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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methodology
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Abstract– Samples returned by the Stardust mission from comet 81P/Wild 2 provide an unequaled opportunity to investigate cometary formation and evolution. Crystalline silicates have been identified in impact craters in Stardust Al foil, yet their origin is ambiguous. They may be original cometary components, or they may have grown from melt generated by impact. We have now studied experimental impacts of the calcium silicate mineral wollastonite, using scanning and transmission electron microscopy to document the relationship between impact feature shape and crystal lattice orientation in impact residue. Wollastonite can have a characteristic acicular habit, forming crater shapes that indicate crystal orientation upon impact. From extracted impact residue, we determined the lattice orientation of crystalline material for comparison with the whole particle orientation. We assume that crystallization from melt, without surviving seed nuclei, should result in randomly oriented crystallite growth, with no preferred direction for individual crystals. However, we find that the majority of crystalline material in the residue retains b-axis orientation parallel to the long axis of the crater form. This, together with impact parameter calculations and lack of Al incorporation by the residue (suggesting melting did not occur), indicates that these crystals and, by analogy, the majority of Al-free crystalline silicates in Stardust foil, are surviving remnants of the impactor. Furthermore, amorphous wollastonite residue probably did not form via melting and subsequent quenching, but instead by high-pressure amorphization or degradation of unquenchable phases. Finally, one crystal studied appears to be a new high-pressure/temperature polymorph of CaSiO3, indicating that such polymorphs may be observed in Stardust residues in craters.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methodology
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Stardust, a NASA sample return mission to comet 81P/Wild 2, provided an unequaled opportunity to analyze particles from a known cometary source in terrestrial laboratories. These materials were captured via impact into silica aerogel cells and Al foils as the spacecraft flew through the comet’s coma. Since its return, many grains captured in the aerogel and residues from numerous craters in Stardust Al foils have been analyzed by transmission electron microscopy (TEM). They commonly contain a mixture of crystalline and amorphous material (e.g., Leroux et al. 2008, 2010; Stroud et al. 2010). It has been tentatively assumed that crystalline grains are relict cometary dust, and amorphous material is shock-induced melt that subsequently underwent rapid quenching. However, the manner in which these materials were collected (via impact with perpendicular incidence at approximately 6.1 km s−1, Brownlee et al. 2003) means that it is important to know whether the impact process might also be responsible for the production of at least some of the crystalline material. To make interpretations of preaccretional or parent body processes, we require knowledge of complete impactor mineralogy, that is, chemistry and structure. Therefore, while this uncertainty remains, the value of these samples to our understanding of cometary grains remains unclear. Determining their value requires that we understand the impact process and be able to distinguish those materials inherent to the impactor from those that are the product of its capture.

The Stardust encounter conditions were a well-constrained, relatively low velocity of approximately 6.1 km s−1 and a perpendicular angle of impact incidence (Brownlee et al. 2003). Such conditions can be replicated in the laboratory using a two-stage light-gas gun (LGG). We can therefore investigate the effects of capture on the Wild 2 cometary grains by taking known, well-characterized minerals, impacting them into flight spare Stardust collection media and making direct comparisons between the preimpact and postimpact materials.

A large number of analyses have been conducted on laboratory impact analogs of flight spare Al foils and have shown them to be indispensible in the investigation of original impactor properties such as size, density, gross morphology, and even bulk chemistry (e.g., Kearsley et al. 2006, 2007, 2008, 2009a; Wozniakiewicz et al. 2009, 2011; Price et al. 2010). Through these investigations, it has become clear that the Al foils are a valuable and complementary collection medium to the aerogel. Unlike in aerogel, where captured material is often spread along the length of tracks and volatiles are seen to diffuse into surrounding aerogel (e.g., Ishii et al. 2008), the material captured by the foils is kept highly localized in the form of impact residues that line the impact craters. While molten aerogel can mix with impacted materials and modify original Si ratios (e.g., Leroux et al. 2008b), there is no significant external source of silicate in the foil (the Al foils contain <<1 wt% Si, Kearsley et al. 2007). Aerogel is an exceptionally effective thermal insulator and, as such, serves to maximize and prolong the effect of impact heating, while Al acts as a heat sink conducting heat away very quickly. However, one of the possible criticisms of interpreting mineralogy of material preserved on the Al foils is that their crystalline structure may not be a true primary feature, and may have been not only modified by the impact process, but even grown from impact melt.

