Ordered mixed-layer structures in the Mighei carbonaceous chondrite matrix

Geochimica et Cosmochimica Acta Vol. 46. pp. 419 (P Pcrgamon Press Ltd. 1982. Printed in U.S.A.

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Ordered mixed-layer structures in the Mighei carbonaceous chondrite matrix IAN D. R. MACKINNON Lockheed, C23, Johnson Space Center, Houston, Texas 77058 (Received July 27, 198 1; accepted in revised form November 17, 198 1) Abstract-High resolution transmission electron microscopy of the Mighei carbonaceous chondrite matrix has revealed the presence of a new mixed layer structure material. This mixed-layer material consists of an ordered arrangement of serpentine-type (S) and brucite-type (B) layers in the sequence . . SBBSBB. . . . Electron diffraction and imaging techniques show that the basal periodicity is -17 A. Discrete crystals of SBB-type material are typically curved, of small size (e 1 pm) and show structural variations similar to the serpantine group minerals. Mixed-layer material also occurs in association with planar serpentine. Characteristics of SBB-type material are not consistent with known terrestrial mixedlaver clav minerals. Evidence for formation bv a condensation event or by subsequent alteration of preexisting material is not yet apparent.

using high resolution transmission electron microscopy (HRTEM) (Mackinnon and Buseck. 1979a; Akai,l980). This layer structure mineral (christened “SSB-type” based upon the ordered arrangement of serpentine-type (S) and brucite-type (B) structural repeats), has also been observed in CM2 Antarctic meteorites (McKee and Moore, 1980) and appears to be pervasive in the CM2 matrices studied. This paper describes in more detail electron microscope observations of the SBB-type material as it occurs in the fine-grained matrix of the Mighei carbonaceous chondrite.

INTRODUCI’ION THE fine grainecl matrices of carbonaceous chondrites have been the focus of attention for students of solar system processes for many decades. Considerable interest in the matrices is generated because they contain a record of events which occurred early in the history of the solar system. Discussion regarding the nature of the matrices has centered upon formation by (a) ~ndensation processes in a presolar nebula or (b) the alteration of pre-existing planetary or planetismal material. Previous investigators, using optical microscope, electron microprobe, X-ray diffraction and bulk chemical data, have described the general nature of carbonaceous chondrite matrices at a scale, on average, greater than a few microns (DuFresne and Anders, 1962; Nagy, 1975; Fuchs et al., 1973; Bunch and Chang, 1980). Electron microscope techniques have also been used to describe in greater detail (~1 micron) the mineralogy of fine grained CM2 meteorite matrices (Kerridge, 1969; McKee and Moore, 1979; Mueller et al., 1979; Mackinnon and Buseck, 1979a, b; Mackinnon, 1980a; Barber, 198 1). These detailed studies have, in general, confirmed earlier tentative identifications of matrix minerals (e.g. Fuchs et al., 1973). In addition, some studies have chemically and/or structurally characterized the most abundant matrix phases within specific meteorites (e.g. McKee and Moore, 1979; Mueller et al., 1979; Mackinnon and Buseck, 1979b). Nevertheless, the debate regarding the origin of matrix material still fluctuates from one opinion, embracing condensation processes (Fuchs et al., 1973; Wilkening, 1978), to the other, which favors alteration mechanisms (DuFresne and Anders, 1962; Kerridge, 1964; McSween, 1979; Bunch and Chang, 1980). Phyllosilicates, particularly serpentine-type minerals, are the major phase of CM2 carbonaceous chondrite matrices (Bunch and Chang, 1980; Barber, 198 1). However, a new layer structure mineral type has been observed in the Murchison CM2 matrix

