& Planetary Science 41, Nr 5, 681–688 (2006) Abstract available online at http://meteoritics.org

Polyhedral serpentine grains in CM

Thomas J. ZEGA1*, Laurence A. J. GARVIE2, István DÓDONY2††, Heiner FRIEDRICH2‡, Rhonda M. STROUD1, and Peter R. BUSECK2, 3

1Materials Science and Technology Division, Code 6360, U.S. Naval Research Laboratory, Washington, D.C. 20375, USA 2Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287–1404, USA 3Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287–1604, USA ††Permanent address: Department of , Eötvös L. University, Budapest, H-1117, Pázmány sétány 1C, Hungary ‡Current address: Inorganic Chemistry and Catalysis, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands *Corresponding author. E-mail: [email protected] (Received 01 August 2005; revision accepted 10 December 2005

Abstract–We used high-resolution transmission electron microscopy (HRTEM), electron tomography, electron energy-loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS) to investigate the structure and composition of polyhedral serpentine grains that occur in the matrices and fine-grained rims of the Murchison, Mighei, and Cold Bokkeveld CM chondrites. The structure of these grains is similar to terrestrial polygonal serpentine, but the data show that some have spherical or subspherical, rather than cylindrical morphologies. We therefore propose that the term polyhedral rather than polygonal be used to describe this material. EDS shows that the polyhedral grains are rich in Mg with up to 8 atom% Fe. EELS indicates that 70% of the Fe occurs as Fe3+. Alteration of cronstedtite on the under relatively oxidizing conditions is one probable pathway by which the polyhedral material formed. The polyhedral grains are the end- member serpentine in a mineralogic alteration sequence for the CM chondrites.

INTRODUCTION structures and compositions of serpentines in primitive carbonaceous chondrites using transmission electron The carbonaceous chondrites are primitive rocks that microscopy (Zega and Buseck 2003; Zega et al. 2003; Zega formed over 4.5 billion years ago in the solar nebula (Amelin et al. 2004). We report on a polyhedral form that occurs in et al. 2002), the cloud of gas and dust from which the Sun and matrices and fine-grained rims (FGRs) of the Murchison, planets formed. The minerals within them are relics from the Mighei, and Cold Bokkeveld CM chondrites. Polygonal early solar system and therefore provide information about serpentine is the closest terrestrial analog to the polyhedral the chemical and physical processes that occurred during its grains discussed in this study, and so we briefly describe its formation. structure. The CM carbonaceous chondrites experienced varied Polygonal serpentine consists of tetrahedral (T) sheets degrees of aqueous alteration early in their histories joined to M2+-centered octahedral (O) sheets (where M is (Browning et al. 1996). The mineral types characteristic of primarily Mg and Fe), which give rise to a 1:1 (TO) layered alteration are diverse and include, for example, oxides, structure with a 0.7 nm layer periodicity (the 00l d-spacing). hydroxides, carbonates, sulfates, and phosphates (Brearley The structure is similar to chrysotile in that polygonal and Jones 1998; Buseck and Hua 1993; Zolensky and serpentine contains concentric lizardite layers wrapped McSween 1988). Sheet silicates, particularly those of the around the tube axis (Fig. 1a). However, unlike the rolled-up serpentine group, are abundant. Serpentines can form as chrysotile, the tetrahedral sheets of the lizardite layers in several different structures with a wide range of solid polygonal serpentine are periodically inverted and kinked, solution, which makes them useful recorders of the chemistry producing sectors (Fig. 1b). Dódony (1997) showed that the and progress of aqueous alteration. Information on their inversion of the tetrahedral sheet manifests itself in high- structures and compositions is important for understanding resolution transmission electron microscope (HRTEM) alteration processes in the early solar system. images as a bending in the layers (discussed below). The Here we expand on previous efforts to determine the relative angles between sectors result in 15- and 30-sided

681 © The Meteoritical Society, 2006. Printed in USA. 682 T. J. Zega et al.