Analyses of Wild 2 samples indicate that the two predominant mineral components of this comet are anhydrous silicates and sulfides (e.g., Hörz et al. 2006; Zolensky et al. 2006). Previous impact analog work on sulfides by Wozniakiewicz et al. (2011) found that shock may induce melting in these minerals when impacted into foils, generating new crystalline material via subsequent recrystallization. Melting of the sulfide results in loss of S producing impact generated crystalline material with telltale S-depleted, nonstoichiometric Fe to Ni to S ratios. Similar investigations into anhydrous silicates (Wozniakiewicz et al. 2012) and previous Raman studies (Burchell et al. 2008) show that stoichiometric crystalline material remains in silicate impact analog residues. However, the most common silicates of cometary interest (olivines and pyroxenes) do not usually contain the abundant markedly volatile elements whose depletion might be used as an indicator of volatile loss during melting. In fact, minor elements such as Mn were found in high quantities in Stardust olivines (e.g., Zolensky et al. 2006). However, there is very little evidence for elemental fractionation between major and minor elements of very differing volatility in impacts of silicate glasses (such as the basalt powder experiments of Kearsley et al. 2007). It therefore seems that even the retention of apparently original compositional stoichiometry in mafic silicate crystals is insufficient to rule out their origin by melting and subsequent recrystallization even if this is somewhat unlikely, given the rapid quench rates expected for comet dust captured on the Stardust Al foils.

Over the past few decades, a variety of collection media have been exposed in low Earth orbit, capturing materials passing through the local Earth environment (see Graham et al. [2001]; Kearsley et al. [2009b] and Wozniakiewicz et al. [2009] for more complete discussion of previous LEO impact work). In foils exposed on the Long Duration Exposure Facility (LDEF), crystalline silicates were found that exhibited cosmic ray tracks (e.g., Brownlee et al. 1993), providing evidence that these are preserved (unmelted) interplanetary dust particles. However, the unknown impact conditions (e.g., possibility of low impact velocity and oblique angle of incidence on LDEF) mean that these findings may not be a reliable indicator of the probability for survival of crystalline material on Stardust foils. Nevertheless, they do suggest that structural properties could be used as indicators of surviving materials. Therefore, it was the aim of this study to investigate preimpact and postimpact samples of a silicate with some distinguishing structure whose retention can be used as an indicator of survival. There is no more fundamental structural characteristic in a mineral than the crystal lattice itself, and here we employ lattice orientation to distinguish secondary, isotropic crystallite growth in impact melt, from the survival of solid remnants to demonstrate the extent to which original crystalline material may survive the capture process on Stardust Al foil.

Wollastonite is an anhydrous silicate typically formed through thermal metamorphism (Deer et al. 1992) and has a melting temperature (approximately 1544 °C/1817K after an initial phase change to pseudowollastonite [Osborn and Schairer 1941]) that lies within the range of those for silicates commonly identified in comet 81P/Wild 2 samples. It commonly occurs as characteristic acicular or “needle-like” crystals (Deer et al. 1992) and, from Raman analyses of previous LGG impacts, it is known to generate residues that contain crystalline material (Burchell et al. 2008). Wollastonite craters can have a range of morphologies that are indicative of crystal orientation upon impact (Kearsley et al. 2008): vertically impacting wollastonite needles generate deep, “bowl-shaped” craters, while horizontally impacting needles create shallow, “boat-shaped” craters elongated in the same direction as the impactor (Fig. 1). We prepared TEM sections taken perpendicular to the direction of elongation in both an unshot wollastonite projectile powder grain and a resulting crater and compared crystal orientations to determine whether impact residue crystals were original as preserved residue crystals should exhibit the same orientations as those in the projectile section.

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Figure 1.  The relationship between wollastonite projectile orientation upon impact and resulting crater shape. Vertical impacting needles generate deep circular bowl-shaped craters, while angled impacting needles form more complex, multipit craters and horizontal impacting needles create shallow elongated boat-shaped craters.