EXPERIMENTAL El~ron-transparent samples from chips of the Mighei meteorite were prepared using a Technics ion-mill following the procedures outlined in Mackinnon (1980a). A JEM 1OOb electron microscope and techniques described by Mackinnon (1980a) and references therein were used in this study. The experimental error on measurements of lattice spacings from the JEM iOOb (using a top-entry goniometerf is less than 10%. A Syntex automated optical densitometer was used to perform line scans on TEM negatives. A step scan of 0.01 I mm was used with a spot diameter of 0.011 mm. The spot diameter corresponds to a spot size of 0.3 A (denendinn unon the actual TEM magnification) on the negative film.* Radiation damage is a difficult problem with CM2 meteorites during specimen preparation and electron microscopy. Damage to the thin regions of a specimen will occur during the initial high angle (30°-400), high voltage (4 kV-5 kV) ion-beam thinning process. At this stage, ionbeam damage can obscure all structural detail in thin areas. However, if the initial thinning process is followed by gentle thinning at a low incidence angle (1 0°- 1So), and low voltage (600-800 volts), then the amorphous layer (seen most clearly at the edge of the specimen) can be reduced significantly (see Fig. Sa). On the other hand, electron beam damage of matrix minerals is often rapid (loss of structure can occur within seconds) and variable. The relationship between rate of electron beam damage and type of mineral is not well known. However, qualitative studies of serpentine polymorphs suggest that composition may be important in determining the rate of electron beam damage at a given accelerating voltage. For example, Mg-rich, cylindrical serpentines appear less stable than planar (Mg, Fe, Al) serpentine varieties. In general, this trend is also applicable

479

1 M4C‘KINNC)h

4x0

to SBB-type material; the curved variety is less stable than the planar variety in the electron beam. A close comparison of the region to the right of the stacking fault in Figs. 5a and 5b will reveal subtle differences in image quality due to the encroachment of electron beam damage in the SBBtype material. Exposure times for Figs. 5a and 5b were approximately two seconds with less than five seconds between each exposure.

OBSERVATIONS

HRTEM studies of both Murchison and Mighei matrices have shown that SBB-type material and phyllosilicates rarely occur in large grains (i.e., >I micron; Mackinnon and Buseck, 1979a; Mackinnon, 1980a). The small grain size hinders complete characterization of material by conventional selected area electron diffraction (SAED) techniques. However, in all cases where high resolution images have been obtained, at least the basal periodicity has been confirmed by SAED. The small grain size and the limitations of the experimental technique make it difficult to precisely define all of the mineralogical associations for SBB-type material. The most common associated mineral is a planar serpentine-type mineral (Mackinnon, 1980a; Fig. 5). The occurrence of SBB-type material with respect to chondrules and/ or isolated mineral grains is not known. In the following observations and discussion, the terms serpentine-type and brucite-type are used to indicate a primarily structural identification of material. This structural identification is based upon measurement of characteristic basal spacings of materials previously identified in CM2 meteorites (DuFresne and Anders, 1962; Fuchs et al., 1973). For example, serpentines show a basal spacing of 7.3 A, while micas and talcs have a layer repeat of 10 A. The compositional emphasis placed in the text (e.g. serpentine, rather than kaolinite) is based upon bulk chemical analyses of CM2 matrices (Fuchs et al., 1973; McSween and Richardson, 1977). Curved structures Fig. 1 shows TEM

images

of SBB-type

material

in the matrix of an ion-thinned

specimen of Mighei. Fig. la is a low magnification photo of two grains of SSB-type material each approximately 1000 A by 6000 A in size. The SAED pattern (inset) from the area circled suggests that the material consists of two distinct crystals. The crystals, or grains, appear to have grown in close association with a common

