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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedure
  5. Results
  6. Identification of Reactions Recorded in the DSC Spectra
  7. Infrared Spectra of the Samples
  8. Astronomical Implications
  9. Conclusion
  10. Acknowledgments
  11. References

Abstract– Exothermic reactions during the annealing of laboratory synthesized amorphous magnesium-bearing silicate particles used as grain analogs of cosmic dust were detected by differential scanning calorimetry (DSC) in air. With infrared spectroscopy and transmission electron microscopy, we show that cosmic dust could possibly undergo fusion to larger particles, with oxidation of magnesium silicide and crystallization of forsterite as exothermic reactions in the early solar system. The reactions begin at approximately 425, approximately 625, and approximately 1000 K, respectively, and the reaction energies (enthalpies) are at least 727, 4151, and 160.22 J g−1, respectively. During the crystallization of forsterite particles, the spectral evolution of the 10 μm feature from amorphous to crystalline was observed to begin at lower temperature than the crystallization temperature of 1003 K. During spectral evolution at lower temperature, nucleation and/or the formation of nanocrystallites of forsterite at the surface of the grain analogs was observed.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedure
  5. Results
  6. Identification of Reactions Recorded in the DSC Spectra
  7. Infrared Spectra of the Samples
  8. Astronomical Implications
  9. Conclusion
  10. Acknowledgments
  11. References

Crystalline or amorphous magnesium rich silicate minerals are widely and abundantly distributed in the solar system and also in many other astrophysical objects. Based on infrared spectra of the diffuse interstellar medium, Kemper et al. (2004, 2005) estimated that the fraction of crystalline material along a sight line to the galactic center is 2.2 ± 2.2% of the interstellar silicate dust population, while Li and Draine (2001) have estimated that a maximum of 5% of interstellar silicates might be crystalline. In the case of dust formation in evolved stars, the fraction of crystalline silicates is typically 10–20% in the outflows of asymptotic giant branch (AGB) stars with high mass outflow rates, whereas most AGB stars show no crystalline features in their infrared spectra (Molster and Kemper 2005). On the other hand, observations show that some fraction of silicates are crystalline around young stars (Waelkens et al. 1996), in comets (Crovisier et al. 1997), and around evolved stars (Waters et al. 1996), in addition to previous suggestions of crystalline materials in comets (Hanner et al. 1994), in interplanetary dust particles (IDPs) (Bradley et al. 1988), and around β-Pictoris (Knacke et al. 1993; Fajardo-Acosta and Knacke 1995) based on the shape of the 10 μm silicate feature. Thermal annealing is a very efficient process for crystallization of silicate grains. In this study, we focused on the thermal crystallization of magnesiosilica grains and discuss the implications for both presolar materials and grains that may have condensed in the solar nebula.

The presence of both amorphous and crystalline silicates around AGB stars suggests that formation conditions, such as cooling rate or the temperature at which grain nucleation begins, in such objects may vary, or that the crystalline silicates are the result of annealing of previously formed amorphous silicates, because whether a silicate grain becomes crystalline or remains amorphous depends sensitively on its thermal history. Crystalline silicates can also result from heterogeneous nucleation in the solar nebula, because silicates are easily able to condense onto previously formed refractory minerals and/or metals such as corundum and tungsten by heterogeneous condensation at temperatures that are higher than those required for crystallization (approximately 1000 K) from amorphous grains. In contrast, homogeneous nucleation does not occur near the equilibrium temperature (Kimura et al. 2010) but can require much cooler vapors. Indeed, homogeneously condensed samples that are produced in the laboratory from a high-temperature vapor via reactions in gas mixtures of H2, O2, SiH4, and Mg (Hallenbeck et al. 1998) and that form by laser vaporization of natural minerals in an O2 gas of 10 mbar (Rotundi et al. 2002) are amorphous. In contrast, the vaporization of olivine (Nagahara et al. 1988) or the evaporation of Mg-Si rich glass (Toppani et al. 2006; Kobatake et al. 2008) produced crystalline silicates when the evaporated gases heterogeneously condensed onto a high temperature substrate.