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Samples and Methodology

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methodology
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Sample Production

The wollastonite impact analog foils were generated using the two-stage light-gas gun (LGG) in the Centre for Astrophysics and Planetary Sciences at the University of Kent, Canterbury. Details of the powder shot and velocity measurement techniques employed here can be found in Burchell et al. (1999). In each shot, the target chamber was evacuated to a few ×10−1 mbar, or better, to minimize velocity loss in flight.

The target was a sample of flight-spare Stardust Al foil (provided by F. Hörz of NASA, JSC), an Al 1100 series alloy approximately 101 μm thick (Kearsley et al. 2007). For the experimental shots, this foil was wrapped around a 1 mm thick square Al alloy plate (Al 6082) measuring approximately 1.5 × 1.5 cm, to simulate the mounting on the Stardust collector. The plate was then held with conductive adhesive putty onto an Al base-plate (10 × 10 cm), drilled with a hole in each corner for support within the target chamber and to enable handling throughout the shot and subsequent analysis without damage or contamination to the craters and their residues. Throughout this preparation procedure, gloves and clean pliers were used to avoid contamination of the foil.

The wollastonite specimen was provided from collections at the Natural History Museum, London (specimen awaiting cataloguing). It was prepared as a projectile powder by crushing a small subsample with a pestle and mortar, and passing through a 53 μm sieve to remove coarser grains. These particles, impacted at a velocity of 5.92 km s−1, generated craters up to approximately 250 μm in diameter (consistent with the particle–crater diameter correlation by Kearsley et al. 2007). The resulting wollastonite foil was catalogued as shot G190706#2.

Analytical Instrumentation and Sample Preparation and Analysis

Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) was used to locate an elongated crater and identify regions of residue within it (Fig. 2). The SEM imaging and EDX analyses for this investigation were primarily conducted using the JEOL 5900 LV SEM in the Electron Microscopy and Mineral Analysis Division (EMMA) of the Department of Mineralogy at the Natural History Museum, London (NHM). EDX microanalyses were obtained through an Oxford Instruments X-sight Si(Li) EDX microanalyzer, running INCA software (version 16). Additional imaging was conducted using the JEOL JSM-7401F SEM at Lawrence Livermore National Laboratory (LLNL).

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Figure 2.  The chosen elongated wollastonite crater. A) SEM secondary electron image. B) Higher magnification of region in (A), showing presence of needles with long axes close to the same direction as the crater elongation. C–E) SEM-EDX maps for O, Si, and Ca respectively, showing the presence of abundant residue. These maps exhibit shadows as a result of restricted line-of-sight from portions of the crater floor to the detector due to the extreme topography of the crater.

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A focused ion beam (FIB) was then used to prepare electron-transparent sections of the preimpact and postimpact samples for analysis by the transmission electron microscope (TEM). The FIB sections were produced using the FEI Nova600 NanoLab dual-beam FIB microscope at LLNL. This method utilizes a micromanipulator needle to move the cut section from the bulk sample to the TEM grid for thinning and analysis and therefore requires that surface topography be minimal. Unshot wollastonite projectiles were embedded in epoxy, polished down and a suitably orientated needle (with elongated axis parallel to the surface) identified. Previous investigations into impact residues also employed embedding foils edge on in epoxy and polishing down until a crater cross section, and therefore residue, outcropped on the surface (Wozniakiewicz et al. 2011, 2012). However, as directional information was of upmost importance, this method could not be used and access to the residue by FIB was made possible by flattening crater rims by hand (Fig. 3). This permitted the micromanipulators to reach into the crater and extract the section. Both sections were prepared perpendicular to the direction of needle elongation so that subsequent TEM observations should be looking down the elongation axis if the crystal structure, and therefore orientation, has been preserved (see Fig. 4).

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Figure 3.  Pre-FIB preparation. Secondary electron images of the crater from above and either side before (top row) and after (bottom row) the rims were flattened to allow access to the residue/needles by FIB.

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Figure 4.  Samples prepared via FIB. A and B) SEM secondary electron images of the projectile (A, preimpact sample) and crater (B, containing postimpact residue). White arrows indicate the direction of needle elongation in each case. C and D) Close-up of locations on projectile (C) and crater (D) where sections were taken (black dashed boxes in A and B). Both preimpact and postimpact sample sections were extracted perpendicular (dashed white lines) to the direction of needle elongation (solid white arrows). E and F) TEM sections prepared by FIB from projectile (E) and crater (F). Subsequent TEM analyses look down the axis of elongation.