boundary perpendicular

to c*. Both crystals show a

curvature of . IO-- I?“ of arc (measured Iron: (1~ image). This curvature of the SBB-type structure I> common and gives rise to arcuate diffuse sslrcaking roughly perpendicular to the c* direction. Another noticeable feature of the SBB-type diffraction pattern is the relative intensity of the third laker diffraction spot (arrowed, inset ). In all difYractic,n patterns obtained from SBB-type material, !h~ third diffraction spot (003) is the most intense 01 the rcro layer reflectiona. Fig. lb is an HRTEM image from the region circled in Fig. 1a. In Fig. 1b. the ordered arrangement of serpentine-type and brucite-type layers of the fashion SBBSBB are noted by - 17 zA marker arrows. These layers correspond to -7 A. -- F A and -5 A repeats, respectively. The boundary between the two SBB-type crystals (arrowed) is not well resolved along its length due to variations in imaging conditions. However, it is apparent that the boundary region contains minor amounts of small period (-5 8, and - 7 A) material in addition to SBB-type material. Variation in image contrast and fringe appearance between the crystals is due to a slight mismatch of grain orientations. Note the wavy material (along the layers) in the top left corner of Fig. 1b. This type of structure is similar to the “corrugated” layers described in Fig. 3. An unusually large area containing SSB-type material is shown in Fig. 2a. The low magnification image reveals a large, curved grain(s) - 1.5 micron in size (Region A). The grain(s) at the top of Fig. 2a (Region B) also consist(s) in part, of SSB-type material. Much of this area contains material too thick to produce interpretable diffraction patterns. However, images and diffraction patterns can be obtained from the thin edges of the grains. Fig. 2b is a HRTEM image from the area circled in Fig. 2a. Note the almost planar appearance of this segment of the grain (indicated by the SAED pattern in the inset) except for an area closely related to a “hole” in the crystal. This area has lattice fringes (arrowed) at low to moderate angles to the predominant layer direction of the crystal. The non-periodic reversals in curvature (indicated by the arrow in Fig. 2b) may be due to shortening approximately parallel to the layer surfaces, or a growth structure perhaps caused by chemical variations within the layers. The low magnification view in Fig. 2a shows that the SBBtype crystal is curved along the layer direction. Thus, the reversals in curvature, shown in Fig. 2b are probably in response to stress along the layer direction at the most acute point of curvature.

FIG. 1. T&W images of SBB-type material in the matrix of the Mighei meteorite. (a) A low magnification image of two grains of SBB-type material (A and B) with a common boundary perpendicular to c*. The SAED

pattern (insets) from the area circled shows two distinct basal periodicities of - I7 to the two crystals. (b) A HRTEM image of the boundary region between grains A and B showing small period (- 5 A and -7 A) lattice fringes. In contrast, the lattice spacings in grains A and B are - 17 A; within each -17 A repeat are smaller lattice fringe spacings of -5 A and -7 A.

A corresponding

MIGHEI MATRIX

481

MIGHEI MATRIX

483

FIG. 3. A WRTEM image of SBB-type material in which the layers bend and form corrugations. The corrugations along the layers are best viewed at a low angfe to the page. In an analogous manner to antigorite, the corrugations in SBB-tyw material may be related to the stoichiometry of the tetrahedral and octahedral layers.

A similar type of microscopic bending of serpentine layers has been observed in an anthophyllite asbestos specimen from Pelham (Veblen, 1980). In the Pelham specimen, unusual bending of layers is due to non-periodic reversals in curvature of the classic antigorite structure. Curvature of silicate structures is one mechanism for relieving misfit between sheets of different sizes (Wicks, 1979). The degree of misfit between tetrahedral and octahedral sheets of layer silicates is a function of the composition of each layer. In a similar manner, curvature of SBBtype material may be attributed to the degree of misfit between tetrahedral and octahedral layers. Another response to chemical variation within SBB-type layers is shown in Fig. 3. The basal fringes in this instance appear “corrugated” along the layers (best viewed at an angle to the page) and end in a small kink to the left of the image. Antigorite commonly shows “corrugated” lattice images when viewed perpendicular to c* (Veblen, 1980). In the classic antigorite structure, the wavy, or “corrugated,” nature of the layers is due t,o the stoichiometry of the

