference in the compositions of olivines from olivine vs. pyroxene
dominated IDPs. The degree of heterogeneity of these olivine
compositions from anhydrous IDPs (Fo44-100) is great, significantly
exceeding that of the olivines from the hydrous IDPs (Fo76-100) we
examined. We observe the same relationship for orthopyroxenes (En57-100
for anhydrous, En79-100 for hydrous). Interestingly, we encountered true
diopsides predominantly in pyroxene-dominated hydrous IDPs. Some
anhydrous IDPs also displayed restricted compositional ranges for
olivines and pyroxenes. For sulfate- containing anhydrous IDPs, olivines
and orthopyroxenes had the ranges Fo91-100 and En94-100; for melted
(atmospherically ablated) IDPs these ranges were Fo90-100 and En91-94.
Are hydrous and anhydrous IDPs genetically related? With the exception
of some from the serpentine class (Bradley and Brownlee, 1991), all
chondritic IDPs have nearly identical mineralogies (Zolensky and
Lindstrom, 1992). The compositional ranges of olivines and pyroxenes in
these materials are quite dissimilar, with those from hydrous IDPs being
relatively magnesium-rich; however this could be explained by a
relatively greater ease of destruction of iron-rich silicates during
aqueous alteration. It is known that increasing the Fe^2+ content of
olivine decreases the temperature at which serpentinization occurs (Deer
et al., 1982). This is apparently due to greater dislocation densities
for Fe-rich olivines, and oxidation of Fe^2+. If a similar phenomenon
affects alteration of pyroxenes and olivines to smectite, then the
genetic relation of hydrous to anhydrous IDPs is permitted by our
results. We have already reported that the Mg-Fe compositional range
exhibited by phyllosilicates in hydrated IDPs is nearly identical to
that of olivines and pyroxenes in anhydrous IDPs (Zolensky and
Lindstrom, 1992); this observation also permits a genetic link between
these IDP types. However, it is also possible that hydrous are not
genetically related to anhydrous IDPs. We note that the relatively
Mg-rich compositions of olivines and orthopyroxenes in
sulfate-containing IDPs can also be explained by preferential
destruction of Fe-rich olivines by oxidation of Fe^2+. The ablated IDPs
are merely showing the results of equilibration. Are chondritic IDPs
merely small samples of the parent bodies of chondritic meteorites?
Aside from differences in bulk composition and physical properties, IDPs
differ from all chondritic meteorites except CIs and CMs in the
compositional ranges of olivines and pyroxenes (for olivines: CV3-
Fo40-60; CO3- Fo40-70; H3, L3 and LL3- Fo30-80; Scott et al., 1988;
Dodd, 1981). Although a few hydrous IDPs (a subset of the serpentine
class particles) are apparently related to CMs (Zolensky and Lindstrom,
1992), the majority are mineralogically dissimilar. Small carbonaceous
clasts contained within certain meteorites (e.g., CR chondrites, the
Bholghati howardite, LEW 85300 polymict eucrite and Kaidun carbonaceous
breccia) have similar mineralogies to some IDPs (Zolensky et al., 1992),
although more detailed analyses of these materials are still required.
In summary, most chondritic IDPs are compositionally and mineralogically
distinct from chondritic meteorites, but have apparently experienced
similar conditions of origin and evolution both in the solar nebula and
on parent bodies. References: Bradley and Brownlee (1991) Science 251,
549; Deer, Howie, and Zussman (1982) Rock-Forming Minerals, p. 77; Dodd
(1981) Meteorites, A Petrologic-Chemical Synthesis, p. 45; Scott et al.
(1988) in Meteorites and the Early Solar System, 721; Zolensky and
Lindstrom (1992) Proc. Lunar and Planet. Sci. Conf. 22, 161; Zolensky et
al. (1992) Lunar and Planet. Sci. XXIV, 1587.