In our solar system, several comets show 10 μm emission features characteristic of a mixture of amorphous and crystalline silicates (Hanner 1999). In addition, mixtures of amorphous and crystalline silicates are observed in stratospherically collected IDPs (e.g., Bradley et al. 1988) and their infrared spectra are similar to those of comets (Sandford and Walker 1985). Crystalline silicates have also been identified in comet Wild 2 (Stardust) samples (Brownlee et al. 2006). Since most interstellar silicates are in an amorphous state, it has been proposed that the amorphous silicates gradually crystallized via thermal annealing in the hot inner solar nebula over time and then were transported outward and incorporated into comets (Shu 1996; Boss 2004). Using this thermal annealing model, Nuth et al. (2000) discussed the chronology and formation age of comets in the early solar system based on laboratory experiments involving the annealing of amorphous magnesiosilica grains (Hallenbeck et al. 1998, 2000; Rietmeijer et al. 2002a, 2002b, 2004). In our solar system, there are several possible formation processes to produce crystalline silicates: annealing of presolar grains, direct condensation of crystalline grains, condensation of amorphous grains followed by annealing and oxidation of metallic precursors (Kimura and Nuth 2009). Solar system silicates may have been formed by all of these different mechanisms, i.e., the degree of each contribution needs to be assessed.

In all cases, the thermal evolution of amorphous silicates into minerals is an important component of the history of cosmic silicates. In this study, we examine the behavior of amorphous magnesium-bearing silicate particle analogs and their reaction energy as a function of temperature and time, during thermal annealing in air using differential scanning calorimetry (DSC). These annealing data provide a basis for discussing the kinetics of crystallization of amorphous silicate materials and their implications for the thermal annealing model for the formation of crystalline Mg-silicates.

Experimental Procedure

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedure
  5. Results
  6. Identification of Reactions Recorded in the DSC Spectra
  7. Infrared Spectra of the Samples
  8. Astronomical Implications
  9. Conclusion
  10. Acknowledgments
  11. References

Production of Grain Analogs in the Laboratory

Refractory magnesiosilica particles were produced in the laboratory by vapor phase condensation using the Condensation Flow Apparatus at NASA’s Goddard Space Flight Center (Nelson et al. 1989; Nuth et al. 2002). The particles produced are generally amorphous and those made in any one run generally have several different compositions. Since astronomical conditions cannot be exactly duplicated, the laboratory samples are simple analogs of the more complex materials that might be found in natural systems.

Smoke particles were produced at a total pressure of approximately 90 Torr in an atmosphere dominated by hydrogen at temperatures ranging from approximately 773 K to more than 1500 K as the gas passes through the hydrogen flame front. At the flow rates of the experiment particles transit the high temperature furnace in less than 10 ms. The hydrogen flow rate was always approximately 1000 sccm (standard cubic centimeters per minute). Silane was introduced into the apparatus with a flow rate of 55 sccm and mixed with oxygen at a flow rate of 110 sccm. Since carbon monoxide and aluminum oxides form prior to the formation of silicates in thermodynamic equilibrium models, the abundance of elemental oxygen (e.g., 4.90 × 10−4 in solar gas with respect to H:1) during the formation of silicates decreases in proportion to the amount of carbon (2.69 × 10−4) and aluminum (2.82 × 10−6; Asplund et al. 2009) oxidized previously. The Si/O ratio during these experiments was approximately 0.25, which is somewhat higher than the solar ratio of 0.15. Since the Si/O ratio of dust in the solar system can also be changed by the presence of water ice, the Si/O ratio is close to that in a solar system environment. Magnesium metal was placed into a graphite boat inside an alumina furnace tube. The furnace was heated prior to the start of the experiment to 973 K, which was measured by thermocouples on the outside of the furnace. The hydrogen atmospheric temperature around the Mg is about 773 K initially and becomes higher as the experiment proceeds because of the flame. At the nominal 773 K temperature of the furnace, magnesium metal has a vapor pressure of approximately 0.5 Torr. While the pressure may be higher than that of natural environments, the growth time scale of the particles is shorter than in natural environments, i.e., longer time scales to form natural particles at lower pressure (lower collision frequency) may simply be equivalent to the higher pressure (higher collision frequency) and shorter time scale in experiments. A 15 min run produced about 3 g of smoke sample. The sample was dark gray, indicating an excess of magnesium metal.