tetrahedral and octahedral layers (Wicks, 1979). In fact, the structural formula of antigorite can be related to the superlattice period of the layer “corrugations” (Kunze, 196 1). In a directly analogous manner it is suggested that “corrugations” in the SBBtype structure (Fig. 3) are related to the stoichiometry of the tetrahedral and octahedral layers. However, unlike the classic antigorite studied by Yada (1979). some chemical variation within the crystallite is apparent: there appears to be a variation in periodicity of the corrugations, and parts of the image in Fig. 3 {upper middle) show straight lattice fringes. Planar structures If imaging conditions and microscope resolution are favorable, two dimensional structure may be observed in SBB-type material. For very small grains (i.e. <1 pm) optimum conditions occur by a random process because specimen tilting using conventional methods is very difficult. In addition, rapid radiation damage by the electron beam often precludes the

FIG. 2. (a) A low magnifi~tion TEM image of unusu~ly large, curved grains of SBB-type material. Note that grains A and B are in close association and almost co-planar. (b) A HRTEM image of the region circled in Fig. 2a in which the layers are bent sharply in order to accommodate the extreme curvature of the grain. In the SAED pattern (inset), taken from a planar region of grain A, the intense (003) retleetion is arrowed.

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I MACKINNON

FIG. 4. A HRTEM image of SBB-type material in which the serpentine-type and brucite-type layer\ are clearly resolved. In addition. considerable detail can be seen within the S-type layer and perpendicular to the layer direction

“niceties” of proper orientation and illumination generally afforded stable specimens. Close examination of the high resolution image of SBB-type material in Fig. 4 however, reveals that detailed structural information can be obtained both in the c* direction and perpendicular to c*. The ordered repeat of -5 A, -5 A and -7 A layers is clearly visible in Fig. 4. Within the -7 A layer, there is further structural detail which may correspond to the tetrahedral and octahedral layers within the serpentine-type structure. Perpendicular to the c* direction, a repeat distance of -4.5 A is resolved in parts of Fig. 4. A -4.5 A repeat commonly occurs with serpentinetype lattice fringes (Yada, 1979) and may correspond to the (020) periodicity of the serpentine-type structure. The lack of continuity of the -4.5 A fringes perpendicular to c* could result from variations in imaging conditions due to misorientation of the crystal, or the complexity of the structure in that direction. The diffraction pattern recorded in association with the image in Fig. 4 indicates that the former alternative is more likely. SBB-type material also occurs in close association with planar serpentine. Fig. 5a is a HRTEM image of the Mighei matrix showing planar serpentine fringes (-7 A) which grade rapidly into successive layers of SBB-type material (- 17 A). The inset to the lower left of Fig. 5a is a plot of transmittance

versus distance for part of an optical densitometer scan from a to d. The optical densitometer trace allows an accurate measurement of the relative fringe spacings between the marker arrows of the section u-u’. The relative highs of intensity correspond to regions of low optical density on the HRTEM negative (bright fringes in Fig. 5a). Each point on the plot is approximately every fifth point sampled during the densitometer scan. The precision for each fringe spacing is approximately 0.3 A per fringe measurement. The accuracy of the fringe spacing measurements shown in Fig. 5a depends upon an accurate estimate of the image magnification under the given experimental conditions. The relative fringe spacings in Fig. 5a show that the densitometer scan has sampled a planar serpentine region with c* - 7 A, and a region with -5 A, -5 A and -7 A repeats. The association of planar serpentine and planar SBB-type material is accompanied by a defective region (upper arrow) where the basal fringes are dislocated. A higher magnification image of the defective region (Fig. 5b) shows that three units of SBB-type material are discontinuous at a common plane (CP) in c*. At this common plane, lattice fringes appear to (a) terminate, (b) bend slightly, (c) combine to form a single lattice fringe or (d) continue through the common plane. Examples of

