Interplanetary dust in the transmission electron microscope: diverse materials from the early solar system

0016-7037

XI 060915.29$0?00

0

Cqyrlgh:i‘~19XI Pergamon Prrs, Lid

Interplanetary dust in the transmission electron microscope: diverse materials from the early solar system P. FRAUNDORF McDonnell Center for the Space Sciences, Washington

University,

St Louis.

MO 63130, U.S.A. (Received

6 August

1980;

uccapted

in revisedform

4 February

1981)

Abstract---An analytical electron microscope study of dispersed interplanetary dust aggregates collected in the earth’s stratosphere shows that, in spite of their similarities, the aggregates exhibit significant differences in composition, internal morphology, and mineralogy. Of 11 chondritic particles examined, two consist mostly of a noncrystalline chondritic material with atomic (S/Fe) 2 2 in places, one consists of submicron metal and reduced silicate ‘microchondrules’ and sulfide grains embedded in a carbonaceous matrix, and another consists of submicron magnetite-decorated unequilibrated silicate and sulfide grains with thick low-Z coatings. Although the particles are unmetamorphosed by criteria commonly applied for chondritic meteorites, the presence of reduced chemistries and the ubiquity of mafic. instead of hydrated, silicates confirm that they are not simply Cl or C2 chondrite matrix material. The observations indicate that portions of some particles have not been significantly altered by thermal or radiation processes since their assembly. and that the particles probably contain fine debris from diverse processes in the early solar system.

1. INTRODUCTION

scope capable of sub-nanometer spatial resolutions. In the microscopist’s perspective, the work described IN RECENTYEARS,interplanetary dust particles (IDPs) here amounts to little more than a characterization of _ 10pm in size, collected in the earth’s stratosphere ‘hand specimens’ from several 5 or 10 pm ‘meteorites’. in a program of sampling initiated by BROWNLEEet al. Individual IDPs, once mounted, are ideal candidates (1976; cf. BROWNLEE, 1978133,have become available by virtue of their size for analytical electron microfor laboratory study. The extraterrestrial nature of a scopy. The sophistication of modern techniques (cf. compositionally ‘chondritic’ subset of stratospheric HREN et al., 1979) should thus make much more particles has been verified by measurements of noble detailed observation of IDP structure and chemistry gas elemental and isotopic abundances (RAJAN et ~1.. possible in the future. 1977; HUDSON et ul., 1980). Interplanetary dust is thought to be largely of cometary origin (MILLMAN. 1972; DOHNANYI, 1976; cf. BROWNLEE, 1979), and 2. EXPERIMENTAL METHODS comets in turn provide the most promising reservoir In addition to methods for handling small particles. the for unaltered samples of material present during the major techniques applied in this study were transmission collapse of the solar nebula. Thus IDPs provide posselectron microscope (TEM) brightfield and darkfield imagible clues to the origin of our solar system and the ing, selected area electron diffraction (SAED). secondary interstellar dust from which it was formed, as well as electron imaging, and energy dispersive X-ray (EDX) a source of present day solar system material disanalysis. Although data acquisition methods for all of these techniques are discussed in this section, data unctlysis tinctly different from meteorites or lunar samples. A methods for some of them are not. In particular, techdetailed inventory of the kinds of collected particles niques for analysing SAED data had to be developed which have major element abundances similar to which make fuller use of available information than do those found in chondritic meteorites is a prerequisite conventional methods. and these are presented with the observations in Sections 3.4 and 3.5. to understanding their individual histories. We report here on the detailed microcharacteriz2.1 Sump/e prepurution ation of 11 of the ‘chondritic’ IDPs by various techThe methods adopted to clean and mount individual niques of analytical electron microscopy. Preliminary IDPs are described by FFSW, and discussed further by FS results of this work have been described by FLYNN et and FRAUNDORF (1980a). In brief. individual particles were ~11.(1978) and FRAUNDORF and SHIRCK (1979). These cleaned of the silicone oil (used on the particle collector) by papers will be referred to subsequently as FFSW and immersion in a running stream of xylene or hexanes. Each aggregate was then pressed to -0.5 pm thickness between FS, respectively. In the electron microscope, individclean glass slides which had been coated with _ 10 nm of ual dispersed particles appear as huge objects. Just as Victawet. a water-soluble wetting agent. The slides (and a 3 ton meteorite is large for the field geologist lookdispersed aggregates) were then coated with 15 nm of evaping at hand specimens. so each 5 pm micrometeorite orated carbon and the carbon films (with specimens) were floated onto a 3 mm diameter electron microscope grid. is a large and detailed structure in an electron micro915

P.

916 2.2

Transmitted

electron

imqiny

FRA~~NU~RI

and SAED

Two TEMs were employed in this study: a Philips 3COG and a JEOL 1OOCX.equipped respectively with k45 and +60’ side entry goniometer specimen stages. Liquid nitrogen cooled specimen anticontamination devices on both microscopes, and diffusion pump traps on the JEOL, served to minimize beam-induced sample contamination. General purpose spring-clip specimen holders were used. instead of split C-ring rotation holders, in order to minimize chances of specimen loss or contamination during handling. All measurements of angles and spacings were performed directly on micrograph negatives using a ruler and protractor. The lens rotation angles for specimen images, and SAED images, were determined and checked by standard techniques (cf. HIRSCH et ul., 1967; HEAD ct ul.. 1973; or KERRUJGE,1969). SAED camera lengths were monitored by spacing measurements on patterns from an evaporated aluminum film. Lens settings and goniometer tilt angles were recorded for each micrograph taken, to aid in later image and diffraction analyses, and the self-consistency of all measurements was subsequently verified (spacings: +2”,,; angles: f2”) during the analysis of single crystal SAED data (Section 3.4). Spacing errors might have been decreased by an order of magnitude by means of a diffraction standard evaporated onto the specimens (e.g. HIRSC‘H et ul., 1967; KERRIDGE. 1968). but this was not done because it would reduce the quality of TEM images and hinder EDX analyses of the specimens. 2.3 Secondary electron imuging arid EDS analysis Prior to crushing, the particles were examined in a Cambridge S4-10 scanning electron microscope (SEM) equipped with an Ortec Si(Li) X-ray detector (25 pm Be window). Exposure to the 20 keV electron beam was minimized by limiting examinations to _ 5 min. and by using magnifications of less than 15,000. The complicated geometries, short analysis time, and absence of computer support for the EDX analysis system during these examinations limited the quantitative chemical interpretation of the X-ray data. After mounting for TEM work, secondary electron images and EDX data were obtained with the JEOL 1OOCX. which was equipped with scanning coils and a Princeton Gamma-Tech Si(Li) detector (7 pm Be window). Secondary electron imaging was crucial for recognizing the role of low-Z materials in the chondritic IDPs, as described in FS, and it provided a means of measuring sample thickness as well as the rate of beam-induced contamination. The scanning capability, coupled with a clean specimen vacuum. permitted acquisition of EDX data from very small (e&. lo- ’ 5cc) sample volumes. The thin-sample geometry considerably simplified analysis of these data. Detailed data and a full description of the method are found in FRAUNDORF (1980a). Briefly, the continuum in each region of interest was estimated by using an empirical model for background shape. and measurements of the continuum in regions free of characteristic X-ray peaks. After continuum subtraction, an empirical matrix overlap correction for characteristic peaks was performed. Relative elemental abundances were calculated by the peak-ratio method (cf. CLIFF and LORIMER,1975; GOLDSTEIN.1979: ZALUZEC. 1979) with a (small) first-order absorption correction, using characteristic peak ratios of the unknowns and calibration measurements on terrestrial silicates mounted in a similar fashion. The spread in independent measurements on the terrestrial standards (Fig. 1) gives a measure of uncertainties resulting from geometry effects. Considering uncertainties in the calibration measurements, systematic errors in inferred abundance ratios are probably smaller than 20°, (FRAUNDORF, 1980a). The ratios of characteristic peak intensities to the intensity in a region chosen for the absence of characteristic peaks (Ar K-line; E = 2.96 keV)

were used to monitor relative amounts of non-detectable elements (Z < 11) in the sampled volume. Because of poor control over continuum sources other than sampled Bremsstrahlung, such ‘peak to background’ ratios give ;I measure of ahsolu& abundances in the samplr I-eliablc to no better than a factor of 3. 3. OBSERVATIONS 3.1 The