Silica smoke grains were also produced in order to see the thermal features of the reactions of pure silica. For the production of silica smoke, the magnesium metal was removed. The total pressure and furnace temperature were again controlled under the same conditions used to produce the Mg-bearing silicate smokes. The hydrogen, silane, and oxygen flow rates were 1000, 120, and 80 sccm, respectively.

Annealing Experiments Using Differential Scanning Calorimetry

Two to seven milligrams of the analog grains were placed into a Pt pan, and then heated in air, which is required for good thermal conductivity to detect thermal reactions, up to 1200 K at 1, 5, or 10 K min−1 simultaneously with another Pt pan filled with the same mass of alumina powder using a commercial DSC system (DSC 8270; Rigaku Corp.). The schematic is shown in Fig. 1. The alumina serves as a reference material and is inert. Since the temperature of the Pt pan containing silicate varies depending on the energy of the reactions caused by heating, such as oxidation, crystallization, a crystalline transition, or evaporation, we can measure the reaction energy of a process by measuring the input thermal energy required to keep the sample pan at an equilibrium temperature, which is measured by a thermocouple, with respect to the reference pan. When the process is an exothermic reaction, the DSC spectrum produces a positive peak, while in the case of an endothermic reaction the spectrum produces a negative trough. The reaction energy corresponds to the enthalpy of the reaction for the known sample mass. The base line of the spectrum corresponds to the difference of the heat capacities between alumina and the sample.

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Figure 1.  Schematic of the commercial differential scanning calorimetry (DSC) system (DSC 8270; Rigaku Corp.).

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Sample Analysis

Infrared spectra of the samples, embedded in KBr pellets, were measured with a Fourier-transform infrared spectrometer (Horiba Inc., FT-210). The infrared system utilized a KBr beam splitter and deuterated triglyceride sulfate detector. The wavelength resolution used for this work was 2 cm−1.

Samples of both the condensed and annealed smokes were mounted on holey amorphous carbon thin films or amorphous carbon thin films supported by standard 400-mesh Cu transmission electron microscope (TEM) grids. TEM observations were carried out using a JEOL 2010 TEM operated at an accelerating voltage of 200 kV at the University of New Mexico. The TEM was equipped with an ultra-thin window energy-dispersive X-ray (EDX) analysis system (Oxford ISIS 200 EDS systems) for in situ quantitative chemical analysis of very small grains such as interplanetary dust particles that are typically similar to the size of the laboratory particles produced in these experiments, or even smaller. The samples were also observed using a Hitachi H-7100R TEM operated at an accelerating voltage of 100 kV equipped with an EDX analysis system (Horiba Xerophy) at Ritsumeikan University and an H-8100 TEM operated at an accelerating voltage of 200 kV at Tohoku University.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedure
  5. Results
  6. Identification of Reactions Recorded in the DSC Spectra
  7. Infrared Spectra of the Samples
  8. Astronomical Implications
  9. Conclusion
  10. Acknowledgments
  11. References

Characterization of the Initial Sample

Figure 2 shows a typical TEM image and corresponding electron diffraction (ED) pattern of the initial magnesiosilica particles. The Bragg reflection rings in the ED pattern show that the original particles contain crystalline Mg2Si, MgO, and a small amount of metallic Mg. Crystalline silicates such as forsterite were not observed from ED patterns in the unannealed samples. Although amorphous phases typically exhibit ED patterns with one or two rings of very diffuse intensity (not sharp diffraction rings), identification of the amorphous phase by use of ED patterns is unrealistic in this case since the present sample contains many crystalline particles as well. However, the midinfrared spectrum of the unannealed samples clearly identified the presence of amorphous magnesiosilica from the characteristic 9 μm feature in Fig. 3a. The EDX analysis using an approximately 0.5 μm spot on 23 different random clusters of grains in the sample indicated that the mean atomic ratio of the sample was Mg:Si = 76.85:23.15 with a least-squares error of ±2.2 and with a distribution of Mg:70-87 and Si:30-13. The particles have spherical shapes, and based on measurements of a thousand particles, have a size distribution that ranges from 10 to 550 nm with a mean diameter of 50.8 ± 0.9 nm (Fig. 4). This size range is similar to or somewhat smaller than the sizes of individual components of IDPs (50–500 nm). Larger particles were usually metallic magnesium, as identified by selected area ED patterns and dark field images. Smaller particles are mixtures of magnesium oxide, magnesium silicide, and amorphous magnesiosilica.