Fig. 1. Shape and structure diagrams of polygonal serpentine. a) Oblique projection. Arrow indicates the core. Solid lines indicate sector boundaries. Dashed lines indicate 00l layers. b) Axial projection. Sector boundaries (solid lines) occur at the positions of inversion of sheets of silica tetrahedra (black triangles). The angle between sectors (α) in terrestrial samples is 12° and 24° for 30- and 15-sectored polygonal serpentine, respectively. polygons in terrestrial samples (Baronnet and Devouard correction of the remaining drift using interactive data 2005; Baronnet et al. 1994; Chisholm 1991, 1992; Dódony language scripts (Weyland 2001). The tomogram was 1997; Dódony and Buseck 2004). reconstructed using the sequential iterative reconstruction technique (Gilbert 1972) with 30 iteration steps. METHODS RESULTS We prepared samples for TEM analysis using two methods. TEM support grids were mounted onto regions of High-resolution TEM images reveal a 0.7 nm periodicity interest in petrographic thin section, extracted, and Ar-ion- (Fig. 2a), consistent with the 00l layer spacing in serpentine- milled to electron transparency (for details, see Zega and group minerals (Bailey 1988). The lattice-fringe contrast Buseck [2003]). Also, millimeter-sized chips were exhibits periodic bending along the 00l layers, defining disaggregated using carbide microneedles and dispersed onto sectors that occur radially around the core of the grain, with lacy-carbon TEM grids using a 1 µl droplet of methanol (see angles between sectors that range from 13.5° to 27.5°. Some Zega et al. [2004] for details). grains have nearly complete cross sections (Fig. 2a), whereas We acquired high-resolution images and selected-area others appear intergrown or partially formed (Figs. 2b–d). electron-diffraction patterns with 400 keV JEOL 4000EX (Cs The cores of some grains are elongated (Fig. 2c), and the 00l = 1.0 mm), 200 keV Topcon 002B (Cs = 0.4 mm), and layers are continuous around them. 200 keV JEOL 2010F (Cs = 0.5 mm) TEMs. Multislice image We performed image simulations on chrysotile to simulations were calculated using Cerius2 (Accelrys Inc.). understand the effect that the orientation of a cylinder has on Mineral composition was determined using energy-dispersive the contrast of an HRTEM image. In axial projection, the and electron energy-loss spectroscopy (EDS and EELS, simulation shows that a cylinder has a circular cross section respectively). Energy-dispersive spectra were acquired with a with concentric 00l layers (Fig. 3a). In off-axis orientation, Noran EDS detector attached to a 200 keV JEOL 2200F TEM the simulation shows that a cylinder has an elliptical cross and quantified with standardless routines in the Noran section and elongated core (Fig. 3b). Contrast is lost in the Vantage software. Electron energy-loss spectra were acquired off-axis orientation, and the 00l layers are noncontinuous using a Gatan 766 DigiPEELS spectrometer attached to a around it. 100 keV Philips 400 FEG TEM (see Garvie and Buseck 1998; The polyhedral grains occur isolated or together with Garvie et al. 2004; Zega et al. 2003; and Garvie et al. 2004 for other minerals such as serpentine nanotubes and a chrysotile- details). High-resolution scanning electron microscope like phase (Fig. 2). Grain sizes range from tens to hundreds of (SEM) images were acquired with a Hitachi S-4700 FEG- nanometers, and SEM images show that they have spherical SEM. to subspherical morphologies (Fig. 4). The polyhedral We performed electron tomography with a 200 keV FEI material is less abundant than the other serpentine-group Tecnai F20 TEM (Cs = 1.2 mm). Images were acquired over a minerals such as cronstedtite and the chrysotile-like phase. Of tilt range of ±60° at 2° increments. The raw-image tilt series the three chondrites studied, polyhedral serpentine grains are was aligned by sequential cross-correlation and manual most abundant in Cold Bokkeveld. Polyhedral serpentine grains in CM chondrites 683

Fig. 2. TEM images of polyhedral grains from the Mighei CM . a) A grain occurring within a lizardite matrix. The 0.7 nm layer periodicity is indicated by solid white lines and small white arrowheads. Sector boundaries are indicated by dashed white lines. The large white arrowhead points to the core of the grain. The angle between two sectors (lower left part of image) is 26.2°. b) A partially formed grain sitting on the amorphous carbon (AC) support film. A serpentine nanotube (black arrowhead) occurs with this grain. c) Two grains sitting on the AC support film. Background image is an enlargement of the area delineated by the white box in the foreground image (inset). Sector boundaries are indicated by the dashed lines. d) A partially formed grain occurring with the chrysotile-like phase (white arrowhead) and near to a serpentine nanotube (black arrowhead). 684 T. J. Zega et al.

Fig. 3. Schematic of electron-beam propagation through a cylinder with corresponding image simulations. a) Axial projection. b) 42° off-axis projection. Scales on the x- and y-axes are in Å.