particles

before

dispersiorl

The detailed studies of individual IDPs reported here show that there are fundamental ditTerence> between particles. It is likely that further work will lead to a taxonomy of particles and that certain subsets of particles will prove to be more informative than others, as is the case with meteorites. In conjunction with future experiments on IDPs. it would be highly desirable to have TEM information on the microstructure of each particle. This information would have to be obtained from small fragments broken off from the main IDP. since the material becomes virtually impossible to manipulate further once it has been dispersed on TEM grids. This necessity might be circumvented if it 15 someday possible 16) relate the gross features of the particles, as observed in the optical microscope and SEM, to their detailed microstructure. For this reason. data on the appearance of the particles before dispersion are listed in Table 1. The data are meager and non-quantitatrvc for two reasons. First, measurements were made prior to the availability of computer support for EDX analysis and prior to becoming aware of the wide range of inter-particle differences. Secondly. beam exposure of the IDPs in the SEM was minimized because speckmen interaction with 20 keV electrons, or electronbeam induced contamination, might have obscured the solar flare track record in IDP crystals (cf. FRAUND~RF et cd., 1980a). Data obtained bq the optical microscope include the particle size prior to dispersion. and transmitted and reflected light characteristics. SEM observations provided information on particle surface textures, which are often described as ‘porous. ’ ‘reentrant: or even ‘fairy castle’. The strut-. ture can be envisioned as a core surrounded to varying depths by irregular, rounded nodules or ‘grains’. The reentrant character of the particles can bc quantlfied by an estimate of sphericity: the ratio of surface area of the equivalent-volume sphere to that of the particle (WADELL, 1932). Sphericity estimates arc listed in Table 1, along with the range of surface ‘grain’ sizes. The table also contains a list of mnemonics to aid in future references to each particle. and a qualitative summary of the whole particle EDX data. A model of spatial heterogeneity in the chondritic IDPs based on TEM observations of their fine compositional grain size indicates that most of the variation in bulk composition from one particle to the next is due to differences in source material or atmospheric heating, and does not simply result from the smallness of the samples (FRACNDOKF. 198f)a)

Interplanetary Table

dust in the transmission

1. Summary

of data on IDPs

electron

prior to dispersion

Reflected light*

Grains (pm)

Particle

Mnemonic

Size Wn)

Transmitted light

13-05-01 c 13-OSOK 13-Oh-OSA 13-07-OXA

4 5510 I5 IO 4

Opaque Opaque Opaque

Black Black Black

l-3 0.2 3

I .i-08-09

Sam- 1 Gemini Zodiac Mash Chaugnar

Opaque

0.1

13-08-16 13-10-03 l-t-Ol-06A

Blowup 02 Tut

4x6 8 x II 10x 12

Opaque Opaque Opaque

l-1.O?-OVA

Bounce

15

Opaque

l4-06-09A

Lost + Found

Black/submetallic Black Black Black:submetallic Black; brown Black

14-07-?A5

Tex

Blackisubmetallic

* The reflected

3.2 Inrernd

6 x 8

light observations

IO

Transparent opaque Opaque’ transparent

were made with the particle

morphologies

Table 2 presents data on crystal structure and morphology for portions of each particle which have been mounted for TEM work. The objective is to point out trends in these features which helped to distinguish material in one IDP from that in the next. Included is an estimate of the volume fraction of the original IDP that was examined to provide the information shown in the table. These volume fractions are small for the following reasons: (i) portions of the IDP are often lost for TEM work during preparation of the mounts; (ii) when large fractions of a 10 ,um particle have been successfully mounted for TEM. limitations of time prevent more than a cursory examination of most of the particle; and (iii) voids in the IDP prior to crushing constitute volume which may never be examined in the TEM. Also given in the table are a description of the apparent abundance. structure. and morphology of crystal grain size fractions, as well as some identified crystal types. in each IDP. Identifications made on the basis of EDX analysis or SAED are included. The more rigorous SAED identifications are discussed separately in Section 3.4. The structure and morphology of each crystal type are characterized in Table 2 by ascribing a shape class, the dominant volume defect structure, an edge roundness category. and an edge type. (i) The shape classification is based on the categories of ‘oblate’. ‘prolate’, ‘equant’ and ‘bladed’ proposed by Z~NGC (1935; cf. ROYSE, 1970). (ii) Dominant volume defect structures in crystals can usually be determined only in crystals much larger than 10nm in size. This is because volume defects in TEM imaging are often recognized by features from strain-field contrast mechanisms which operate over such distances in a crystal. Observed defects include lamellae from twinning or exsolution (cf. WENK, 1976), precipitates (cf. HIRSCH et u/., 1967) possibly mosaicism (a condition

q I7

microscope

Sphericity

Major EDX peaks decreasing order

0.6

SI. Fe. Mg. S Si, Fe. Mg Si. S, Fe Fe. Si. S. Ca

I

0.1

S. Fc, Si

0.1 I 0.1 2

0.1 0.1

Fe, Si. S. Ca. Mg Fe. Si. S Si. Fe. S. Mg

0.14

0.2

Si. Fe. S. Mg. Ca

0.3 2

0.2

SI. Fe. Mg. S. Ca

0.1-l

0.1

Si. Fe, S, Mg

lying on a whrte substrate

in which monomineralic grains are found as aggregates of small deformed crystals; cf. ASHWORTH and BARBER. 1975) and radial crystallization like that found in the low-Ca pyroxene chondrules of some meteorites (cf. NELWN et d., 1972; WASSON. 1974). (iii) Edge roundness is conveniently defined in the 2-dimensional (projected) image of the crystal in TEM micrographs as the ratio between the average radius of curvature for particle corners and the radius of the maximum inscribed circle (WADELL. 1932; cf. Rousn, 1970). For simplicity, only three classes of roundness are used: very round (>0.6). round (0.25.Q.6) and angular (<0.25). (iv) Finally, crystal edges were also classified with regard to their probable origins (as opposed to their degree of weathering) as either euhedral (crystallographic), elastic (broken), or droplet-like (surface-tension).

3.3 Intrrnul

chemistries

Compositional data obtained from the particles after dispersion are summarized in Table 3. It lists the estimated volume fraction of each particle from which EDX data were obtained, the average atomic abundance ratios relative to silicon for detectable elements in sampled portions of each IDP, and estimates for several parameters which have proven diagnostic in the study and classification of meteorites. Table 3 also provides values from the literature for various comparison materials. Results from the analysis of over 150 individual regions, most of them _ 10-i’ cm3 in volume, are listed in Table 4 for later reference. Although the spectra are representative of the portions of each particle examined, they do not necessarily result in good averages over the whole particle. Figure 1 shows a ternary diagram of normalized Mg:(Si-Ca):Fe abundances measured on distinct IDP silicate grains. Also included on the diagram are

918

P. FRAUNDORF Table 2. Summary of data on structures internal to the IDPs _I____-.__

Particle 13-0%OIC

13-05-03c

13-06-05A

Mnemonic

-

Sam-l

Gemini

Zodiac

Fraction examined

Noncrystalline

0.02

0.0’

0.001

0.2

0.1

matrix caked

Er-cla

0.95 matrix

0.05

0.4 matrix pliant

13-08-09

Chaugnar

0.4

0.95 matrix

13-10-03

14-Ol-06A

oz

Tut

0.02

d.001

14-03-09A

Bounce

0.001

14-06-09A

Lost + Found

0.02

14-07-2A5

Tex

0.001

0.5 E-LAM a-cla 0.5 -ECT a-dro

0.001

0.06

0.2 Er-cla

0.5 matrix pliant

Mash

Blowup

Olivine

-

13-07-08A

13-08-16

Crystal Crystal Crystals: < 10 nm 10-100 nm >lOOnm

0 I

0.3

l-es

Fe r ”

3 -EC-f .+dro

1

P-UVD v-cla

I

O-LAM r-euh

0.2

?

0.05

P-L-AM a-euh

0.2 coat pliant

0.3 E-MOS -cla

0.5 E-MOS

0.6 matrix pliant

0.3 E-CLR v-dro

0.1 E-CLR v-dro

0.5 matrix pliant

0.3 E-CLR

0.2 E-CLR

0.6 matrix caked

0.2 E-UVD

-ClLi

2 Ea-cla

17,’ E-MOS

1

~’

t 3 -M-OS

-Cl3

1

3

F-t&D \-dl
E-CLR r-dro

I

E-CLR I-drr,

I

E-LAM i3-Ch

0.2

0.9

0.1 coat

a-cla 0.4 matrix

.~

Pyroxene _ 4 E-LAM a-euh

- _-

0.1

0.2

2 0-CLR a-cla

0.2 -PRE r-cla

* Column 3 contains an estimate of the volume fraction of the IDP examined for this table. Columns 4~7 contain volume fraction estimates and descriptions of typical components categorized according to their crystallinity. Columns 8-11 list the number and description of common mineral grains, larger in size than 100 nm. which were identified. The column ‘Fe + ?’ refers to grains whose sole major detectable element was iron. Noncrystalline material is classified as matrix or coating when appropriate, and caked (as opposed to pliant) if it evidenced cracks when crushed. Crystalline material is classified according to shape (P = prolate; 0 = oblate; E = equant; B = bladed), volume structure (CLR = clear, undamaged; LAM = lamellae; MOS = mosaicism; PRE = precipitates; ECT = radial crystallization: UVD = unidentified volume defect), roundness class (v = very round, r = round, a = angular). and edge type (euh = euhedral; cla = elastic; dro = droplet).

the results of numerous

analyses on silicate calibration standards. Stoichiometric olivines should plot on the (SiCa) = 33% line. Stoichiometric pyroxenes should have (Si-Ca) = .50%, or larger if other cations (e.g. Al, Ti or Cr) are present. Silicates in the chondritic IDPs seldom give measureable Ca, Al, Ti or Cr signals except in the case of particle 13-08-16 (Blowup). In this particle, silicate grains which had more silicon than could be expected for olivines generally contained variable amounts of Ca, Al and Cr: common pyroxene cations (e.g. FS, Fig. lOa). Figure 2 provides a histogram of (S/Fe) abundance ratios measured on IDP grains whose major detectable elements are iron and/or sulfur.