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Figure 2.  Representative TEM image and corresponding ED pattern of as-prepared Mg-silicate grains. The ED pattern shows the presence of crystalline Mg2Si, MgO, and Mg. No forsterite grains were detected. Although amorphous silicate was not (and could not be) detected from the ED pattern, midinfrared spectra showed that it is also present in the sample, as shown in Fig. 3a. The EDX analysis using Horiba Xerophy indicated the atomic ratio of the region corresponding to this image as Mg:Si = 80:20.

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Figure 3.  Midinfrared spectra of Mg-silicate smoke particles before and after DSC runs at 1 K min−1 up to 1200 K in air. Each spectrum corresponds to the points indicated in Fig. 5a: a) initial, b) 630 K, c) 750 K, d) 900 K, e) 1000 K, f) 1050 K, g) 1200 K, h) commercial MgO powder, i) forsterite standard (Koike et al. 2003), j) and k) laboratory-synthesized clinoenstatite and orthoenstatite standards (Chihara et al. 2002). Each spectrum has been shifted for clarity.

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Figure 4.  Size distribution of magnesiosilica particles before and after annealing.

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Similar magnesiosilica smoke particles were produced previously using the same smoke generator (e.g., Rietmeijer et al. 2002a, 2002b, 2004; Hadamcik et al. 2007). These previous studies showed that the particles were composed of a mixture of amorphous and crystalline phases similar to our sample, with sizes similar to or smaller than those in our study (Rietmeijer et al. 2002a, 2002b; Hadamcik et al. 2007; Volten et al. 2007). Rietmeijer et al. (2002b) showed that amorphous magnesiosilica smokes had three very specific deep metastable eutectic compositions. Crystalline grains were pure MgO (periclase) and SiO2 (typically tridymite) and rare forsterite. Amorphous silica was often present as well. In the case of our present analysis, although MgO was detected, tridymite and forsterite were not detected in the initial sample. The presence of amorphous silica is also excluded as shown below. In addition, EDX analysis did not detect Si metal without Mg. The infrared spectra of the previous study showed a strong 9 μm feature attributed to silica and amorphous magnesiosilica (Rietmeijer et al. 1986), similar to the feature seen in the present study. Therefore, the main difference between the current and previous samples may be the amount of Si-O present.

Results of the DSC Analysis

We performed annealing experiments on synthesized magnesiosilica and silica smoke particles in the DSC at temperatures up to 1200 K. In the case of magnesiosilica particles, several characteristic exothermic peaks were observed in these spectra while annealing in air. Typical DSC spectra, which were measured in air at 1, 5, and 10 K min−1, are shown in Fig. 5. The spectra have been corrected by subtracting the spectrum using blank Pt pans and by use of a calibration curve, which was determined using the melting points and enthalpies in the DSC spectra of indium, tin, zinc, and silver. As a result, five characteristic features were observed over the range 591–607 K, 675–707 K, 740–779 K, 863–907 K, and 1065–1094 K. Although the peak positions have been labeled in the figure, departure points from the baseline are more important in a DSC spectrum, because they indicate the starting temperatures of the reactions. The peak position shows the temperature at the end of the reaction. The starting and peak temperatures and reaction energies (enthalpies) before and after annealing are shown in Table 1. Here, the tables do not show the exact enthalpies of each reaction, because the measured value depends on the mass fraction of the species that is responsible for the peak, and the samples are composed of several different kinds of particles. When the scanning rate of the annealing temperature is decreased from 10 to 1 K min−1, the peak positions are shifted by about 10–40 K, depending on the peak, toward lower temperature. More accurate initial reaction temperatures are obtained from the spectrum at 1 K min−1. On the other hand, the peaks of the spectrum become broader and the heights become smaller as the scanning rate decreases, because the reactions proceed more slowly and last for longer times, while the total reaction energies do not change. Note the smaller value of the vertical axis in Fig. 5a. As a result of heating at 5 and 10 K min−1, two significant sharp peaks at 894 and 907 K (or 903 K for 10 K min−1) can be seen. Those peaks are invisible due to the breadth of the peak obtained with slower reaction rates in the spectrum heated at 1 K min−1 as shown in Fig. 5a.