1 0

1:o

2:o

3:o

co

Oistanca(mmd

FIG. 5. SBB-type material also occurs as a planar variety in association with planar serpentine. (a) An optical densitometer trace across the boundary region (marked by arrows between a-d) of the accompanying high resolution image is shown in the lower left corner. The trace clearly shows the transition from -7 A S-type fringes to regularly alternating -5 A, -5 A and -7 A fringes. The unusually narrow (x20 A) amorphous region at the edge of the specimen is a result of ion-beam thinning at a low incidence angle and low voltage. (b) A higher magnification image of the region near the upper arrow in Fig. 5a is shown. The discontinuity of lattice fringes is the result of a stacking error in which the C* directions of SBB-type material are reversed on either side of the common plane (CP). See text for details.

i MACKINNON

00

0 W

Figure 0

6a 0

0

feature is best observed by sighting parallel with the fringes at a low angle to the page. It is by no means clear that each fringe spacing in Fig. 5b corresponds exactly to an octahedral or tetrahedral-octahedral layer in either the SBB-type material or the planar serpentine. Extensive computer modelling following the well illustrated procedures by O’Keefe ef al. ( 1978) is necessary to correlate image contrast with atomic positions. In addition, intuitive interpretation of complex microstructural images is shown to be invaluable for known minerals (Veblen and Buseck, 1980). The SBB-type structure is not known in any detail, though a proposed model is given in the next section. Therefore, a detailed interpretation of the atomic displacements at the structural fault shown in Fig. 5b is not attempted. DISCUSSION

0 on 0 0

B

0 0

0

(0

2

I-

0 0

0

0

00

0 0

0

0,

0

M9

l

SI

I

S

Y Figure

6b

FIG. 6. Two possible structural models for a - 17 A arrangement of stacked layer structure units. These idealized

models show ordered arrangements of (a) serpentine (S) and talc (T) layers (B) layers corresponding to an SBBtype structure, and (b) serpentine (S) and brucite, termed an ST-type structure. The SBB-type model is a more likely structural representation of the ordered - 17 8, material.

the first three phenomena are arrowed in Fig. 5b. Assuming that there is some correspondence between the lattice image and the layer structure of the SBBtype material, the common plane in Fig. 5b probably represents a stacking fault. The fault may be described as a reversal of c* (by 180”) in the SBB-type material. This effect is best illustrated by following the structural unit designations (S or B) at the top and bottom of Fig. 5b. At the top of 5b, the planar serpentine fringes adjoin ordered fringes of the form , whereas at the bottom of Fig. . . . BBSBBS . 5b the fringes alternate . . SBBSBB . . after the planar serpentine region. The serpentine fringes common to the SBB-type material are perfectly straight and parallel to the left of the SBB-type material. On the other hand, the two fringes immediately to the right of the planar serpentine region display a pronounced bend in the region of the stacking fault. This

As with all previous descriptions of the ordered mixed-layer phases in carbonaceous chondrites (Mackinnon and Buseck, 1979a,b; Mackinnon, 1980a), the suffix, “type”, is appended to the acronym in order to show that the phase(s) are only partially characterized. In general, for a standard 30 micron petrographic thin section, matrix material is too fine-grained to reveal textural or mineralogical detail using optical microscopy. Therefore, a comparison of electron images with transmitted or reflected light images has not been attempted. Some measure of success has been achieved in comparing Polished Ultra-thin sections with scanning electron microscope (SEM) images of matrix material (Bunch and Chang, 1980). This approach and subsequent comparison with HRTEM images, provides optimum conditions for detailed compositional and structural interpretation of matrix mineralogy (Mackinnon et al., 1981). However, to date, the small size of SBBtype material has made it difficult to obtain comparative optical, SEM or X-ray data. In addition, the electron-sensitive nature of the SBB-type material precludes extensive tilting experiments in the microscope. Therefore, this paper presents characterization of the layer direction only of SBB-type material in the Mighei matrix. It has been suggested (D. Veblen, personal communicarion) that an alteration of talc-like (T) and serpentine-like (S) layers will also produce - 17 A repeats. Examples of the two possible types of periodic repeats of layer structures are shown in Fig. 6. These models are based upon simplified drawings for the trioctahedral group of the 1:I layer silicates from Deer et al. (1963). SAED patterns from all regions of - 17 A material show a similar zero layer intensity distribution. In order to test for each structural model, zero layer structure factors were calculated for both the SBB-type and the ST-type models. The results of this calculation, using the equation