3.4 Single crystal SAED Direct information on the mineral structure of IDP aggregates, prior to this work, has come primarily from the X-ray powder diffraction work of Brownlee and coworkers in Seattle. The X-ray data, obtained from individual IDPs during exposures lasting a day or more in a reduced-diameter Debye-Scherrer camera, provide a relatively accurate measure of strong-line spacings of the major diffracting phases. The X-ray data indicate the existence of magnetite and pyrrhotite in most chondritic IDPs, olivine in two cases, and hydrated silicates in one atypical particle (BROWNLEE et al., 1977; BROWNLEE, 1978). The technique does not permit mineral identifications

Interplanetary Table

Particle 13-OS-OlC 13-0%03c 13-06-05A 13-07-08A 13-08-09 13-08-16 13-10-03 14-Ol-06A 14-03-09A 14-06-09A

Mnemonic

l4-07-2A5

Sam-l Gemini Zodiac Mash Chaugnar Blowup 02 Tut Bounce Lost + Found Tex

Average: Chondritic

IDP average’

dust in the transmission

3. Averages

Fraction examined

Mg/Si

AljSi

0.03

0.71


0.0001 0.0003 0.01 0.004 0.0004 0.0002 0.0001 0.0001

0.83 0.59 0.60 0.82 <1.17 <0.08 0.59 1.51

0.004

0.71

:

0.76 0.85

<0.12 0.06

:

1.06 0.92 1.04 1.08 0.96 1.05 0.94 0.97 0.78

0.09 0.09 0.08 0.12 0.10 0.10 0.06 0.06 0.06

Lunar mare4: Lunar highland4: Terrestrial crust’:

0.30 0.25 0.10

0.38 0.62 0.31

Cl chondrites’ Cl matrix3 : C2 chondrites’: CV chondrites’: CV matrix’: CO chondrites’: L chondrites’: H chondites’: E chondrites’:

electron

of data on IDP internal Atomic

abundance

SjSi

Ca/Si

919

microscope

chemistries ratios CriSi

Fe/Si

Ni/Si

NiS

FOX

0.29 <0.06 <0.06 prior to EDX analysis0.67 <0.05 <0.04 0.35 <0.05 <0.05 0.45 <0.07 <0.07
0.87

< 0.07

1

4

0.48 0.88 0.45 0.62 2.68 0.19 0.92 0.41

<: 0.05
0.14<0.07

<0.04

i

0.52-

0.36 0.35

< 0.06 0.05

< 0.05 0.01

0.80 0.63

.: 0.08 0.04

0.46 0.13 0.23 0.12 0.06 0.06 0.10 0.10 0.28

0.07 0.01 0.07 0.08 0.07 0.07 0.05 0.05 0.04

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.90 0.54 0.84 0.74 0.92 0.80 0.58 0.81 0.75

0.05 0.05 0.05 0.04 0.04 0.04 0.03 0.05 0.04

0.001

0.28 0.37 0.17

0.0004

0.26 0.11 0.23

0.0004

FVA 4

3 2 7 I 3

2 36 3 I 47 24

16 2

58

* Column 3 contains an estimate of the volume fraction of the IDP examined for this table. NiS denotes atomic ‘I0 Ni in iron sulfides, FOX denotes average atomic y0 of Fe/(Mg + Fe) in silicates; and FVA denotes the standard deviation in FOX. References are ‘BROWNLEE (1978a); 2M~~~ (1971); 3M~S~~~~ and RICHARDSON (1977); “T~JRKEVICH (1973); and 5FAIRBRIDGE (1972).

0

13-c6-CK San-1

+

I3-06-o%

(SiCa)

bdlac

)s[ I3-ce-09anWar .

1>06-16

&WJP

0

13-10-03

02

[3

14-01-o6pTul

+ A A

14-03-OWBOU~W 14-06oM

Lost *Found

l4-0?2kSTu

l OlibW Smndods +

!+mmr

Standards

0

0. 0

/

\

\

/ M9

Fe

Fig. 1. Mg:(Si-Ca): Fe atomic abundance ratios in IDP silicate grains. The terrestrial standard clusters represent 6 clinopyroxene spectra and 11 olivine spectra. The arrow beneath one of the analyses from 14-06-09A (Lost + Found) indicates that measured Si is probably an overestimate because of fluorescence of the silicon peak caused by a gold grid bar (Table 4, analysis 2A-F25).

Interplanetary

Atomic Particle 13-0x- 16 Blowup

Analysis number

Si

of detected S

Ca

Cr

l-F68 I -F64


l-F71 l-F72 l-F73 I -F74 l-F75 l-F76 1-F77 l-F81 l-F82 l-F83 l-F84 l-F85 1-F86 l-FX7 2A-F34 ZA-F35 2A-F40 2A-F41 2A-F42 2A-F43 2A-F44 ZA-F45 2A-F46 2A-F47 2A-F70 ZA-F72 ?A-F73 Mean:

0.27 <0.17 0.27 0.02 0.34 0.24 0.25 0.04 0.29 0.03 0.13 0.11 0.26 <0.06 0.29 0.03 0.25 0.04 0.22 0.08 0.36 <0.04 to.32 <0.23 0.46 <0.02 0.10 <0.07 0.19 0.04 0.37 0.03 0.10 <0.03 0.39 <0.03 0.30 0.04 0.19 0.04 0.47 <0.02 0.38 0.03 0.42 <0.02 <0.04 <0.03 0.33 <0.04 0.41 0.03 <0.13 <0.09 0.31<0.04 0.1 I 0.03

0.47
<0.60 <0.29 <0.37 <0.39 <0.34 <0.43 <0.36 0.66 <2.2 <0.27 <0.09 <0.07
<0.35 0.21 0.40 <0.23 iO.20 <0.25 <0.21 0.34 <1.3 0.27 0.12 <0.04 <0.13 <0.02 <0.03 0.37 0.05 0.56 0.39 0.16

lA-F126 lA-Fi27 lA-F128 IA-F129 lA-F130 lA-F131 lA-F133 lA-F134 lA-F135 2A-F2’ ?A-F’3 ‘A-F24 ?A-F27 3A-F48 2A-F49 2A-F50 2A-F51 :A-FKO ?A-FXI IA-FX, Mean :

I-Fly l-F20 l-F?1 I-F44 3-F45 ‘-F46 2-F47 Z-F48 2.F4Y

<0.43 <0.21 <0.27 t0.28 <0.25 <0.31 <0.26 <0.39 <1.6 <0.19 <0.06 <0.05 <0.16 <0.02 <0.04 iO.01 <0.04 0.03 <0.03 <0.09

<0.10so.o5<0.170.23

0.0x

0.20

< 0.07 0.37 0.21 0.04 0.03 -co.04 <0.03 0.04 <0.04

<0.05 co.03 <0.06 0.05 0.04 0.04 0.05 0.06
0.71 0.73 0.44 0.70 0.58 0.69 0.67 0.80 0.82

electron

elements

0.38 <0.02 0.22 0.05 0.23 0.12 0.34 <0.24

CT:

I4-Ol-06A Tut

Al

fraction

l-F64 l-F65 I -F66 l-F67

0:

13-10-03 oz

Mg

dust in the transmission

Fe

Ni

0.15 0.47 0.1 I 0.27 <0.51 0.09 <0.09 0.09 <0.12 0.05 0.55 0.16 0.28 0.44 0.05 0.15 0.21 0.27 0.14 0.19 0.18 0.19 0.30 0.20 0.16 0.18 0.16 0.23 0.29 0.44 0.18 0.08 0.07 0.7?zE

co.01 <0.02 co.04 <0.13 <0.53 co.01
0.13

0.0’

10.23 0.55
921

microscope

Det. mass fraction

S:Fe

0.3 0.2 0.1 co.1 co.1 0.2 co.1 0.4
<0.08 <0.05 <0.45 co.61 <0.18

0.36 ~~ <0.26 <0.03 <0.26 <0.14
Ni,‘( Fe + S + Nil

39:47: 15 25:24:52 26:61:13 34:39:27 :99: <52 32:58:11 27:73: < 10 34:55: 12 44~56: < 16