Olivine-rich chondritic interplanetary dust particles (IDPs) are an important subset of fluffy chondritic IDPs collected in the earth's stratosphere. Particles in this subset are characterized by a matrix of nonporous, ultrafine-grained granular units. Euhedral single crystals, crystals fragments, and platey single crystals occur dispersed in the matrix. Analytical electron microscopy of granular units reveals predominant magnesium-rich olivines and FeNi-sulfides embedded in amorphous carbonaceous matrix material. The variable ratio of ultrafine-grained minerals vs. carbonaceous matrix material in granular units support variable C/Si ratios, and some fraction of sulfur is associated with carbonaceous matrix material. The high Mg/(Mg+Fe) ratios in granular units is similar to this distribution in P/Comet Halley dust. The chondritic composition of fine-grained, polycrystalline IDPs gradually breaks down into nonchondritic, and ultimately, single mineral compositions as a function of decreased particle mass. The relationship between particle mass and composition in the matrix of olivine-rich chondritic IDPs is comparable with the relationship inferred for P/Comet Halley dust.
This paper presents a synopsis of current investigations on the mineralogy of chondritic micrometeorites obtained from the lower stratosphere using flat-plate collection surfaces attached to high-flying aircraft. A compilation of detailed mineralogical analyses for 30 documented chondritic interplanetary dust particles indicates a wide variety of minerals present in assemblages which, as yet, are poorly defined. Two possible assemblages are: (1) carbonaceous phases and layer silicates and (2) carbonaceous and chain silicates or nesosilicates. Particles with both types of silicate assemblages are also observed.
Data on the nature of dust in the upper atmosphere have been provided by a stratospheric dust collection program. This program makes it possible to study in detail materials related to early solar system history. A comprehensive overview of the stratospheric collection program provides a basis for a better understanding of stratospheric dust origins. It is important for the development of this understanding that easily accessible criteria for classification be established. The present investigation is concerned with the development of a suitable classification scheme for the obtained stratospheric materials. Three sets of eight collectors were flown on WB57 high-altitude aircraft during August and September, 1981, as part of the considered collection program. Three of the 24 impactors (flags) have undergone preliminary examination. The current investigation is concerned with selected data on materials from flags W7010, W7017, and W7029.
The petrology of a massive olivine-sulphide interplanetary dust particle shows melting of Fe,Ni-sulphide plus complete loss of sulphur and subsequent quenching to a mixture of iron-oxides and Fe,Ni-metal. Oxidation of the fayalite component in olivine produced maghemite discs and cellular intergrowths with olivine and rare andradite-rich garnet. Cellular reactions require no long-range solid-state diffusion and are kinetically favourable during pyrometamorphic oxidation. Local melting of the cellular intergrowths resulted in three dimensional symplectic textures. Dynamic pyrometamorphism of this asteroidal particle occurred at approx. 1100 C during atmospheric entry flash (5-15 s) heating.
Synthetic enstatites were prepared at 800°C/25 kbar from six gels along the binary joins Mg2[Si2O6] (= En) - Mg2Al[AlSiO6] (= MgTsch) and En - □0.5Mg0.5Al[Si2O6] (= Mg-Es = "Mg-Eskola pyroxene") as well as in the ternary range between these joins. Microprobe analyses of the run products exhibit aluminum contents in pyroxene between 0.132 and 0.333 Al p.f.u. (based on 6 oxygens), which is much more than indicated by the classical equilibrium isopleth (0.08 Al p.f.u.). In all these enstatites the ratios of Si/Mg were larger than 1 so that they clearly plot off the En - MgTsch join towards SiO2. As the calculated structural formulae show cation deficiences, and as no OH-bands were observed by infrared spectroscopy, these ternary enstatite compositions are explained by a combination of the two substitutions Al[6] + Al[4] for Mg + Si and 2 Al[6] + □ for 3 Mg[6], essentially in the ratio 2:1 thus plotting along the join MgSiO3-Al2SiO5. Stacking faults in the plane (010) in synthetic enstatites with clinoenstatite lamellae, but no amphibole bands were detected by TEM. With rising degrees of substitution the cell parameters a, c, and V increase, while for the b direction a contraction was observed.
We describe the nanometer-sized phases in millimeter-sized spheres produced in a triggered lightning-strike experiment characterized by ultra-high heating of ~ 108 degrees s-1 followed by similarly rapid quenching. The compositions of the aluminosilica glass spheres define a metastable Al2O3-SiO2 eutectic at ~ 40 wt. % Al2O3. The other phases are defined by the liquidus in the SiO2-Al2O3-Fe3O4 (wt %) phase diagram whereby hercynite formed at ~ 1750°C and domains of Fe3+-rich cordierite glass that represent a ternary minimum melt at ~ 1400°C with cotectic glasses linking this glass and spinel. Metastable eutectics are potentially important to understand the phase relationships due to flash heating events.