The polyhedral grains are rich in Mg (Fig. 5) and contain structural difference from terrestrial counterparts. Chisholm up to 8 atom% Fe. Energy-loss spectra show that the Fe L3 (1991) showed that terrestrial polygonal serpentine contains edge is broad and asymmetric toward higher energy loss either 15 or 30 sectors, with angles of 24° ± 3° and 12° ± 3° (Fig. 6). The edge shape is similar to the serpentine nanotubes between them (360°/n where n is the number of sectors), but markedly different than cronstedtite (Fig. 6). respectively. He later suggested that a grain in off-axis Quantification of spectra from several grains reveals that orientation, or one with an incomplete structure, could approximately 70% of the Fe occurs as Fe3+. explain the ±3° deviation (Chisholm 1992). Although we do We combined TEM imaging and electron tomography on observe grains with incomplete structures (Fig. 2) and those one grain from the Mighei CM chondrite (Fig. 7) to determine that appear to be in off-axis orientation (cf. the cores of the its three-dimensional structure. The core of the grain is grains in Fig. 2a and Fig. 3b), the angles that we measure in elongated (Fig. 7a). The external structure is tapered at the top some of them deviate beyond those reported for the terrestrial and bottom, indicating a subspherical, prolate shape (Fig. 7b). samples. For example, the angles between two adjacent The surface of the grain was made transparent to visualize the sectors from a grain in Cold Bokkeveld are 14° and 22°. interior. At the core is a structure that runs the length of the Our experimental images show that the cores of the grain (Fig. 7b). When viewed in axial orientation (Fig. 7c), polyhedral grains are elongated, but the 00l lattice fringes are this structure is hollow from one end to the other, suggesting continuous around them (e.g., Fig. 2a). Such elongation of the that it is tubular. It contains several kinks along its length core and continuity in the lattice-fringe contrast is (Fig. 7b) and exhibits a branching shape in some parts. inconsistent with the simulations and the cylindrical structure from which they are calculated (Fig. 3). We hypothesize that DISCUSSION these grains are roughly spherical. We combined SEM and electron tomography to test this The measured angular variation among the sectors in hypothesis. The SEM images reveal grains that have roughly these polyhedral grains is important because it suggests a rounded morphologies and diameters that range from tens to Polyhedral serpentine grains in CM chondrites 685 hundreds of nanometers (Fig. 4). The EDS measurements indicate that their composition is consistent with a serpentine- group mineral. A TEM image from a grain in Mighei shows that its core is elongated (Fig. 7a), suggesting an off-axis orientation. However, it has a circular rather than elliptical cross section, suggesting a non-cylindrical shape (cf. Figs. 3a and 3b). The tomographic reconstruction reveals that the outer surface of the grain is tapered at the top and bottom, indicating a prolate shape (Fig. 7b). If the grain were a cylinder it would have parallel rather than the tapered walls that we observe. The SEM and electron-tomography data therefore support the hypothesis that some of these grains have spherical and subspherical forms and are therefore not polygonal serpentine in the strict sense. Because a sphere can be approximated as a polyhedron with an infinite number of sides, we propose the term polyhedral rather than polygonal serpentine be used to describe grains exhibiting the characteristics discussed above.

COSMOCHEMICAL IMPLICATIONS

Inferring how and under what conditions the polyhedral material formed is important for gaining insight into the history of the CM chondrites and alteration processes in the early solar system. Zega et al. (2003) used HRTEM and EDS to show that alteration of cronstedtite on the meteorite parent body is a possible pathway by which the polyhedral material formed. The EELS measurements indicate that the Fe3+/ΣFe ratio in these polyhedral grains is 0.7, which is similar to the ratio measured from serpentine nanotubes (Zega et al. 2004) but approximately 40% greater than that measured from cronstedtite (Zega et al. 2003). Because the Fe3+/ΣFe ratio is a measure of redox state (Frost 1991), the EELS data suggest that alteration of cronstedtite occurred under relatively oxidizing conditions to produce the polyhedral material. Alteration of amorphous material is a plausible alternative pathway to produce the polyhedral grains. Huertas et al. (2004) showed that hydrothermal treatment of amorphous Si-Al gel produced spherical kaolinite with a structure similar to that of the polyhedral serpentine. Mesostasis glass in , FGRs, and matrix is highly susceptible to and among the first solids that undergo aqueous Fig. 4. FEG-SEM secondary-electron images of polyhedral grains in alteration in CM chondrites (Barber 1981; Brearley 1995; Cold Bokkeveld. Chizmadia and Brearley 2003; Hanowski and Brearley 2001; Lauretta et al. 2000; Metzler et al. 1992; Richardson and previous mineralogic observations (Tomeoka and Buseck McSween 1978). Therefore, mesostasis glass could have 1985). This sequence, in order of increasing Mg crystallized or was pseudomorphically replaced during concentration, consists of cronstedtite, serpentine nanotubes, alteration to produce the polyhedral grains. a chrysotile-like phase, and the polyhedral serpentine. We TEM images reveal that the polyhedral material is among propose that there is a correlation between the composition the coarsest-grained sheet silicates in the CM chondrites, and and grain size of the polyhedral material in CM chondrites. compositional measurements indicate that it is the richest in The coarse-grained polyhedral material appears to be the Mg. In our report on serpentine nanotubes (Zega et al. 2004), end-member serpentine product in the alteration sequence. we suggested a reaction sequence for CM chondrites that That such coarse grains could form and survive to the present parallels bulk compositional trends (McSween 1979) and is surprising given the level of brecciation that many CM 686 T. J. Zega et al.