< 0.06 <0.4 <0.X <0.33
32:61: 7 31:13:57 15:68: 18 26~46128 31:24:46 33:61: 7 28:54: 19 36:44:21 <32:73:27 48:38:15 10:71:19 21:59:20 38:42:20 17:32:51 40:40:21 32:52:17 20:60:20 47:37: 16 40:36:24 43:28:30 <9:14:86 36:45:‘0 46145: 9
m~m:~64199 <49:35:65 <49:53:47 <70:<41:99 <74:<44:99 <73:143:99 <72:<43:99 66:34: ~22 - ‘p :99 <41;40:60 <11:14:86 <8: <5:99 <37:<22:99 <8: t5:99 <9: <5:9Y 63:37:<1 <7: 5:95 41157: 3 61:39: <2 < IX:‘?:78

<9:89:11 27:73: <3 22:44:35 5:85: 10 5:78:18 <5:88:12 <4:86: 14 4:89: 7 <5:94: 6

0.45 0.41 0.24 0.44 0.54 0.41 0.51 <0.27
<0.25 <0.13 10.15 <0.16 <0.14 10.18 <0.15 <23 <0.92
<0.24 <0.12 <0.15 <0.16 <0.14 <0.1x <0.14 <0.22 <0.90

0.83 1.1 0.69 0.80 1.2 0.70 1.0 ~~
0.47

<0.20 10.13
0.21 <0.03 <0.06 0.1 I 0.19 0.17 0.15 0.05 0.14

KO.04
-co.04 0.08 -co.04 io.03 <0.03 -co.03 <0.05 0.34 <0.05
0.1 0.1 0.1 0.4 0.7 0.2 0.4 0.5 0.2

2.7 ~~ <0.16 1.4 1.5 1.9 1.4 0.84 2.x

<0.12 < 1.0 <0.13 0.09 0.04 0.06 0.07 < 0.07 <0.07

<0.03 <0.41

Mg:Si*:Fe

922

P. FR.~UNWRF Table 4--tcontinued) __....._~__.~ Atomic

Particle

(continued)

14-03-09A Bounce

Analysis number 2-F50 2-F51 2-F52 2-F53 2-F54 2-F55 2-F56 2-F57 2-F58 2-F59 Mean : 0:

Mg

Al

<0.03 0.05 0.05 0.04 <0.03 0.04 <0.04 0.06 <0.04 <0.03 0.10 0.27 <0.03 0.03 <0.04 0.05 0.08

0.03

2A-F14 0.22 0.06 2A-F15 0.26 0.06 2A-F16 0.29 <0.06 2A-F17 0.17 0.06 2A-F20 0.23 0.09 2A-F21 0.22 0.06 2A-F28 0.20 0.06 2A-F29 0.19 0.07 2A-F30 0.21 0.04 2A-F3 1 0.24 0.04 ZA-F32 0.19 0.11 2A-F33 0.20 0.08 Mean : 0.220.05 0: 0.03 0.02

14-06-09A Lost + Found

2A-F13 2A-F25 Mean:

0.52 0.02 0.41 <0.08 --G-la

14-07-2A5 Tex

l A-F90 lA-F5 lA-F6 lA-F7 lA-F8 lA-F9 IA-F11 lA-F12 lA-F13 Mean :

0.36 0.04 0.29 0.08 0.25 <0.04 0.35 <0.08 0.21 0.06 0.33 0.04 0.26 0.04 0.32 0.04 0.27 0.09

0:

fraction Si

of detected S

Ca

elements Cr

Fe

Ni

0.04 0.08

0.38 0.30 0.47 0.27 0.25 0.27 0.40 0.42 0.38 0.35 0.12 0.31

<0.02 <0.02 <0.03 0.02 <0.04 0.04 <0.03 <0.03
0.06

0.01

0.84 0.70 0.83 0.79 0.16 0.23 0.45 0.60 0.66 0.87

0.35 0.39 0.24 0.47 0.43 0.40 0.34 0.33 0.35 0.37 0.58 0.41

<0.03 <0.02 40.04 <0.03 <0.05 iO.03 < 0.03 40.03 0.02
<0.02 <0.02 <0.03 <0.02 <0.04 <0.02 <0.03 <0.03
<0.02 <0.02 <0.03 co.02 <0.04 co.02 <0.02 <0.03 <0.01 co.01 <0.05 co.02

0.01

0.01

0.01

0.33
0.05 co.02 <0.03
0.01
0.01
0.02

0.06

0.02 io.01


Ni/‘(Fe + S + Ni)

.___.~..._

Mg:Si*:Fe

0.3 0.5 0.3 0.2 0.4 0.1 0.4 0.2 0.2
2.0 1.6 2.5 1.2 0.83 2.2 1.3 2.3 0.86

< 0.08 < 0.04 < 0.08 < 0.09 0.03 co.10 0.03 0.05 <0.07 10.3’

c.3.Y5: 5 7:84: 10 t4:95: 5 15:Yl: Y c-7:26:74 13.52:25 <5:67:33 %?:xs:lj lO:75: 16 ; 17 99: -z?

0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.6 0.4
<0.07 <0.07 CO.08 <0.09 <0.19
< 0.06 CO.05 <0.06 0.08 .:0.13 0.14 <0.06 <0.06 <0.02 < 0.03 <0.31 < 0.06

13:37:40 X:41:32 .?0:2j:47 I’)-51:30 26:47:2X Jj:35:30 ” 36:43 --.. ll.35:45 ‘2:38:40 ‘6:3Y:36 71.65:14 ??:4j:34

~ 0.2 0.13
0.5 0.1 0.2

<0.07 <0.28

co.05 CO.18

j.i.34:l-l 41:4O:lY

0.13 0.20 0.38 0.16 0.34 0.20 0.10 0.26 0.31

0.3 0.3 0.3 0.5 0.3 0.2 0.4 0.2 0.2 0.3

0.37 <0.08 <0.07 co.08 co.05 <0.14 0.54
< 0.06 < 0.06 < 0.05 iO.06 < 0.04 co.10 < 0.06 < 0.07 c 0.05

41:44: I5 3’.46:?3 26:36:3Y X:44:18 ‘?:42:36 34:46:21 io-59:ll 34:39:2X j(!: !7:34


0.290.060.42<0.03<0.02~0.22<0.01 0.05

S:Fe

_._

0.01

0.37
Det. mass fraction

0.09 co.01

Typical volume sampled is c lo- I4 cc. Col umn 11 contains an estimate of the mass fraction of elements in the volume which were detectable (Z r ll), based on peak to background ratios. Although it is only attributed factor-of-3 significance, it does show trends in the presence of low-Z materials from particle to particle. Mean values are written as upper limits only if upper limit terms contribute more than lo’.” of the sum. * For purposes of ternary diagram (Fig. l), the pyroxene cation Ca has been subtracted from the Si value for these ratios. t The measured Si is probably overestimated because of fluorescence of that peak by the gold grid bar.

requiring single crystal diffraction on IDP crystallites, and it does not allow one to correlate diffraction spacings with specific IDP constituents. Because the interaction of electrons with atoms is stronger than that of X-rays (cf. COWLEY, 197.3, and because of the superior imaging capabilities of modern electron optics, single crystal diffraction measurements on submicron crystallites, and darkfield imaging of diffracting crystals with subnanometer resolutions, are possible on a routine basis in the TEM. An added advantage is that electron scattering is less biased toward heavier atoms than is X-ray scattering. Thus lines

from low-Z silicates compete more effectively with lines from iron oxides and sulfides. A major hindrance to SAED work on examined IDPs is the rarity of single crystals larger than 0.2 pm in size. Concomitant problems include the absence of visible Kikuchi lines or extended spot arrays. and the frequent need to verify by darkfield imaging that a given diffraction spot was indeed from the crystal of interest, and not from an adjacent one (cf. FRAUNDORF, 1980a). These problems were countered by the development of an approach to the analysis of SAED data from single crystals which takes into account not

Interplanetary dust in the transmission electron microscope

0 0

05 S/Fe

Fig. 2. Sulfur-to-iron

ratios

10 atomic

in iron and iron-sulfur

only information on spacings and angles between diffraction spots on a given micrograph. but also information on the angles between spots found on diffraction micrographs taken at d@rent sample orientations (FRAUNWRF. manuscript in preparation). This approach maximized the usefulness of diffraction patterns with only one diffraction spot, and allowed identification of crystals that would otherwise have been too small. For the large fraction of IDP crystallites which are still too small for single crystai work, polycrystal SAED patterns are shown in Section 3.5 to provide information complementary to that provided by the above technique. The basic data required for SAED analysis are the relative spatial coordinates of a set of electron diffraction spots from the crystal of interest. In this work, relative coordinates were obtained by measuring the length of the line segment drawn between a given diffraction spot and the central (incident-beam) spot on a TEM diffraction negative. as well as the angle that the line segment makes with respect to a reference direction on the negative. The coordinates of each spot are then transformed into a common three-dimensional coordinate system. referenced to the specimen. by an Euler angle transformation. The three Euler angles for each m~~rograph are. in sequence. the magnetic lens image rotation angle for the particular lens setting, the tilt angle of the goniometer stage, and the rotation angle of the grid holder (if a tiltrotate holder is used). Proposed crystal structures can be eliminated at the outset if they do not have a set of lattice spacings consistent with those seen in the microscope. For candidate structures not eiiminated in this way. the observed array of spots is tested against all possible arrays from the reciprocal lattice of the candidate structure by comparing the M measured spacings, and the m(m-1)/Z angles subtended at the origin by lines drawn to each pair of spots. For this work, alternate indexing possibilities were tested. within specified tolerances on measured spacings and interspot angles. using a PDP-BA computer. As a result of this test, the proposed structure is classified either as ‘inconsistent’ with the set of spots, in ‘marginal agreement’. or in ‘good agreement’. The criteria for marginal agreement are that no interspot angle disagrees by more than 5 ‘, and no measured spacing disagrees by more than 5”,. These criteria define an empirical

15

ratio----grains

identified

in the chondritic

IDPs.