The mineralogy of interstellar grains is established in the fiery death throes of the stars and characterized to varying degrees by telescopic observations of absorption bands against the background of other stars, by studies of the infrared emission of dusty circumstellar shells, and by analyses of materials separated from meteorites and Interplanetary Dust Particles (IDPs) captured in the lower stratosphere of the Earth. The mineralogy of interplanetary dust was established at the birth of the solar system. These topics have been reviewed on many occasions from the perspective of the analysis of IDPs (Sandford, 1986; Bradley, 1988; Rietmeijer, 1998) or the theoretical interpretation of Interstellar Extinction Spectra (Dorschner and Henning, 1995). We intend to take a very different perspective: that of an experimental chemist using the full range of laboratory studies of IDPs, presolar grains and analog materials to set limits on the processing history of solid materials in astrophysical environments. We warn the reader that some of the conclusions drawn from this perspective are at odds with currently accepted models of grains in the interstellar medium or with interpretations of specific features in IDPs and meteorites. These areas are fertile ground for experimental studies, and analytical or observational programs that will look for the subtle details by which one might distinguish between these alternate hypotheses.
Orthopyroxene megacrysts (OPM) are examined from three field associations (leuconorite, anorthosite and anorthositic veinlets) within the anorthosite massif at St-Urbain, Quebec, Canada. Field characteristics, petrographic characteristics and mineral chemistry of OPM are given for each association. REE patterns of OPM are similar but enriched over other orthopyroxenes in the massif. OPM have values of Mg/(Mg + FeT) in the range of other orthopyroxenes in the massif but have elevated Ca and Al contents. Exsolved plagioclase and oxide lamellae are associated with OPM and have An contents, REE patterns, Sr contents and Sr isotopic compositions virtually identical to plagioclase material found elsewhere in the massif. These characteristics and other features indicate that OPM crystallized from the same anorthositic magma that gave rise to the rocks of the massif as a whole and that they are not vestiges of a basaltic parent. -R.A.G.
Non‐spherical chondritic interplanetary dust particles (IDPs) have been routinely collected in the stratosphere at ∼17–19 km altitude since May 1981 by the NASA Cosmic Dust Program. These IDPs show distinct morphological subtypes, viz. chondritic porous (CP) and chondritic filled (CF) aggregate particles, glazed CP and CF IDPs with ‘softened’ contours, and non‐aggregate chondritic smooth and rough‐textured (CR) IDPs. The distributions of these IDP subtypes show distinct abundance maxima as a function of collection time. Albeit conservatively at this time, these distributions support systematic intra‐ and inter‐annual variations. Rigorous statistical treatment of the data is disabled by gaps in the collection periods and uncertainties in collection and curation. This study recommends monthly sampling of the lower stratosphere during a two year period to quantify these temporal variations in the non‐spherical chondritic IDP subtypes. With frequent and systematic collection we will be able to combine temporal variations in IDP subtype distributions with their petrologic and chemical properties. It will then also be possible to correlate the temporal variations sampled in the lower stratosphere with events that deliver chondritic IDPs to the Earth’s atmosphere. Hence, the study of micrometeorites collected in the stratosphere might substitute for in‐situ sampling of asteroids and comet nuclei.
Ultrathin microtome sections,using new techniques, has permitted elemental analysis of IDP components that are smaller than 100 nm. These studies indicate the glass component of GEMS has very low Fe content and the residual Fe content may reside in nm-size sulfides.
Non-equilibrium gas to solid condensation produced three distinct groupings of ferrosilica solids with compositions ∽15, 30 and 87.5 wt.% FeO (or, ∽17, 33 and 97 wt.% Fe2O3). These solid compositions define metastable eutectic points in the FeO/Fe2O3–SiO2 binary system. The position of a silica-rich metastable eutectic is sensitive to the ratio FeO:Fe2O3 during gas-to-solid quenching. The Fe-rich metastable eutectic indicates a two-liquid field in this revised pseudo-binary equilibrium phase diagram.