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The FIB sections were then analyzed using the 300 kV FEI TITAN field emission gun (FEG) monochromated, aberration-corrected (scanning) transmission electron microscope ((S)TEM) at LLNL. Both sections were imaged in bright field (BF) and dark field (DF) TEM. TEM-EDX atomic% data was obtained using an EDAX Genesis 4000 Si(Li) energy-dispersive X-ray spectrometer system running Emispec ES Vision (FEI Co., version 4.1) for projectile and residue bulk chemistry comparisons, and selected area electron diffraction patterns were obtained to identify and compare the crystal structures and orientations.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methodology
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Scanning electron microscopy imagery of the floor of the elongated crater revealed the presence of needle-like features (several micrometers in size, see Fig. 2B) amongst the areas shown by SEM-EDX maps to be rich in Ca-, Si-, and O-rich residue (see Fig. 2C–E). There is minor angular dispersion caused by the impact, but the long axes of the majority of these needles are aligned close to the overall long axis of the crater itself and therefore closely match the orientation of the impacting crystalline needle: 90% of the needle-like features have their long axes within ±30° of the crater long axis, with >60% being within ±10° (see Fig. 5). This macroscopic orientation alone is highly suggestive of survival of original crystalline material. For confirmation, we also require determination of crystallinity and subsequent electron diffraction indexing.

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Figure 5.  Orientations of needle-like feature observed in the chosen elongated crater. A) SEM secondary electron image of chosen elongated crater. B) Higher magnification SEM secondary electron image of area in (A) (white dashed box) showing location containing needle-like features. C) Orientation map of needle-like features in (B). A color-coded line represents each needle. Directions relative to the long axis of the crater (and hence the long axis of the impactor upon impact) are indicated inset in all images: <10° from crater long axis in black, between 10° and 20° in dark gray and 20° to 30° in light gray. Needles with long axes more than 30° from the crater long axis are shown in white and appear to occur along the periphery of two large (>10 μm across) mounds on the crater floor, therefore if these are original crystals, their orientation may result from slumping around these features.

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The wollastonite preimpact and postimpact sections were first imaged in bright and dark field (BF TEM images shown in Fig. 4E and 4F, and in more detail for the residue section in Fig. 6). The projectile section is completely crystalline while the residue is composed of a mixture of crystalline and amorphous material (Fig. 6). The location of crystalline material includes those regions where needles were previously observed in SEM. In addition, this material appears concentrated on the surface of the residue, where it may correspond to late-arriving rear-projectile material, which is likely to be less heavily shocked (e.g., Pierazzo and Melosh 2000).

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Figure 6.  Bright field TEM image of the wollastonite residue section. Upper right corner: a map highlighting the location of Al foil (spotted), protective Pt (diagonal stripes) and crystalline (black) and amorphous (gray) residue.

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Transmission electron microscopy EDX analyses were then obtained to confirm the bulk chemistry of the preimpact projectile for comparison with that of the postimpact residue. During the collection of EDX spectra, the beam was focused down to an area <200 nm in diameter allowing for separate compositional analysis of amorphous and crystalline regions in the residue. Figure 7 is a ternary diagram displaying the atomic% data for Si, O, and Ca, and shows that both preimpact and postimpact compositions are close to the theoretical composition of wollastonite (indicated by the white cross). The minor variations in O are attributable to X-ray absorption effects resulting from variable section thickness: the multicomponent nature of the postimpact samples (resin, foil, crystalline residue, amorphous residue, etc.) leads to varied milling rates and therefore complicated, varied thickness residue sections. A small number of the TEM-EDX spectra were found to exhibit minor Al peaks. We have previously observed the incorporation of Al (either completely intermixed or as discrete nm-sized Al metal beads depending on its miscibility in the melt) in silicate and sulfide residues where the temperatures generated by impact have been estimated to exceed their melting temperatures (Wozniakiewicz et al. 2011, 2012). We have also observed Al in the silicate glasses of anhydrous silicate-bearing aggregate residues where melting occurs, either in the form of flux melting or due to increased temperatures that may result from the smaller grain size and multiple impacts involved (Wozniakiewicz et al. 2012). We therefore attempted to obtain a STEM-EDX map over the residue to determine whether the wollastonite residue had also incorporated Al. However, this resulted in electron beam damage to amorphous regions due to higher local current densities. Nevertheless, the few STEM-EDX spectra obtained from both crystalline and amorphous residue components were Al-free, and high-resolution imaging showed no sign of discrete nm-sized Al beads in any part of the residue. This indicates that the impactor did not incorporate Al on impact, which suggests that melting did not occur. Those TEM-EDX spectra that exhibited minor Al peaks are therefore likely the result of proximity and interference of Al foil as we note that they were obtained close to the residue Al foil boundary.