‘safety zone’ separating the distribution of measurement errors found when testing the correct structure. and the distribution of errors in interspot angles and spacings found when testing structures dissimilar to that of the specimen. In addition to the above. the criteria for good agreement are that the root mean square disagreement of interspot angles be less than 2 . and that the standard deviation of percent errors in lattice spacing not exceed I!“,,. These criteria were chosen to be as restrictive as possible. based on the observed size of measurement errors for known crystals, to maximize the ability of the test to distinguish between crystal structures which were simtlar to one another. The lattice parameters and space groups for various crystal structures were obtained from standard literature, and are listed in FRAUNDORF(1980~1). For this work spots expected to be absent due to space group symmetry elements (cf. NIJFFELD.1966) were allowed in the proposed indexing because such non-allowed spots often occur tn diffraction from tiny crystaiiites. The primary of causes of such ‘anomalous reflections’ {cf. HIRSCH cr tti.. 1967: KERRIDGE, 1968) are probably: (i) spot broadening from very thin crystals: (ii) dynamical diffraction in which diffracted beams are themselves diffracted; and (iii) twinning or the presence of overlapping phases. Even wtth such nonallowed spots permitted in the rectprocal lattice of the candidate crystal, the data were adequate to distinguish between dissimilar crystal structures. In most cases. it &as possible to go a step further and infer the conventional Bravais triplet. within errors. for the identified structures.

The following paragraphs summarize the major crystal identifications. Positive identification required an array of at least 3 diffraction spots not coplanar with the origin, and good agreement with the proposed structure. To ensure against spurious agreements, several alternate structures not ruled out on the basis of EDX analysis, or other considerations, were often tested. Except in the case of structures similar to the identified one (as in the case of pyroxene or pyrrhotite polymorphs). none of the alternative structures met the criteria for marginal agreement with the observed spots. The raw spot data, in the form of spherical coordinates. are listed in FRAUNDORF (1980a) should further testing be of interest.

Four olivine crystals have been identified. These include a rounded 1 x 0.2 pm crystal (FFSW, Fig. 4a) from 13-06-05A (Zodiac) with a composition near Fog, (Table 4. analysis 2-F78). an irregular 0.3 pm crystal (FFSW. Fig. 4d) from 13-08-16 (Blowup) which was too near a grid bar for EDX analysis, 9 rounded 0.3 pm crystal from 13-10-03 (0~) with composition near Fog9 (Table 4. analysis ‘A-FSO), and a I x 0.6 pm crystal (FRAIINDORE, 1980a. Figs 3~ 5 and 3 7; FKALJNDORFet (I/., 1980a, Fig. 3c. d) in 14-06-09A (Lost +- Found) with a composition near FoTO (Table 4. analysis 2A-F25). In addition to these SAED identifications. two identifications of olivine by EDX data are supported by single crystal analysis of _7-spot diffraction arrays: a portion of the non-stoichiometric grain of EDX analysis 2A-F46 (Table 4: also Fig. 4) from 13-08-16 (Blowup). and the olivine ‘microchondrule’ from 13-10-08 (Or) of EDX analysis l A-F134 (Table 4: also Fig. 5). Three pyroxenes have been identified by SAED. In all three cases, (100) streaking in the diffraction pattern was probably associated with (100) lamellae seen in the direct image. The presence of diffraction spots with (h + k) odd argues against the pigeonite C2/c space group, and the absence of enstatite ‘18 A’ spots thus suggests the clinoenstatite P2,ic space group for all three crystals (cf. DEER et LI/.. 1963; NUFFELD. 1966). The first of these pyroxenes, found in particle 13-06-05A (Zodiac), was a euhedral lath well over a micron long (FFSW. Fig. 4~). with rounded edges. The proximity of the crystal to a Cu grid bar prevented EDX analysis. The second pyroxene was one of several angular euhedral laths (FS. Fig. 3a) in particle 13-08-09 (Chaugnar). EDX analysis of the crystal (Table 4. analysis O-F02) indicated nearly pure enstatite. In all cases the laths were elongated in the clinopyroxene [lOO] direction. The third crystal was a tiny angular crystal barely 0.2 pm in size (FFSW, Fig. 4b) from the particle 14-Ol-06A (Tut). Only magnesium and silicon were detected (Table 4. analysis l-F20). but the huge silicon excess in this spectrum over that expected for pyroxene is unexplained. unless the coating of the crystal contains silicone oil (from the particle collector) which has not been removed by the cleaning process. Two iron sulfide crystals in particle 13-10-03 (Oz) have been identified as pyrrhotite by SAED. The first was part of a 0.2 pm polycrystalline clump, which as a whole was enriched in iron relative to pyrrhotite (Table 4. analysis 2A-F27). Because of the large number of diffraction spots coming from other portions of the clump, finding only 3 spots whose origin in a single crystal could be verified by darkfield imaging was a considerable task. The 3 spots could be indexed using pyrrhotite subcell indices (cf. DEER er ~1.. 1966; MORIMOTOet ul.. 1970; PIERCE and BUSECK. 1976), and hence any pyrrhotite pseudomorph might have given rise to them. The second example was provided by a largely single crystal grain, 0.2 pm in size.

with a S/Fe atomic ratio consistent with pjrrhotite (Table 4, analysis 2A-F48). All observed spots could be explained in terms of the hexagonal pyrrhotite subcell. In particular. an [OIO] reciprocal lattice patlern evidenced no superlatticc spots. This c~b~ci-vati~)tl argues against the large superlattice structures corn-mon in terrestrial pyrrhotitcs. On the other hand. the presence of subcell (001) reflections is consistenr with the 2C superlattice structure of troilitc. lhc common meteoritic form of pyrrhotite. Further Mark I\ necesxary to determine whether the trollitc sis 2A-F.51) with the high-temperature tacmtr rtructure has been identified by SAED in i3-10-08 iOz). 9 polycrystalline taenite droplet m the same partictc i< discussed in the next section. Finally. the magnctrlr crystal structure has been identified in ihi- textured volume of a grain (see Fig. 6) from 14-03-09A (Bounce). Surprisingly. EDX analysis Indicated the bulk composition of a silicate. suggesting thal the magnetite pattern was from tiny oriented crystallites associated with a silicate grain (Table 4., ;malysr? 2A-F33). Similar structures were characteristic o! much of the particle examined in the TEM. Although the single-crystal identification was based on onl) three magnetite spots, all with Miller indIces divisible by 4 when referenced to the con\entlonal diamond face-centered unit cell. a full set of polycrystallinc magnetite rings associated with the particle add weight to the structure identification.

Evidence indicates that diffractIon on 0.1 jrm and larger crystals may not provide ux with a balanced picture of IDP mineralogy. For example. ihe X-rah powder data identify magnetite and pyrrhotite a> dominant diffracting phases (BKOWNL.L:I, 1978a). Although 0.1 pm Fe, .S crystals have been found in many IDPs, single-crystal magnetite pattern> have been obtained only from the large. sulfur-pool. ironrich IDP 14-03-09A (Bounce). ‘Thus if magnetite is abundant in the examined particles. it probably cxlsts as crystals too small to be identified by single crystal SAED. Diffraction patterns from polycrystallin< portions of the IDP aggregates contain the only information available on mineralogy of small crystallitea. which sometimes make up the bulk of the qstalline material. Polycrystal patterns frequently consist of a series of discontinuous, concentric rings (e.g. FFSW. Fig. 3d). Since many individual crystallites contribute to such a pattern, it is difficult to correlate individual spots with individual crystals. The information provided by polycrystal diffraction micrographs is therefore primarily contained in the observed spacings. The usefulness of the spacing information is limited by the large uncertaintics in camera length (as compared to X-ray powder diffrac-