Phase equilibria in the system CaMgSi2O6-H2 in the pressure and temperature ranges 10-9 to 10-6 bar and 1200-1500°C, respectively, have been experimentally determined. The vaporous curve was determined by monitoring evaporation rate and compositional changes as a function of temperature. Crystalline diopside condenses from a gas phase along a vaporous curve with a positive slope of approximately 16°C/log unit PH2 (bars) from about 1360°C at PH2 = 10-9 bar to 1388°C at 3 × 10-7 bar. Partially evaporated diopside at temperatures above its vaporous shows but systematic increase in Ca/Mg with time (and degree of evaporation). There were no chemical changes with time (and degree of evaporation) of diopside within its own stability field. At higher pressures, a liquid condenses from the vapor, with a slope of the liquid-vapor curve of about 50°C/log unit PH2 (bars). The liquidus temperature decreases from 1388°C near the triple point to about 1350°C at PH2 = 10-6 bar and is nearly vertical to PH2 = 1 bar. The Ca/Mg of the liquid exceeds 1 indicating incongruent melting behavior although no additional crystalline phases were observed. Liquid diopside probably also evaporates incongruently. Provided that the phase equilibria of the diopside composition mimic those of other refractory phases condensing from the early solar nebula and that only H2 gas was present, the experimental results indicate that the pressures in the nebula during condensation of crystals from vapor were significantly lower (< 10-6 bar) than that frequently suggested (> 10-4 bar). Alternatively, if the higher pressure estimates are correct, liquid silicate will condense from the vapor, and vapor-liquid and crystal-liquid equilibria were more important during the petrogenetic processes resulting in aggregated silicate materials than suggested previously. Also at: Geological Institute, University of Tokyo, Tokyo, Japan.
An analytical and transmission electron microscope study showed that nonequilibrium gas-to-solid condensation in an AlO–SiO vapor yielded solid units with well defined compositional peaks at 11.5 wt % Al2O3 and 41–47 wt % Al2O3. These peaks coincide with metastable eutectic points in the Al2O3–SiO2 phase diagram. The compositions of thermally annealed units support the presence of a (metastable) miscibility gap between ∼15–∼60 wt % Al2O3. The important finding in this analysis is the determinative role of metastable eutectics in nonequilibrium gas-to-solid condensation. © 1999 American Institute of Physics.
The present work discusses topics in the source regions for meteorites,
their secondary processing, irradiation effects on meteorites, solar
system chronology, the early solar system, the chemistry of chondrites
and the early solar system, magnetic fields in the early solar system,
the nature of chondrules, micrometeorites, inhomogeneity of the nebula,
the survival of presolar material in meteorites, nucleosynthesis, and
the relationship between extinct radionuclides and
nucleocosmochronology. Attention is given to igneous activity in the
early solar system, principles of radiometric aging, the cosmochemical
classification of the elements, highly labile elements, the potential
significance of pristine material, the astrophysical implications of
presolar grains, boundary conditions for the origin of the solar system,
and iodine-xenon dating.
The paper reviews the aspects of cosmic dust that are important to its collection and utilization as a resource of extraterrestrial material. Most of the collected dust particles appear to be primitive and sometimes unique solar system materials that contain important clues to early solar system processes and environments. Collected dust can also be studied to investigate selected properties of the interplanetary medium and the terrestrial environment. The treatment of the origin and evolution of dust in the solar system is focused on aspects important to the interpretation of sample results. The bulk of the paper reviews the results of laboratory studies of dust samples collected in the stratosphere and deep-sea sediments.
Results are reported concerning the use of an energy dispersive X-ray detector to carry out the analysis of thin foils in the electron microscope. The combination of a thin specimen and the extreme stability of the energy dispersive X-ray detector enables the experimental determination of a calibration curve of X-ray production—detection efficiency vs characteristic X-ray energy. Quantitative analysis can be carried out using the calibration curve without reference to standards at the time of analysis.
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.