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Figure 5.  The DSC spectra of Mg-silicate smoke particles (4.3 mg) heated at a) 1, b) 5, and c) 10 K min−1 up to 1200 K in air. Left and right y-axes correspond to the spectral curves (black lines) and linear lines for temperature (gray lines), respectively. The numbers above the features indicate the peak temperatures. The spectra have been corrected for blanks and calibrated using the melting points and enthalpies in the DSC spectra of indium, tin, zinc, and silver. The labels a)–g) correspond to the midinfrared spectra indicated in Fig. 3: a) initial, b) 630 K, c) 750 K, d) 900 K, e) 1000 K, f) 1050 K, and g) 1200 K.

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Table 1.   Starting and ending temperatures of the reaction and reaction energies of each feature in the DSC spectra shown in Fig. 5.
Heating velocity591–607 K feature675–707 K feature740–779 K feature863–907 K feature1065–1094 K feature
Start (K) (J g−1)Start (K) (J g−1)Start (K) (J g−1)Start (K) (J g−1)Start (K) (J g−1)
End (K)End (K)End (K)End (K)End (K)
  1. aDouble peaks at 894 and 907 K with reaction energies of 6.91 and 1.78 J g−1, respectively.

  2. bDouble peaks at 894 and 903 K with reaction energies of 26.29 and 18.00 J g−1, respectively.

1 K min−1477727.916273527.571822.29805106.25100374.66
5916757408631065
5 K min−1424321.676353772.673681.7284350.131023113.29
596693763a1080
10 K min−1429249.776394151.074895.4386792.921034160.22
607707779b1094

Identification of Reactions Recorded in the DSC Spectra

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedure
  5. Results
  6. Identification of Reactions Recorded in the DSC Spectra
  7. Infrared Spectra of the Samples
  8. Astronomical Implications
  9. Conclusion
  10. Acknowledgments
  11. References

In order to understand the origin of each exothermic peak, annealing experiments on amorphous magnesiosilica were performed by heating up to 630, 750, 900, 1000, and 1050 K at 1 K min−1. The minerals detected using TEM and infrared spectroscopy in each temperature range are listed in Table 2. Experiments with pure silica were also performed up to 1200 K at 5 K min−1 to determine the contribution of silica in the sample. Since, in the case of the silica experiments, only one broad feature was observed at 818 K in 5 K min−1 runs (Fig. 6), the features seen in Fig. 5 can be completely attributed to the reactions of magnesiosilica grains. Since the mass of the magnesiosilica sample increased by about 14% after annealing up to 1200 K, it can be assumed that the sample oxidized during annealing. The increase in mass was seen after annealing to 750 K and the mass did not change between 750 K and 1200 K. Therefore, we assigned the origin of the 675–707 K feature to oxidation of Mg2Si. Indeed, TEM observations showed that the ED pattern corresponding to Mg2Si crystallites was hardly seen in the samples annealed at 750 K (Fig. 7c). Since an increase in mass was not observed after oxidation of magnesium (the peak at 894 K), we expect that the amount of Mg is less than 1% by mass. Although these samples were oxygen deficient before annealing, the sample can be treated as an oxygen-enriched silicate after the oxidation reaction.