925

Interplanetary dust in the transmission electron microscope tion). On the other hand. the relatively lower bias of electron scattering processes for heavy atoms, the ability to take patterns from various portions of an IDP, and the darkfield technique of imaging only the material responsible for a given diffraction ring, all allow polycrystal SAED to provide information not available by other means. For some polycrystal patterns, EDX data greatly limit the set of possible minerals. However. for a ‘chondritic’ aggregate the set of minerals consistent with EDX data is still large. A reasonable objective for the chondritic IDPs is thus to test polycrystal SAED data against the set of minerals which have already been identified in the IDPs by single crystal diffraction. This probides information on the relative abundances of these minerals in the fine-grained portions of the IDPs. Further, if the dominant features of the polycrystal patterns are fully explained by a mixture of such minerals. then one can argue that other crystalline phases arc not abundant. In order to guide the analysis of the chondritic materials. a list of key spacings for six minerals was prepared (Table 5). The list was compiled on the basis of X-ray powder diffraction files, structure factor calculations. and SAED measurements on analog materials. A ranking on the basis of probable relative intensities is given in column ‘I’, the convention being that 9 denotes the brightest spot. and 8. 7. 6. 5. 4, 3. 3. I, w(aeak) denote the other spots in decreasing order of brightness. The clinopyroxene list has been compiled using information on the three Mg, Ca. Fe monoclinic pyroxenes: clinoenstatite. pigeonite. and diopside. For the orthopyroxenes: enstatite, hypersthenc, and ferrosilite. most of the predicted key spacings are, given the 2”,, uncertainties, already found in the clinopyroxcnc list. Hence the set of clinopyroxene spacings should suttice as a test for pyroxencs in general. Tahlc (,

5. Kq =

I d

spacings I hkl

for minerals (,

=

considered

I tf

I hkl

Analysis of chondritic IDPs is carried out In the follouRing spacings are measured from TEM negatives of the diffraction patterns. Often SAED micrographs taken at different goniometer orientations and at diRerent ing manner.

locations on the particle are combined to obtain a larger sample of spacings. The spacings are listed in order of increasing length. and then ranked according to relative intensities according to the convention: A = 30 IOO”,, 01 the brightest spot: B E l&30”,,; and C = 0 IO”,, Adjacent to this list, there are six columns: one correrpondlng to each of the six candidate minerals listed In Table 5. Each observed spacing is compared with the spacmgs m the kq spacing list. When an obsrrbed spacing nor range c>f spacmgs) falls within 2”” of a key spacing for a gl~en mineral. the relative intensity for the key spacmg I:, \\rltten on the same line as the observed spacmg in the column iorresponding to the key mineral. This is done in Table 6 fol polycrystal patterns from 3 crushed. rncmo-mlneralic terrestrial standards. and in Table 7 for 7 chondritic IDPb. If more than one key spacing for a particular mineral cornsponds to an obserbcd spacing. then the rno\t Intense is listed. When the listing IS completed. the numbers in a row corresponclmg to each measured spacing denote the various crystal structures which might yield that apacing. Each vertical column lists the spacings for each mineral which were deteced in the pattern. A simple crl!t‘rIon can then be established. If 4 or 5 of the most intense hpaclngb for a given mineral are detected, and if at least one of those spacings is not explicable by another dominant mineral. then the data are considered evidence for the presence of the mineral. If. further. one or more of the more intcnsc observed spacings (category A or B) are correlated with the 3 most intense key spacings of a mineral. then the mlneral is considered a major diffracting phase. Even though a given spacing might be explained by a number of different mineral>. these criteria provide unamhigu~>u~ ldentltications of the major diffracting species in the crushed monomineralic standards (Table 61 This ih in spite of the fact that. in comparison to the IDPs, the standards were in the form of very coarse. crushed rragments L+hich almost certainly evidenced some preferential orientation\ ~cith respect to the crushing plane.

Analyses of the IDP data lead to clear cut interprctations which are summarized in Table 8. in which in the analysis