IFA ==(Ah + &I)“~

481

MIGHEI MATRIX where, for A ,,,,, = 2 f, cos 27rlz, and Bu,i = 2 f, sin 27rlz, J I are given in Table 1. Neither model can be adequately distinguished using the calculated intensities of the diffracted beams along the zero layer. In addition, the calculated lFoolI values for each model do not correspond to the observed intensity distribution for the - 17 A material in Mighei (see insets of Figs. 1 and 2). The intense (003) diffraction spot may arise from deviations in structure or composition from the idealized models in Fig. 6. Composition is another parameter which would distinguish between SBB-type and ST-type models. Assuming no substitution of Fe for Mg in the structure, the ratio of Mg/Si should be a factor of -3 higher in the SBB-type model, due to the two brucitetype layers. However, preliminary chemical investigations of the - 17 A material (Mackinnon, 1980a,b; McKee and Moore, 1980) suggest that Fe is a major element in the structure. Minor amounts of Al are also qualitatively identified. Such departures from the idealized chemical models emphasizes the tenuous nature of the designations “SBB-type” and/or “ST-type”. It has not yet been possible to unambiguously define the chemistry of the - 17 A material. However, it is apparent from the various structural forms shown in Figs. 1 to 5, that there is some compositional variation within the layers. An analogy can be drawn between the serpentine group minerals and the - 17 A material. The relatively Mg-rich cylindrical or curved form of serpentine (i.e., chrysotile), the stoichiometric, corrugated antigorite and the more compositionally diverse planar lizardite, all have analogous structural forms in the SBB-type material. However, the exact substitution pattern within the serpentine group may not be followed by the SBB-type structure. The only available bases for comparison of the SBB- and ST-type models are the high resolution structure images in Figs. l-5. In all images of the - 17 A material, a well defined sub-set of three “lattice fringes” are visible. These images usually alternate as one long layer (-7 A) and two short layers (-5 A), respectively; apparently over a range of defocus conditions. Intuitively, this consistent alternation of layers of the same approximate dimensions suggests that the SBB-type model best fits the observed data. By comparison, an ST-type structure would show alternations of -9 A and -7 A layers. SBB-type material occurs primarily as well-ordered crystals with structural and morphological nuances similar to terrestrial layer silicates. A review of the literature on terrestrial layer silicates indicates that mixed-layer clays may be a suitable analogue for this meteoritic SBB-type material. However, very few, if any, - 17 A mixed-layer clays have been observed. The majority of mixed-layer clays (usually detected by X-ray diffraction techniques) are Al-rich and form with longer spacings in the layer direction

actors structure Table 1. Calculated for the idealized models of ~17 mixedlayer material.