Table 2.   Detected minerals using TEM and infrared (IR) techniques at each annealed temperature.
Annealing temperatureMgMgOMg2SiAmorphousForsterite
TEMIRTEMIRTEMIRTEMIRTEMIR
(a) As-preparedYN/AYYYN/AN/AYNN
(b) 630 KYN/AYYYN/AN/AYNN
(c) 750 KNN/AYYNN/AN/AYNN
(d) 900 KNN/AYYNN/AN/AYNN
(e) 1000 KNN/AYYNN/AN/AY Stall
(f) 1050 KNN/AYYNN/AN/ANYY
(g) 1200 KNN/AYYNN/AN/ANYY
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Figure 6.  The DSC spectrum of amorphous silica smoke particles (4.3 mg) heated at 5 K min−1 up to 1200 K in air. Left and right y-axes correspond to the spectral curve and linear line for temperature, respectively. The peak temperature is at 814 K. The spectrum has been corrected for blanks and calibrated using the melting points and enthalpies of indium, tin, zinc, and silver.

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Figure 7.  Representative TEM images and the corresponding ED patterns of Mg-silicate particles annealed to b) 630, c) 750, and g) 1200 K, in air, respectively. The labels correspond to the spectra in Fig. 3 and annealing points in Fig. 5a. The ED patterns show the existence of Mg2Si, MgO, and Mg in (b), MgO in (c) and MgO and Mg2SiO4 in (g).

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From the TEM observations, it was determined that the mean grain diameter of the samples increased from 50.8 ± 0.9 to 65.3 ± 1.2 nm after annealing to 630 K at 1 K min−1. The exothermic reaction energy in the 591–607 K feature increases as the annealing rate decreases as seen in Table 1. Because slower annealing allows more time for the fusion of grain surfaces, we believe that the origin of the feature at 590 K is due to fusion of grains in the sample. This also suggests that the time scale for fusion is slower than the time scale for oxidation. The increase in the mean diameter shows that approximately 2.1 particles coalesced to form each larger particle. On the other hand, the origin of the shoulder on the higher temperature side of the 591–607 K feature is still unknown, as is the origin of the 907 and 903 K features.

Crystallization of forsterite produces the 1065–1094 K feature. It has been reported that the crystallization of forsterite occurs after the stall state, which is a metastable intermediate state between amorphous and crystalline forsterite observed in the infrared spectra of magnesiosilica particles that were produced using the same apparatus as our sample. The stall was observed during annealing (Hallenbeck et al. 2000), but this phenomenon has not been confirmed in other experiments (Fabian et al. 2000; Thompson and Tang 2001). It is possible that nucleation and/or partial crystallization of forsterite occurs on the surfaces of individual grains before the later growth of crystallites within the grains (Kamitsuji et al. 2005). The reaction energy of crystallization decreases from 160.22 J g−1 at 10 K min−1 to 74.66 J g−1 at 1 K min−1, and we believe that the difference in the reaction energy between those two runs is a result of nucleation or crystallization corresponding to the stall state. Indeed, the infrared spectra of the samples heated to 1000 K show a significant feature caused by forsterite in addition to amorphous magnesiosilica (Fig. 3e) in spite of heating to a point lower than the crystallization temperature. This result is in agreement with the annealing experiment of similar amorphous magnesiosilica particles by Hallenbeck et al. (2000).

The formation of forsterite nanocrystallites on the surface of an amorphous magnesiosilica particle was captured by live observation during heating in a TEM (Kamitsuji et al. 2005). Figure 8 shows that crystallization on the surface of the particles was also seen in the present study. During annealing, brighter contrasts appeared over parts of the surface, and then a surface layer 5–10 nm in thickness was observed at 1000 K, as indicated by arrows in Fig. 8e. At 1200 K, several forsterite crystallites have grown at the surface of the particles as indicated by arrows in Fig. 8g. This crystallization process may correspond to the stall state. When the surface of magnesiosilica crystallizes to forsterite, spaces or voids are formed between the surface forsterite and interior amorphous silicate due to the differences in their molar volume (densities). Thermal conductivity decreases at the voids and as a result, crystallization of forsterite can be suppressed. If the composition of the starting magnesiosilica particles is intermediate between forsterite and enstatite, excess SiO in the inner part of the grains may evaporate and the composition of the interior may approach a forsteritic composition. Finally, we observe that entire magnesiosilica particles become crystalline forsterite.