of polycrystallinlz

q = I [I

1 hkl

SAED patterns

y = I ri

I hhl _.

~~~ magnetite 0.206 0 337 0.395 0.413 0.476 0.5X3 0.6 I Y 0.673 0.7x1

= m ’ III ; 220 Y 311 I 221 5 300 4 322 6 511 X 440 3 533

taenite 0.4x7 0.557 0.78X O.Y,4 0 Yh5

Y III X 002 7 0’2 6 I13 5 221

pyrrhotlte 0.174 0.214 0.336 0.378 0.404 0.47X~O.487 0.587 0.623 0.693 0.775

= 3 5 7 K 2 Y 6 I 4 w

s 001

= Y x 7 6

k 101 010 112 220

2Nl 101 202 170 203 401 104

= t kamacite 0.493 0.697 0.x53 0.986

* The diffraction spacing (q = I tl) is the con\ention: Y = most intense: 8 = \\eahrr. The pyrrhotite Miller indices spacings were chosen in light of a range

olivine 0.196 0.233 0.257 0.186 0.334 0.361 0.398~0.407 0.44lLO.444 0.463 0.533 0.57lLO.578 0.618 0.668

=

0

4

0’0 w 110 x 071 2 Ill I 001 7 130 Y II2 6 17’ M 1’0 3 150 5 __271 w I33 u Oh2

clinopyroxenc 0.158 0.216 0.217 02x4 0.300 0.305 0.312 0.315 0.331 -0.336 0.345 0.34X 0.40x 0.453 0.465 0.471 0.509 0.623

= c M I IO v, 200

I 020 b 3 7 X Y h M 1 2 i

III 021 221 2’1 310 002 Cl40 33 I 241

531

listed in reciprocal A. Relative Intensities (II are hstcd by next most intense: : l-very low intensity: and w-t’ben (hhl) are referenced to the 1C hexagonal cell although of pyrrhotite structures.

P. FRAUNDORE Table 6. Polycrystal patterns from crushed terrestrial Pyrrhotitc

c c B B A c B C C C C c

m k

0.091 0.17~0.171 0.217 ~0.218 0.330-0.334 0.374 0.3X1 0.403 0.489 -0.4Y3 0.584 0.594 0.628 0.652 0.692-0.703 0.775 0.7x3

3

. 7

51; X 2Y6 YYY

4 6

65

I \v 5 8

4 u

.~___--.-* Letters A, B and B = 10 30”,,: and C spots ia within 2”,, of key spacmgs m Table

mktsoc

Olivine

t s0 c c C A A .A B A A .4 A c A C A c B B (’ C C

0.196.-0.198 0.234-0.243 0.261-0.266 0.289 0.299 0.339 0.346 0.360 0.402 0.41 1 0.441 0.451 0.464 0.474 0.529 0.577 0.586 0.618 0.664 0.696 0.824

C 0.203~0.207 C 0.303 0.229 c 0.24 I C 0.263 0.764 B 0.3060.3 I7 B 0.331 0.313 ‘4 0.350 A 0.389 0.404 c‘ 0.424 B 0.466 0.473 c 0.490 C 0.51 1 C 0.530 C 0.574 0.57x B 0.613 0.617 B 0.662 0.681 C 0.722 0.723 C‘ 0.764 0.774

mk

1%07-08A Mash (71

t but

7 \\ I

Y

1 7 I X Y 2 Y 6

5

Yw 4

7

Y9Y 3-

7

4 6 X

65 1H 5 3 M

3

H

C‘ O.lY3 0.195 B 0.203 C 0.208 &I.210 C B C c A A C c B c B c C C B c

c * Notation

4 x X

t i 0 i’

C 0.156-0.15’) C 0.219--0.2’9 C 0.277-0.2X I c.302 B 0.310 A 0.3440.352 .4 0.406~0.409 c 0.450 B 0.464-0.466 c 0.475 B 0.502- 0.504 B 0.631 0.63’

719

5

In h

7 296 9 6 6~ H.4 Y 3 65 H u 4

_ _ ..__.~._

C denote relative spot intensities: A = 30 loo”, of the brlghte\t >pot. = 0 lo”,,. Numbers under ‘mktsoc’ indicate that the observed range of a key spacing for the corresponding mineral column, as prr the 11~ ot 5. Spacings (y-values) are listed in reciprocal Angstroms.

unanalysed IDPs and the crushed standards are listed as well. Occasionally a key unexplained spacing might be explained by the presence. in small amount, of another mineral from the set listed in Table 5. In these cases that other mineral has been named in parentheses. However, the presence of these minerals is in no way verified by that extra spacing, which may be caused by an anomalous reflection from one of the

I?-05.1C Sam-II31

Diopside 4 fi 8 2%

7

standards

O.T’X 0% 0.257 0.265 0.284 0.330-0.343 0.388~ 0.401 0.426 0.440-0.443 0.479~0.488 0.532 -0.533 0.571 0.584 0.613 0.623 0.649 0.672 -0.6X0 0.727 0.77x 0.7Y2

follows

m k

t s o I: 4

’ 5

c 0.193 O.lYS C 0.25&0.257 C 0.277-0.290

5 I x

7 Y

2\* 71x 2 9 6 6

5 9 9 9

3 5

4

4 6

65 1 w5

8

VV

3

major minerals, or any of myriad other possible trace constituents. All of the more intense observed spacmgs (A or B) in polycrystal patterns obtained from the chondritic IDPs are explained, within the 2”,, tolerances of the current data, as spacings from major phases ldentitied from the set of 6 minerals listed in Table 5. In all IDPs for which single and pnlycrystal SAED data

B B A C C c C B B B C C c C

0.329-0.334 0.359-0.364 0.398..0.401 0.438 0.4660.4X0 0.507 0.569-0.573 0.613~0.619 0.665 0.674-0.682 0.702 0.732 0.751 0.788 0.7Y6

7 Y 5 4 6 X X

3

4 8 , u 71x7 296 6 9 YR 4 -, 6 5 1 w5 : 4

7 \c

(‘ \l,lYJ (‘ iI.Jli (’ (1.24(l 4 0,106 (’ \I.?.! I B 0.330 B 0.139 ( 0.36X 21.i!. 176 A 0.391 c 0.44’ .4 0.475 B 0.527 (’ 0.553 <’ O.i7h C‘ 0.613 (‘ 0.662 <‘ 11.75 I CO771

.: .>

0.33 I 0.331

.* 7

0. ii?, 0.;12

‘)

0.5U5 0.533

i

0.5Yl 0.634 0.6X1

4 (1 K

,f

~’ 1 x . I ‘i ,x _’ ‘1 h h ‘J ‘J CJ J 1 ‘5 (1 ’ I is i ‘,:

-

il

7w

that used in Table

6. The number

in parenthesis

IS the number

of pattern\

corn-

Interplanetary

dust in the transmission

were available on silicates, the data indicate that the same silicate (either olivine or pyroxene) dominates in both single crystals and fine-grained material. Also, note that the 7.3 and 10 A lattice spacings (corresponding to diffraction spacings of 0.137 and 0.100) associated with the layer-lattice silicates of the Cl and C2 carbonaceous chondrites (cf. KERRIDGE. 1968; MCKEE and MOORE. 1979; MACKINN~N and BUSETK, 1979) are absent from the measured spacings, although a weak diffraction spacing of 0.134 was measured from 14-07-2A5 (Tex). Applications of polycrystal SAED are not restricted to the characterization of whole IDPs. as is the case for X-ray powder diffraction. Like most of the grains from the dispersed aggregate 13-08-16 (Blowup), the iron sulfide whose EDX analysis is given in 2A-F47 (Table 4) exhibits a damaged crystal structure. But unlike most of the silicate grains, the structure of this 0.5 pm iron sulfide is so damaged that the diffraction pattern looks like a powder pattern with, in many cases, relatively continuous concentric rings. The analysis of a composite of two diffraction micrographs from this grain is given in Table 9. following the format of Tables 6 and 7. Although the low intensity spacings are not observed, and the presence of other mineral phases is suggested by some of the spots. pyrrhotite is the dominant diffracting phase on the basis of the criteria established above. The usefulness. in combination, of darkfield, brightfield. and diffraction imaging techniques for the structure analysis of localized phases is demonstrated by a second example. The interior of the rimmed grain of 13-10-08 (Oz) shown in Fig, 3 was determined to be

polycrystalline

l4-Ol-06A Tut (4) C B A A c B

0.337 0.350 0.389 0.397 0.401 0.415 0.47X-0.4X0

c 0.519 c 0.537 B 0.554 A 0.675 c 0.740 C 0.766

SAED

14-03-09A Bounce (9)

718

9 9 1 5

6 4 8

bined for the list.

3

taenite by the measurement of spacings. but not interspot angles. Interspot angles were not used because the interior of the droplet consists not of a single crystal, but of several taenite crystals connected by planar interfaces. A rim, typically 15 nm in thickness, surrounds this droplet and exhibits an unusual ‘striped’ contrast in brightfield micrographs, probably due to diffraction effects. The beam exposure of the droplet for EDX analysis (Table 4, 2A-F24) created a sizeable amount of contamination. After discovery of the taenite structure in this low-Ni iron droplet. micrographs taken prior to the EDX analysis were re-examined. On the diffraction micrograph corresponding to the darkfield image in Fig. 3. two inner diffraction rings were noticed. A bright spot on the outermost, more intense ring had been used for the darkfield image, and hence the ring must have originated from the droplet rim. The spacings of the two rings are given in Table 10. along with a list of 10 minerals which contain iron as the only element detectable by EDX. The Miller indices of all diffraction spacings of each mineral which match the observed spacings within 2”, are also listed. Only one mineral, iron carbide or cohenite, has spacings which match those of both rings, the ring spacings constituting the two innermost allowed spacings for the mineral. Cohenite is sometimes associated with metallic iron in terrestrial, lunar, and meteoritic samples (cf. FRONDEL, 1975; PALACHE er (II., 1944) and iron grains coated with graphite or cohenite have been predicted as a ‘universal’ condensate at _ 1000 K from a cooling circumstellar gas with an 0:C ratio less than 1 (LEWIS and

patterns

mktsoc I

')?I

electron microscope

9

‘96 9 9 lH.5 3 65 w u

6 4

c c c c A A A c B c c

0.‘04- 0.209 0.234 0.244 0,308 0.328 0.347 0.385 0.398 0.408 0.447 0.467- 0.485 0.513 0.57X-0.588

B 0.602&0.628 A 0.663 0.681 C 0.760 c 0.783m0.788 C 0.X29 0.835

14-07-2A5 Tex (5) mktsoc

mktsoc 2

7 9

7 7 19 8 9

c 0.134 c 0.195 c 0.245 c 0.325so.335 A0.381-0.389 A 0.39X

1

2

c 0.430

w

5999 4 6 R 3

6 6u 4

6 5 -. IWS H u 7

4 7 9 9

B 0.455 0.479 0.469 0.483 5 c 0.5 I 1 c 0.531 c 0.555 c 0.584 0.595 4 C 0.607-0.625 6 A 0.657- 0.675 8 c 0.718 C 0.733~~0.778 3

718 8 29

9 9w4

2 3 8 6 1WS H W

P.

928 Table 8. Summary Particle 13-05-OlC 13-05-03c 13-06-05A 13-07-08A 13-08-09 13-08-16 13-10-03 l4-Ol-06A l4-03-09A

14-06-09A 14-07-2A5

of composite

FRALINLx)R~

polycrystal

analyses

Mnemonic

based on criteria Evident

described In the text --__-~

diffracting

phases

Maynetitr, pyro.wVw Lost prior to analysis Mostly noncrystalline ~~ ;Wuynetite, oliciw Mostly noncrystalline Olioine, mugnetirv Pw%otitr. olivine, pyroxene. (taenite, magnetite1

Sam-l Gemini Zodiac Mash Chaugnar Blowup 07. Tut Bounce Lost + Found Tex

Mugnetite, Milgnrtitr.

pyroxme

Magnetite,

(olivine. taenite)

pyroxenr Not yet analysed~

_ Standard analyses

_________---_.

.._--. --

Pyrrhotirr

Pyrrhotite Olivine Diopside

Olicinr

PJrouent, -___* Major diffracting phases are in italic, while phases for which evidence is only suggestive are in parenthesis.

NEY, 1979). The presence of cohenite puts constraints on the presence of oxygen during the time when the coating on the droplet was formed. The micrographs taken prior to the EDX analysis contain yet another clue to the process by which the droplet was coated. As is apparent in the darkfield micrograph of Fig. 3. the rim of the droplet is not Table 9. Polycrystal analysis of an FeS grain from 13-08-16 (Blowup) ____ __~ mktsoc Spacing c 0.198 C 0.275 B 0.331-0.335 A 0.379 C 0.417-0.422 C 0.461 A 0.493 c 0.551 A 0.577 C 0.647-0.657 C 0.685-0.695 C 0.726 c 0.745

1 7

71X x

3 w4 5 999 X 4 65 H 3

* Notation follows that used in Table 6. Table 10. Polycrystal analysis of the rim surrounding taenite droplet in 13-10-03 (02) Observed spacings (A _ ‘): Possible Miller indices: Magnetite Fe& Hematite Fez% Magbemite F&O3 Fe1 _,O Wustite Goethite FeO(OH) Lepidocrocite FeO(OH) Siderite FeCO, (Fe. Ni) Taenite Kamacite (Fe, Ni) Cohenite (Fe. N&C

0.265

0.295 -

120

101