Fig. 5. An Mg-Fe-Si ternary diagram showing the composition of the polyhedral grains, measured using EDS. Those from Murchison (black diamonds), Mighei (black hexagons), and Cold Bokkeveld (black triangles, Zega and Buseck 2003) are plotted together with other serpentine- group minerals in the CM chondrites, including cronstedtite (gray circles), a chrysotile-like phase (gray triangles, Zega and Buseck 2003), and serpentine nanotubes (gray squares, Zega et al. 2004). End-member compositions of greenalite [Fe3Si2O5(OH)4], cronstedtite 2+ 3+ 3+ [Fe2 Fe [SiFe O5](OH)4], and chrysotile [Mg3Si2O5(OH)4] are shown for reference together with connecting joins. The data are normalized to 100%. chondrites have sustained and the high degree of compactness (1988) reported polygonal serpentine with a globular of some FGRs and matrices (Metzler et al. 1992). Although morphology in samples of hypabyssal kimberlite and polyhedral grains occur in all the studied, that they suggested that they formed between 400 and 600 °C. are more abundant in Cold Bokkeveld relative to Murchison Although kimberlites probably experienced such and Murray is consistent with compositional trends temperatures, equilibrium thermodynamic calculations and (McSween 1979; Tomeoka and Buseck 1985) and degrees of oxygen isotopes indicate that the matrices and FGRs of CM alteration (Browning et al. 1996). Thus, the presence of the chondrites were altered at far lower temperatures (Clayton polyhedral material is likely an indicator of intensive aqueous and Mayeda 1999; Zolensky et al. 1989). The data from these alteration, e.g., stages 3 and 4 in the alteration model of CM chondrites therefore suggest that such high temperatures Hanowski and Brearley (2001), and also perhaps the lack of (and presumably pressures) are not necessarily needed to extensive regolith gardening that would have fragmented produce polyhedral serpentine with a spherical to such grains. subspherical morphology. Plate-like, layered, and tubular serpentine are common in terrestrial environments (Dódony and Buseck 2004) and were CONCLUSIONS also observed in the CM chondrites (Barber 1981; Brearley and Jones 1998; Buseck and Hua 1993; Zega and Buseck Polyhedral serpentine occurs in the matrices and FGRs of 2003; Zega et al. 2003; Zega et al. 2004). Mitchell and Putnis the Murchison, Mighei, and Cold Bokkeveld CM chondrites. Polyhedral serpentine grains in CM chondrites 687

Fig. 6. Electron energy-loss spectra. Fayalite [(Mg,Fe)2SiO4] and 2+ 3+ hematite (Fe2O3) spectra are shown for reference to Fe and Fe , respectively. A representative Fe L2,3 spectrum of the polyhedral material (Cold Bokkeveld) is shown with that from serpentine nanotubes (Mighei; Zega et al. 2004) and cronstedtite (Cold Bokkeveld; Zega et al. 2003).

It is similar to terrestrial polygonal serpentine, but it has a spherical to subspherical rather than cylindrical morphology. It is the coarsest-grained and most Mg-rich serpentine in the CM chondrites and likely constitutes an end memnber in a mineralogic alteration sequence for these meteorites. The Fig. 7. A tomographic reconstruction of a polyhedral grain from the presence of polyhedral serpentine can be used as an indicator Mighei CM chondrite. a) TEM image. Lattice fringes, with periodicities of 0.7 nm, occur around the core. b) Tomogram viewed of intense aqueous alteration. Alteration and oxidation of perpendicular to the core. The isosurface is made semitransparent, cronstedtite on the parent body is one pathway by which the allowing visualization of the interior. c) Tomogram viewed down the polyhedral grains could have formed. core axis. 688 T. J. Zega et al.

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