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Figure 8.  Representative TEM images of analog particles after annealing at e) 1000 K and g) 1200 K. The labels correspond to the spectra in Fig. 3 and annealing points in Fig. 5a. The regions of bright contrast inside the surface layer can be seen in (e), as indicated by arrows. Crystallites of forsterite can be seen at the surface of the particles in (g), as indicated by arrows.

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Infrared Spectra of the Samples

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedure
  5. Results
  6. Identification of Reactions Recorded in the DSC Spectra
  7. Infrared Spectra of the Samples
  8. Astronomical Implications
  9. Conclusion
  10. Acknowledgments
  11. References

The reaction energies (enthalpies) required for the evolution of the silicate spectrum can be understood by measuring the infrared spectra before and after each thermal peak in the DSC spectra. In order to observe the midinfrared spectra of the samples over several different temperatures, annealing experiments of magnesiosilica grains were performed up to 630, 750, 900, 1000, and 1050 K at 1 K min−1. In addition to the spectra of the unannealed samples and those annealed at 1200 K, five infrared spectra were taken after the characteristic DSC features shown in Fig. 3 were measured. From these spectra, it is obvious that the feature at 1065–1094 K corresponds to the energy of crystallization of forsterite. It was also found that the 10 μm feature attributed to amorphous magnesiosilica barely changes up to 900 K. The shape of the spectra after annealing up to 1000 K is close to that of the stall state (Hallenbeck et al. 1998). Two bumps in the 10 μm feature of magnesiosilica in the stall state roughly correspond to those observed in comets (Hallenbeck et al. 1998). Although the 10 μm feature shows little change up to 900 K, it rapidly changes to the crystalline feature of forsterite as annealing temperatures approach the peak at 1065 K. This is consistent with the crossover temperature predicted in previous annealing experiments (Hallenbeck et al. 2000) where there is no stall expected at 1067 K and the total annealing time at that temperature is on the order of minutes.

Astronomical Implications

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedure
  5. Results
  6. Identification of Reactions Recorded in the DSC Spectra
  7. Infrared Spectra of the Samples
  8. Astronomical Implications
  9. Conclusion
  10. Acknowledgments
  11. References

Since the gas flow rates of silane and oxygen were 55 and 110 sccm, respectively, the atomic ratio of silicon and oxygen in the gas atmosphere was about 1:4, similar to the atomic ratio of silicon and oxygen of forsterite (Mg2SiO4). Nevertheless, a moderate amount of Mg2Si exists in the sample, because oxygen may react with hydrogen to make water or with magnesium to make MgO particles in addition to silicates. The presence of very small amounts of water in condensed MgSiO smokes has been demonstrated (Rietmeijer et al. 2004). Similar situations can be considered in the outflows of evolved stars. If silicates condense homogeneously in the gas ejecta of evolved stars, many minerals, such as forsterite, enstatite, magnesium oxide, and magnesium silicide in addition to magnesiosilica, could form under supersaturated conditions at sufficiently high temperatures. In such a disequilibrium environment, chemical reactions should affect nucleation as these reactions will control the instantaneous abundances of the species in the gas phase. Nevertheless, since magnesium-rich amorphous silicates have been detected around evolved stars by observation and there is no evidence for Mg2Si, we focus here on the thermal evolution of amorphous magnesiosilica particles after the oxidation of Mg2Si.

The present experimental results suggest that the percentage of olivine in the gas ejecta of a high-mass loss AGB star, typically 10–15%, may reflect the percentage of olivine that formed by heterogeneous condensation. For example, condensation temperatures of tungsten and nickel are 1390 and 1030 K, respectively, for 103 yr of the cooling time at 10−5 atm, for a solar composition gas (Tanaka et al. 2002). In the case of substrate free (homogeneous) condensation, since the condensation temperature must be considerably less than the equilibrium condensation temperature (and can be much lower than the crystallization temperature depending on the concentration of SiO: Nuth and Ferguson 2006), condensates remain in the amorphous phase. The difference between the condensation and equilibrium temperatures is the driving force that overcomes the barrier to forming a solid phase from a gas: condensation and growth velocity are much faster when the system is supersaturated than they are at the equilibrium temperature. Thus, in rapidly condensing and growing systems products tend to form in the amorphous state. In the case of the primitive solar nebula, since there are many solid particles present in the gas, the fraction of crystalline olivine might increase as a result of heterogeneous condensation after evaporation, because crystalline silicates can condense on refractory particles at temperatures higher than the temperature (1003 K) required for crystallization by annealing of amorphous grains. Accordingly, most solar system crystalline silicates in cometary IDPs and Wild 2/Stardust probably formed by condensation (and growth or oxidation) in the solar system (Messenger and Keller 2005; McKeegan et al. 2006).