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Figure 7.  Bulk chemistry of preimpact and postimpact (projectile and residue) wollastonite. The ternary diagram displays O, Si, and Ca atomic% data from TEM-EDX analyses of projectile and residue samples. Preimpact and postimpact compositions are close to the theoretical composition of wollastonite (white cross), albeit with minor variation in O attributable to variable section thickness. Lines of constant Si:Ca, O:Si, and Ca:O are from the average measured projectile composition.

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Selected area electron diffraction patterns were obtained from the preimpact projectile section and were indexed as triclinic wollastonite, with acicular needle elongation along the b-axis (Fig. 8). In general, patterns obtained from the residue section were also consistent with triclinic wollastonite with b-axes that were within a few degrees of each other and in the same orientation as the direction of crater elongation (Fig. 9A). No diffraction patterns were obtained for other orientations of wollastonite in the residue section. We interpret this as evidence that the residue crystals largely retained original impactor orientation with their elongated b-axes parallel to that in the preimpact projectile. In addition, there is evidence of partial crystallographic modification in the postimpact crystals including blurring of diffraction pattern spots, consistent with the effects of the stress experienced during impact. It is likely that these materials remained crystalline throughout the impact process, although much of the impactor material was amorphized, and minor rotation of remaining superficial needle shapes occurred during the final stages of crater excavation. From this structural evidence alone we cannot entirely rule out the possibility that the observed crystals grew upon small crystalline nuclei surviving within an extensive fluid melt. However, we can say that it is very unlikely that crystals as large as the needle-like features observed by SEM (typically, 5–10 μm in length and 1–2 μm in width) would grow from a melt under quench conditions: Crystal growth on this size scale for silicates from seed nuclei requires quench times on the order of seconds (e.g., forsterite; Finch and Clark 1971), but calculations for our previous Stardust impact analogs (Wozniakiewicz et al. 2011) suggest that cooling times are on the order of 10−5 s. Considered together with the observation that Al was not incorporated, which suggests that melting did not occur, it appears that a (partial) melt origin would not have been possible. Estimates of the peak pressure Ppeak and peak postshock temperature Tpps (the temperature of the decompressed shocked material that results from waste heat deposited after shock and release) experienced by the wollastonite impactor are also useful in predicting the occurrence/extent of melting. Utilizing Hugoniot data for wollastonite and Al 1100 (Marsh 1980) in the graphical method outlined in Melosh (1989), Ppeak was determined to be approximately 69 GPa. Tpps was estimated to have been approximately 1700K (to the nearest 100K) using the method described by Artemieva and Ivanov (2004) and Fritz et al. (2005) with available Hugoniot data (Marsh 1980) and specific heat capacity data (derived from Krupka et al. 1985). See Wozniakiewicz et al. (2011) for an example of calculating Ppeak and Tpps for pyrrhotite in this manner. These calculations therefore suggest that Tpps approached, but may not have exceeded, the melting temperature of wollastonite (approximately 1800K). Together with the composition and diffraction evidence, this indicates that the crystals within the residue represent surviving impactor mineralogy and also that the bulk of the amorphous material (discussed later) does not represent solidified melt.

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Figure 8.  Wollastonite projectile section. Left: Bright field TEM image. Right: Projectile electron diffraction pattern indexed as an [010] orientation of triclinic wollastonite.