The fusion crust of meteorites is modified by many factors (orientation during flight, heat conductivity in the whole meteorite and in the components, chemical reactivity, etc.). In spite of that, some properties can be astonishingly alike, and the zonal arrangement of mineral associations shows many relations nearly everywhere. Of special interest may be the formation of magnetite, wüstite and a ‘high temperature maghemite’, the alterations in chromite, ilmenite, iron, alabandite and niningerite, ‘Henderson phase’ and graphite.
An automated 200 keV analytical electron microscope was used to obtain elemental analyses from over 4000 points on ultramicrotomed thin sections of eight “layer silicate” class interplanetary dust particles (IDPs). Each analysis yielded major and minor element abundances from a volume approaching that of a cylinder 50 nm in diameter. Multi-element cluster analysis was employed to identify mineral phases and their relative abundances in the thin sections. Petrographic characteristics were determined using brightfield and darkfield imaging, lattice fringe imaging and electron diffraction. Three of the particles contained smectite (1.0–1.2 nm basal spacing) and two contained serpentine (0.7 nm basal spacing). The mineralogy of three of the eight IDPs appears to have been modified by heating, possibly during atmospheric entry. The point count analyses and Mg—Si—Fe ternary diagrams show that one of the serpentine-containing IDPs is similar to CI and CM chondritic meteorites, but the fine-scale heterogeneity among the other seven is distinct from these meteorites, which are more uniform on a 50 nm scale. The IDPs exhibit evidence of aqueous processing, but they have typically experienced only short range, submicrometer scale alteration. In comparison, CI and CM chondrites are compositionally more uniform, presumably as a result of more extensive aqueous alteration. The hydrated “layer silicate” IDPs may provide a broad sampling of the asteroid belt, including samples of the outer P and D asteroid classes.
The extreme antiquity and lack of evidence for significant chemical processing of the chondritic meteorites since they were formed suggest the possibility that their chemistry and mineralogy may have been established during the condensation of the solar system. By using equilibrium thermodynamics, the sequence of condensation of mineral phases from a cooling nebula of solar composition has been calculated. Applying the predictions of these theoretical models suggests that (1) the chemistry and mineralogy of Ca-Al-rich inclusions in C2 and C3 chondrites were established during condensation at temperatures >1300°K; (2) fractionation of such inclusions is necessary to account for the refractory element depletions of ordinary and enstatite chondrites relative to the carbonaceous chondrites; (3) the metal-silicate fractionation in ordinary chondrites took place in the nebula at T < 1000°K and Ptot ∼ 10−5 atm; (4) the volatile element depletion of C2 and C3 chondrites relative to C1 chondrites took place during chondrule formation; (5) the most volatile elements are depleted in ordinary chondrites because they accreted before these elements were totally condensed; and (6) many chemical features of planetary rare gases and organic material in carbonaceous chondrites could have been established during condensation. Chemical fractionation during condensation may also be responsible for the heterogeneous accumulation of the earth, the refractory element enrichment of the moon, and the varying Fe/Si ratios of the terrestrial planets.
The present numerical solutions for the atmospheric entry of 10 micron-1 mm diameter micrometeoroids gave attention to ablative mass loss and cooling, together with gravitational and curvature effects, for entry velocities in the 11.2-72 km/sec range. Maximum temperature and mass-loss rates are found to generally occur at altitudes between 85 and 90 km, during about 1 sec of peak heating; the survival of all particles in the 70 micron-1 mm size range is noted to be limited to those with minimal entry velocity. Virtually all of the 'cosmic spherules' of more than 70-mm diameter, as well as giant unmelted micrometeorites, are implied by the present results to be of asteroidal origin.
Since May 1981, the National Aeronautics and Space Administration (NASA) has used aircraft to collect interplanetary dust particles (IDP's) from Earth's stratosphere. Specially designed dust collectors are prepared for flight and processed after flight in an ultraclean (Class-100) laboratory constructed for this purpose at the Lyndon B. Johnson Space Center (JSC) in Houston, Texas. Particles are individually retrieved from the collectors, examined, and cataloged, and then made available to the scientific community for research. Interplanetary dust thereby joins lunar samples and Antarctic meteorites as a critical extraterrestrial material being curated at JSC.