The laboratory synthesized silicate particles are several tens of nanometers in diameter. The particles were annealed in a manner that could potentially correspond to the thermal evolution of protostellar dust particles evaporated near the protostar and recondensed in the bipolar outflow (Patel et al. 2000) and/or evaporated by shockwave heating and recondensed behind it (Kimura et al. 2008; Miura et al. 2010). The results of our annealing experiments show that amorphous silicate dust has never been heated to more than 1003 K in an oxidizing atmosphere (after it recondensed from the vapor), because amorphous silicate particles quickly crystallize above this temperature. Although this result is in agreement with previously reported annealing experiments (Hallenbeck et al. 1998), the temperature corresponding to the stall state in our experiments was about 30 K lower than in these previous experiments. This difference between the crystallization temperatures may be explained by the degree of oxidation that occurred in each set of experiments, i.e., by the different atmospheres (in air or in vacuum) used during the annealing. The initial magnesiosilica particles are completely oxidized during annealing and will have compositions corresponding to the Mg2SiO4 composition. Indeed, Fabian et al. (2000) performed a similar annealing experiment in an oxygen atmosphere of 1 atm and obtained crystalline silicates at 1000 K, which is very close to our result (1003 K). Our results imply that crystallization of forsterite from amorphous grains depends to some degree on the atmosphere in which heating occurs. During annealing, enstatite and crystalline SiO2 particles were not detected either by TEM observations or infrared measurements in contrast to previous studies which suggest that enstatite and tridymite form during thermal annealing (Rietmeijer et al. 2002a). This difference may be due to the difference in experimental conditions during the formation of the initial particles, i.e., the amount of SiO molecules in our experiment may be smaller than in the previous study.

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedure
  5. Results
  6. Identification of Reactions Recorded in the DSC Spectra
  7. Infrared Spectra of the Samples
  8. Astronomical Implications
  9. Conclusion
  10. Acknowledgments
  11. References

Annealing experiments of amorphous magnesiosilica cosmic dust analogs showed that grain fusion and oxidation take place before the crystallization of forsterite. In addition, nucleation and/or the formation of nanocrystallites of forsterite produce a significant peak in the infrared spectrum, which corresponds to that observed in the spectra of comets, due to partially crystallized forsterite. This characteristic spectrum was obtained by annealing in air at 1000 K, which is about 30 K lower than previously reported for annealing experiments done in vacuum. The crystallization temperature of forsterite was determined to be 1003 K under oxidizing conditions using DSC and the reaction energy (enthalpy) is at least 160.22 J g−1.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Procedure
  5. Results
  6. Identification of Reactions Recorded in the DSC Spectra
  7. Infrared Spectra of the Samples
  8. Astronomical Implications
  9. Conclusion
  10. Acknowledgments
  11. References

Acknowledgments— Adrian J. Brearley and Ying-Bing Jiang provided technical support for TEM analysis at the University of New Mexico. Grants from the Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowships for Research Abroad from April 2004 to March 2006 and from NASA’s Cosmochemistry Research and Analysis Program made these studies possible. This work was also supported in part by a Grant-in-Aid for Young Scientists (Start-up) from KAKENHI (19840048) of JSPS, by Tohoku University GCOE program for “Global Education and Research Center for Earth and Planetary Dynamics,” and by the “Program Research” in the Center for Interdisciplinary Research, Tohoku University, Japan. Joseph Nuth is grateful for support from the NASA Cosmochemistry Program. We thank Christine Floss for useful suggestion and A. Brearley and H.-P. Gail for careful and constructive reviews.

Editorial Handling— Dr. Christine Floss

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  9. Conclusion
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