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Figure 9.  Wollastonite residue electron diffraction data. A) The location of several crystals in the residue indexed as triclinic wollastonite and in the same orientation as the original projectile (looking down the b-axis). A1–4) Diffraction patterns from locations 1–4 in (A). B) The location of the crystal identified as a possible new shock-generated polymorph of wollastonite. B1) The diffraction pattern from location 1 in (B).

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One diffraction pattern obtained from the wollastonite residue, however, does not match the projectile (Fig. 9B); While it matches the “c-axis” of the triclinic wollastonite projectile, the “a-axis” seems to have undergone 30% compression. This crystal appears to be an impact-generated high-pressure/temperature polymorph of wollastonite. Many experimental and theoretical ab initio investigations into wollastonite and its temperature-pressure phase transitions have been undertaken (e.g., Osborn and Schairer 1941; Essene 1974; Gasparik et al. 1994; Akaogi et al. 2004; Jung and Oganov 2005; Komabayashi et al. 2007) due to interest in their role as Ca-bearing constituents of the Earth’s lower mantle and transition zone (Ita and Stixrude 1992; Fiquet 2001). At low pressure (approximately 100 kPa) and temperatures above approximately 1125 °C/1398K, wollastonite transforms to pseudowollastonite before melting at approximately 1544 °C/1817K (Osborn and Schairer 1941). Data of Essene (1974) show that the wollastonite-pseudowollastonite transition slope continues up to approximately 1600 °C/1873K at approximately 2.5 GPa, where over a brief pressure range of approximately 0.2 GPa wollastonite transforms directly to melt. High-pressure experiments conducted at room temperature find that wollastonite begins amorphizing above approximately 10 GPa, becoming almost completely amorphous above approximately 20 GPa and remaining amorphous after pressure is released (Tamai and Yagi 1989). However, experimental data for increasing pressure at temperatures >600 °C/873K find that wollastonite transforms into the walstromite structure (wollastonite II) at approximately 3 GPa (Essene 1974), then dissociates into larnite (Ca2SiO4) and titanite-structured CaSi2O5 at approximately 10 GPa (identified as an ε-CaSiO3 phase by Tamai and Yagi [1989] and later as larnite and titanite structure by Kanzaki et al. 1991) before recombining to form CaSiO3 with perovskite structure above approximately 12–16 GPa (Gasparik et al. 1994; Shim et al. 2002; Akaogi et al. 2004). The Ca-perovskite is cubic at high temperatures and pressures, but transforms into a tetragonal phase upon temperature quenching (e.g., Shim et al. 2002; Kurashina et al. 2004; Komabayashi et al. 2007). When quenched to ambient temperatures and pressures, pseudowollastonite, wollastonite II, and larnite and titanite-structure phases are preserved, while both Ca-perovskites convert to amorphous phases. The impact parameter calculations suggest that the pressures and temperatures required for all of the aforementioned phase transformations may have been achieved in some regions of the projectile. (We note that Ppeak is not experienced by the entire projectile: The rear is likely to be less heavily shocked, resulting in the observed preservation of some crystalline material e.g., Pierazzo and Melosh 2000). Despite this, the unknown impact phase does not correspond to any of the quenchable equilibrium wollastonite polymorphs: d-spacings were calculated for pseudowollastonite, wollastonite II, larnite, and titanite structured CaSi2O5 from cell dimension data in Yamanaka and Mori (1981), Trojer (1968), and Joswig et al. (1999) using the equations provided in Mackay (1979); however, none of these corresponded to the single clear diffraction pattern obtained to date from the unknown impact phase. It is possible that a new high-pressure/temperature polymorph of wollastonite was created during impact. The production of a new polymorph is perhaps not surprising as our impact experiments result in almost instantaneous application and release of pressure and temperature, very different conditions from the relatively gradual increase and decrease adopted during the laboratory phase transition experiments that produced the polymorphs known to date. This highlights an important caution for future analyses and interpretations of cometary Stardust collections: We must be aware that new mineral polymorphs may be identified that are not attributable to comet 81P/Wild 2, but are instead the result of capture processes.