lFhlcl' a 000 001 002 003 004 005

ST

SBB

INDEX 100 96 91 83 75 66

0 2.4 8.0 7.2 10.4 11.8

lFhkll 100 95 90 82 74 65

a 0 2.6 5.2 7.8 10.4 12.1

BOOl Phase angle, c1= TAN-l _ *001 For both models, intensity (F) and phase angle (a) are similar for each layer reflection (e.g., mica-montmorillonite; -27 A; Sudo and Shimoda, 1978). A Mg-rich mixed-layer chlorite vermiculite has been reported in weathered soils of a small serpentinite body in France (Duclous et al., 1976). However, the layer spacing for a chlorite-vermiculite mixed-layer clay (-28 A) is considerably different from the - 17 A SBB-type spacing. At this time, there does not appear to be a study of any terrestrial environment which provides evidence for the formation of a - 17 A SBB-type layer structure. It is tempting to construe the apparent absence of SBB-type material in terrestrial environments as evidence for the formation of an extra-terrestrial mineral under conditions unique to the early solar system. These conditions could then be related to a condensation event or a subsequent alteration of preexisting material. However, this line of reasoning is not fruitful. In the words of a reviewer of this paper: “ . no evidence is just that: no evidence . . .“. In addition, it is difficult to assemble evidence for either formation process from one technique alone. In order to reconstruct geochemical arguments for the formation of CM2 matrices, it is necessary to use techniques which attain spatial resolutions smaller than the predominant grain size of the constituent minerals. Therefore, generalized conclusions regarding the chemistry of matrix minerals awaits further investigations using a combination of micro-analytical techniques (analytical electron microscopy, HRTEM, ion probe microscopy). Alternatively, very detailed textural evidence for a particular formation process in localized environments can be provided by HRTEM (see e.g. Mackinnon, 1980a). In the observed occurrences of SBB-type. material, it is difficult to interpret HRTEM images in terms of alteration processes. One possible clue to the origin of SBB-type material lies in its association with planar, -7 A phyllosilicate (Fig. 5). Bunch and Chang (1980) argue that the various types of phyllosilicate (primarily 1: 1 trioctahedral varieties) are the product of an alteration process. This would then imply that the SBB-type material is an alteration product. However, a single-event origin of the matrix

48X

I. MACKINNON

phyllosilicates is not so evident to the author. There is some indication that CM2 chondrites contain generations of phyllosilicate, or at least generations of alteration processes (Mackinnon et al., 198 1) which may give rise to different types of phyllosilicate. In addition, experiments by Meyer ( 197 I ) suggest that phyllosilicates can be formed from a vapor under conditions present in circumstellar space. If Meyer’s ( 197 1) experimental results can be related to early solar system conditions, the possibility that phyllosilicates can form via a condensation process should be entertained. Thus if phyllosilicates did form from the solar nebula, primary phyllosilicate may still be present within some CM2 matrices, provided parentbody alterations are of limited extent. Explicit evidence for the origin of each type of phyllosilicate, especially the fine grained (61 Frn) minerals, awaits a thorough investigation using techniques which can perform chemical analyses at or beyond a resolution on the scale of the individual grains. It therefore seems prudent to suggest that the origin of SBB-type material in association with planar serpentine (Fig. 5) cannot be adequately explained at this time. Detailed characterization of the phyllosilicates in CM2 matrices may eventually provide insight as to their origins, in addition to circumstantial evidence for the origin of SBB-type material. CONCLUSIONS An ordered, mixed layer material with periodicity - 17 A occurs in the fine-grained matrices of the Mighei and Murchison CM2 carbonaceous chondrites. HRTEM images of the material suggest that the layers alternate regularly in the fashion serpentine-type (S), brucite-type. (B), brucite-type (B) (i.e., . . SBBSBB . . .). Discrete crystals of SBB-type material in the Mighei meteorite are typically curved, of small size (~1 pm) and show similar structural variations to the serpentine group minerals. The structural variations are probably a result of compositional fluctuations within the tetrahedral and octahedral layers of the SBB-type unit. Mixed layer - 17 A material also occurs in association with planar serpentine; in which case the SBB-type layers are also planar. The SBB-type material in Mighei does not correspond to mixed layer minerals commonly described in terrestrial alteration sequences. Since the origin of CM2 matrix material is still a matter for debate, the derivation of SBB-type material and its significance to carbonaceous chondrite history may be a fertile area for future research. Acknowledgments-Useful