Melting and degassing of interplanetary dust particle L2005B22 at approx. 1200 C was due to flash heating during atmospheric entry. Preservation of the porous particle texture supports rapid quenching from the peak heating temperature whereby olivine and pyroxene nanocrystals (3 nm-26 nm) show partial devitrification of the quenched melt at T approx. = 450 C - 740 C. The implied ultrahigh cooling rates are calculated at approx. 105 C/h-106 C/h, which is consistent with quench rates inferred from the temperature-time profiles based on atmospheric entry heating models. A vesicular rim on a nonstoichiometric relic forsterite grain in this particle represents either evaporative magnesium loss during flash heating or thermally annealed ion implantation texture.
An analytical electron microscope study of dispersed interplanetary dust aggregates collected in the earth's stratosphere shows that, in spite of their similarities, the aggregates exhibit significant differences in composition, internal morphology, and mineralogy. Of 11 chondritic particles examined, two consist mostly of a noncrystalline chondritic material with an atomic S/Fe ratio equal to or greater than 2 in places, one consists of submicron metal and reduced silicate 'microchondrules' and sulfide grains embedded in a carbonaceous matrix, and another consists of submicron magnetic-decorated unequilibrated silicate and sulfide grains with thick low-Z coatings. Although the particles are unmetamorphosed by criteria commonly applied for chondritic meteorites, the presence of reduced chemistries and the ubiquity of mafic, instead of hydrated, silicates confirm that they are not simply C1 or C2 chondrite matrix material. The observations indicate that portions of some particles have not been significantly altered by thermal or radiation processes since their assembly, and that the particles probably contain fine debris from diverse processes in the early solar system.
The ways of establishing the extraterrestrial nature of different subsets of interplanetary dust collected in the stratosphere by high-altitude aircraft are discussed. Consideration is given to microanalytic techniques which make it possible to obtain detailed experimental information on the mineralogical and petrographic characteristics, the mid-IR absorption spectra, the Raman spectra, and the isotopic properties of individual particles. The implications of data obtained by these techniques for the origin of interplanetary dust are examined, showing that the particles are less altered than those solar-system material samples found in meteorites. It is suggested that many of the particles come from comets, although an unknown fraction originate from asteroids. Small regions of isotopically distinct material suggest that part of the dust consists of interstellar-cloud material that predates the solar system.
Pulse heating experiments on magnesium-rich olivine and pyroxene, two silicates often found in micrometeorites collected in the stratosphere, show that iron ion tracks remain detectable in the transmission electron microscope for temperature maxima up to approximately 600 C. Assuming thermal emissivities near unity, this implies that micrometeorites with surface densities above 1 mg/sq cm are likely to have their track record erased, and that tracks in reentrant or low-density micrometeorites smaller than 7 microns in size are likely to survive atmospheric entry.
Nonstoichiometric grains with depletions of magnesium and silicon (relative to oxygen) and inclusions of iron-nickel metal and iron-rich sulfides have been identified in interplanetary dust particles from comets. These chemical anomalies accumulate in grains exposed to ionizing radiation. The grains, known as GEMS (glass with embedded metal and sulfides), were irradiated before the accretion of comets, and their inferred exposure ages, submicrometer sizes, and "amorphous" silicate structures are consistent with those of interstellar silicate grains. The measured compositional trends suggest that chemical (as well as isotopic) anomalies can be used to identify presolar interstellar components in primitive meteoritic materials.
Microbeam analysis of clays at low temperature
- I D R Mackinnon
- S A Kaser
Mackinnon, I.D.R. and Kaser, S.A. (1987) Microbeam analysis of clays at low temperature. In RH Geiss, Ed., Microbeam Analysis-1987, p. 332-334. San Francisco Press, California.
Metastable eutectics in the Al2O3-SiO2 system explored by vapor phase condensation
- F J M Rietmeijer
- J M Karner
Rietmeijer F.J.M. and Karner J.M. (1999) Metastable eutectics in the Al2O3-SiO2
system explored by vapor phase condensation. Journal of Chemical Physics,
110, 4554-4558.
Meteorites and the Early Solar System
- J P Bradley
- S A Sandford
- R M Walker
Bradley, J.P., Sandford, S.A., and Walker, R.M. (1988) Interplanetary dust particles.
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