The impact parameter calculations also have implications for the origin of the observed amorphous material. We observed that the amorphous component has not incorporated Al and determined that Tpps approached, but did not exceed, the melting temperature of wollastonite, indicating that the amorphous material observed cannot be the result of melt quenching. Instead, from the estimated Ppeak combined with the previously reported wollastonite phase transformations summarized above, there are two other possible origins: parts of the impactor experienced low temperatures, but pressures exceeding approximately 20 GPa directly producing the amorphous phase, or they experienced high pressures and temperatures that induced the perovskite phase transition, which, upon release, converted to the amorphous phase. The amorphous material is therefore a shock-induced “diaplectic” glass, resulting from solid-state high-pressure amorphization or the degradation of unquenchable phases. Likewise, we find that the amorphous phases that lack Al are also observed in olivine residues (Wozniakiewicz et al. 2012). The Ppeak and Tpps calculations for those impacts also indicate that the melting temperature was not exceeded, pointing to a nonmelt origin such as pressure-induced amorphization as has been previously reported to occur for olivine when taken to and released from pressures >56 GPa (Jeanloz et al. 1977; Schneider and Hornemann 1977). However, regardless of their origin, the fact that the amorphous silicates retain original compositional stoichiometry is a useful insight as it suggests that residue glasses that have stoichiometric compositions were originally crystalline. For example, residue in Stardust crater #8 on foil C2054W identified in Leroux et al. (2008a) as being a mixture of crystalline and amorphous residue of Fo83 composition is consistent with the impact of a completely crystalline Fo83 grain.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methodology
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

After impact into Stardust Al foils, wollastonite projectile grains produce residues composed of a mixture of amorphous and crystalline material, a combination commonly observed in real Stardust cometary residues.

Both the crystalline and amorphous material in the wollastonite residue retain the original projectile chemistry, and the crystalline component also largely retains the structure and characteristic orientation of the impacting grain. Both components show no sign of having incorporated Al from the foil, which, together with the calculated impact parameters, is consistent with no melting having occurred. These data therefore indicate that the wollastonite crystals are examples of original preserved mineralogy and that the amorphous material is not the result of melting and subsequent quenching as might previously have been assumed. Instead, the amorphous component probably results from solid-state high-pressure amorphization or from the degradation of unquenchable phases.

One crystal with wollastonite chemical composition, however, exhibited a diffraction pattern that could not be indexed as wollastonite. Neither could this pattern be indexed as any of the known high-pressure/temperature polymorphs of wollastonite, despite calculations of impact parameters that suggest that conditions required for these phase transitions were achieved. It is therefore possible that a new high-pressure/temperature polymorph was created during impact.

As we continue to investigate the preservation of cometary dust analogs impacted on Stardust Al foil, we are able to clarify those residue features that can yield important information. The analytical results and impact parameter calculations detailed here for wollastonite and in Wozniakiewicz et al. (2012) for other anhydrous silicates suggest that Al incorporation can be used as an indicator of whether melting has occurred and therefore whether the residues can be interpreted as representative of cometary mineralogy. We note that for all anhydrous silicate impact analog residues investigated so far, Al incorporation has only been observed to occur in those amorphous residue components where our calculations indicate that the melting temperature of the impactor was exceeded. There is no evidence of crystalline phases which contain Al, either as part of their chemistry or as precipitants within, suggesting that once molten, these residues do not recrystallize. Therefore, we conclude that the majority of the Al-free crystalline silicates observed in Stardust cometary residues are original to the impactor and preserve the mineralogy of comet 81P/Wild 2; however, we may observe unusual or new high-pressure/temperature polymorphs that result from the collection conditions, and caution should be taken in the interpretation of their significance. Furthermore, we conclude that stoichiometric, Al-free amorphous silicates in Stardust residues were likely also originally crystalline and are shock- rather than melt-induced. These conclusions make the foils an even more valuable collection of comet dust grains for investigations of the mineralogy (and consequently the formation and evolution) of the comet 81P/Wild 2 parent body.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methodology
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Acknowledgments–– We thank NASA for providing Al foils, STFC for support of the LGG. We also thank G. Flynn, M. Zolensky, and D. Brownlee for their valuable comments and suggestions during review. Parts of this work were performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344. This work was supported by NASA grant NNH07AG46I and NNH11AG46I to HAI.

Editorial Handling–– Dr. Donald Brownlee

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Samples and Methodology
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References
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