discussions on aspects of this

work were held with Piers Smith, Kazushiga Tomeoka and David Veblen. Valuable comments on an early version of the manuscript were provided by Rick Wendlandt, Michael Baker and David McKay. Reviews by David Barber, John Kerridge and Hap McSween are greatly appreciated. Keith Tovey and David Rieck provided assistance with optical

densitometer scans. The microscopy was performed at the electron microscope facility in the Center for Solid State Science at Arizona State IJniversity, where John Wheatly and Tania Uram were of assistance. Carleton Moore of the Centre for Meteorite Studies kindly provided the MiPhel a---sample. This work was supported by NASA grant NSG 9068 to Peter Buseck and a grant from the Office of Space Sciences to David McKay and the author. REFERENCES Akai J. (1980) Tubular form of interstratified mmeral consisting of a serpentine-like layer plus two brucite-like sheets newly found in the Murchison (C2) meteorite. Mem. Nat. Inst. Polar Res. Spec. Issue No. 17. Proc. Fifth Symp. on Antarric Meteorites 299-300. Barber D. J. (1981) Matrix phyllosilicates and associated minerals in C2M carbonaceous chondrites. Geochim. Cosmochim. Acta 45, 945-910. Bunch T. E. and Chang S. (1980) Carbonaceous chondrites II: Carbonaceous chondrite phyllosilicates and light element geochemistry as indicators of parent body processes and surface conditions. Geochim. Cosmochim. Acta 44, 1543-1577. Deer W. A., Howie R. A., and Zussman J. (1963) RockForming Minerals. Vol. 3. Layer Silicates. Longmans. London. Duclous J., Meunier A., and Velde B. (1976) Smectite, chlorite and a regular interlayered chlorite-vermiculite in soils developed on a small serpcntinite body Massif Central, France. Clay Minerals 11, 121-135. DuFresne E. R. and Anders E. (1962) On the chemical evolution of the carbonaceous chondrites. Geochim. Cosmochim. Acta 26, 1085-l 114. Fuchs L. H., Olsen E. and Jensen K. J. (1973) Mineralogy, mineral-chemistry and composition of the Murchison (C2) meteorite. Smith. Contrib. Earth Sciences No. 10, 39 PP. Kerridge J. F. (1964) Low temperature minerals from the fine-grained matrix of some carbonaceous meteorites. Ann. N.Y. Acad. Sci. 119.41-53. Kerridge J. F. (1969) The use of selected area diffraction in meteorite mineralogy. In Meteorite Research (ed. William) 500-504. Dordrecht Holland: D. Reidel Publishing Co. Kunze G. (1961) Antigorit. Forschritt. Miner. 39. 206324. Mackinnon I. D. R. (1980a) Structures and textures of the Murchison and Mighei carbonaceous chondrite matrices. Proc. Lunar Sci. ConJ: 1 Ith, 839-852. Mackinnon I. D. R. (1980b) Analytical electron microscopy of matrix phases in Murchison and Mighei. Meteoritics 15, 328-329. Mackinnon I. D. R. and Buseck P. R. (1979a) New phyllosilicate types in a carbonaceous chondrite matrix. Nature 280, 2 19-220. Mackinnon I. D. R. and Buseck P. R. (1979b) High resolution transmission electron microscopy of two stony meteorites: Murchison and Kenna. Proc. Lunar Sci. ConjI IOth, 937-949. Mackinnon I. D. R., King T. V. V. and Baker M. B. ( 198 I ) A comparative study of the Murray CM2 carbonaceous chondrite. Meteoritics, in press. McKee T. R. and Moore C. B. (1979) Characterization of submicron matrix phyllosilicates from Murray and NOaova carbonaceous chondrites. Proc. Lunar Sci. Conf cjO;h, 921-935. McKee T. R. and Moore C. B. (1980) Matrix phyllosilicates of the Antarctic C2 chondrite ALHA 77306 (abstract) in Lunar and Planetary Science XI, p. 703-704. Lunar and Planetary Institute, Houston.

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