110

a

illuminated randomly about the perimeter. as might be expected for a rim composed of randomly oriented crystals. Instead, two regions opposite one another on the rim are illuminated. After correction for the lens rotation that occurs in the image when changing from a diffraction image to an image of the specimen. the line drawn between these two regions is parallel to the line drawn between the diffraction spot used for imaging and the incident beam spot. The : I IO; planes responsible for the diffraction spot lie perpendicular to the direction of diffraction. and hence parallel LO the edge of the droplet. The fact that this relationship continues all around the droplet was verified by examining the darkfield image of a different diffraction spot on the [ 1101 ring. Two d$/kwt drametritally opposed regions of the rim were illuminated. Once again the direction of the diffraction spot used in the imaging indicates that the cohenite ; 110; planes are parallel to the droplet edge. Thus the cohenite coating consists of crystals only tens of nanometers in size which lie against the surface of the taenite droplet on cohenite [ 110; planes. This means that the cohenite crystal lattices are not crystallographically coherent with the underlying taenite lattice. It therefore seems likely that the cohenite rim is not the result of solid state diffusion of carbon into the iron lattice, but instead has been deposited from an oxygen-depleted gas phase onto a previously solidified iron surface. An understanding of the conditions under which iron carbide crystals can be nucleated along i 110) planes onto a low-nickel taenite substrate will further constrain the formation history of this particle. 4. RESUL’l’S

In this section. each aggregate is discussed in turn. Particle l3-OS-OlC (Sam-l) is the least reentrant IDP which has been examined. The high sphericity is a possible result

.‘. “’

”

,,..

:1.. ..,_ : :,; :A,,

I

929

Interplanetary

dust in the transmission

of severe heating on atmospheric entry. Sam-l is one of two IDPs whose matrix material evidenced cracks upon crushing, suggesting less plasticity than usual. Sam-l includes distinct fine and coarse-grained sections. with high iron and sulfur concentrations in the former, and magnesium-rich pyroxene in the latter (F’S, Fig. 5). At least some of the pyroxenes are characterized by fine lamellae (FFSW. Fig. 3~). Polycrystal SAED patterns indicate the presence of pyroxene and magnetite in the fine-grained portion (Table 7). and the one sulfide grain examined has (S/Fe) _ 1.3 (Fig. 2; EDX analysis l-FSO). Thus the observations can be explained by a mixture of pyroxene, magnetite, and pyrrhotite, cemented together by a low-2 material. Particle 13-0503C (Gemini) also contained coarse and fine components. The coarse portion consisted of ‘large’ pyroxenes (0.1-l pm in size) which exhibited radial crystallization from a single point (FFSW. Fig. 5). Part of their external surface appeared to have been shaped by surface tension forces. It is possible that some of the crystals examined were portions of micron-sized droplets which fractured upon crushing. The radial structures and droplet morphology are both reminiscent of much larger meteoritic chondrules (cf. WASSON, 1974; NELSON et al.. 1972). The sample was lost after only one examination in the TEM. Particle 13-06-05A (Zodiac) dispersed into a large number of rather thick clumps upon crushing. Although one olivine (FFSW. Fig. 4a) and one pyroxene crystal (FFSW, Fig. 4c) were identified, no other crystals could be located. Most of the IDP consists of noncrystalline, nonstoichiometric clumps of ill-defined shape. As in one other particle which consists almost completely of ‘amorphous’ material, (S/Fe) ratios above 2 were found in some locations (e.g. EDX analysis 2A-F75). Thus the pyrrhotite mineralogy common in other particles cannot explain the state of sulfur here. Only a small fraction of particle 13-07-08A (Mash) dispersed upon crushing. The bulk of the particle flattened into a thick disc. The dispersed clumps, without exception. were also flattened discs containing crystals embedded in a noncrystalline matrix (e.g. FS. Fig. 1). No separate monomineralic grains for EDX or SAED measurements were located. although the SAED analysis showed finely polycrystalline olivine and magnetite to be the dominant diffracting phases. Particle 13-08-09 (Chaugnar) dispersed nicely upon crushing. Although several euhedral enstatite laths and some rounded crystals (including an iron sulfide and a mostly iron grain) were located. most of the IDP consists of angular. apparently noncrystalline flakes (FS. Fig. 3). with detected major elements Si, S. Fe, Mg, Cr. and Al in decreasing order of abundance. As in Zodiac, which is also largely noncrystalline. (S/Fe) ratios above 2 were measured (e.g. EDX analysis l-FOS). Chaugnar and Zodiac are the only two particles whose EDX spectra before crushing exhibit a sulfur peak larger than that due to iron (Table 1). Thus it might be possible to select this distinctive type of particle for bulk experiments (such as mass spectrometry) on the basis of its high (S/Fe) ratio without examination in the TEM. Particle 13-08-16 (Blowup) was one of two particles which ‘exploded’ upon crushing into hundreds of individual pieces, Low-Z noncrystalline material in places > 100 nm in thickness coats individual grains, which are typically lOG300 nm in size. Except for a large single-crystal olivine (FFSW. Fig. 4d). most of the individual grains did not exhibit clear crystal volumes (FS. Fig. 6). Chemistry and SAED patterns of one non-stoichiometric silicate grain indicate a mixture of olivine and an unidentified iron-rich phase (Fig. 4: EDX analvsis ZA-F46). Polvcrvstal SAED analysis of the IDP indicate the presence of’magnetite, even though no magnetite grains were identified among over 30 grains examined. Hence it is likely that the

electron

microscope

9?!

boundaries between the small deformed crystals. which comprise the otherwise monomineralic grains of this particle, are decorated with fine-grained magnetite. Another remarkable property of Blowup is the wide range of silicate grain compositions (Fig. 1). and the completely random spatial distribution of grain types. Although single-crystal and polycrystal SAED show olivines to be the dominant diffracting silicate phase. the minor element composition of the more silicon-rich silicates (e.g. FS, Fig. 10a) and occasional diagnostic diffraction spots indicate that pyroxenes are present, although their internal damage may be more severe than that of some olivines. Also found were a highly polycrystalline pyrrhotite grain (cf. Table 10). a nearly opaque grain whose only detectable element is tin (probably cassiterite. SnO:). and numerous low-Z clumps which have spectra with only very weak Si lines similar to spectra of the coating material (e.g. FS. Fig. lob: EDX analysis 2A-F73). It is possible that the damaged nature of the crystals is a result of shock (cf. ASHWORTH and BARBER. 1975) and that the magnetite decoration is a result of heating on atmospheric entry. On the other hand. the thick coatings and distinct grain chemistries argue against significant mass transport in or out of grains subsequent to assembly of the aggregate, Also, the small sizes. non-stoichiometric composttions. and magnetite-decorated structures of silicates in Blowup match predicted characteristics of silicate cores in coremantle models of interstellar dust (cf. KNACIE. 197X: GREENBERG, 1977). If comets were in part formed from interstellar dust which did not vaporize during the collapse of the solar nebula. then these grains might predate the formation of our solar system. Particle 13-10-03 (Oz) is the second particle which ‘exploded’ upon crushing into hundreds of pieces. Although EDX data on Oz before crushine ._ Indicated roughl; ‘chondritic’ abundances (Table I). areas examined in the TEM were on the average depleted in silicon (Table 3). In most cases, examined pieces are themselves aggregates of tiny droplet-shaped crystals of silicate, iron sulfide, and metallic iron embedded in a low-2 noncrystalline matrix (FS. Fig. 4). The pyrrhotite structure was suggested in the slightly hexagonal form of many Inclusions. and confirmed in single crystal SAED work on two sulfide grains, SAED work on droplets with mainly iron detectable by EDX showed metallic iron with ths high temperasometimes coated with ; 110; ture taemte structure. platelets of cohenite (Fe,C) typically 15 nn: in thickness (Section 3.5: Fig. 3). This observation is remakablr m a number of ways. First. the low-nickel (Ni _ 6”,,) taemte structure is metastable at room temperature (cf. WCH)D. 1964), and hence the samples cannot be the product of a long, gradual thermal metamorphism. Although severe heating of carbonaceous chondrite material in a vacuum can result in the formation of iron sultide and millimetersized iron-metal droplets in an olivme matrix (cf. MAT~A and LIPSCHIITZ. 1978; HASHIMOTU or
932

P. FRAUNDORF

cooling circumstellar gas with an 0:C ratio of less than 1. Other features of 13-10-03 (02) deserve mention. As was described in FS, Oz is the particle richest in low-2 material. In addition to iron-sulfides and taenite droplets. tiny single-crystal silicate ‘balls’ were also identified (e.g. Fig. 5). The typical size of these silicate ‘microchondrules’ is 50nm. Occasionally two (or more) are seen sticking together. Although the remainder of the surface of such sticking droplets looks spherical, the contact boundary is flat as though the droplets were ‘soft’ at the time of first contact. The implied constraints on cooling rates. subcooling depths, and collisional environment of the droplets should prove to, be a profitable subject for laboratory experimentation (e.g. NELSONet al.. 1972). EDX and SAED work on one of these silicate droplets indicated it to be made of iron-poor olivine (EDX analysis lA-F134). SAED ‘powder’ patterns also suggest the presence of pyroxene. Although major magnetite rings in the polycrystal patterns overlap with rings due to other minerals present. Oz is one of 3 IDPs examined in which the presence of magnetite could not be confirmed by our adopted analysis criteria (Section 3.5), and in which the strong magnetite (440) ring at 0.673 reciprocal Angstroms is not a dominant feature of the pattern (Table 7). Thus Oz is by far the least oxidized IDP examined, and the abundant presence of fine-grained magnetite is not a necessary consequence of atmospheric entry. The TEM mount from 14-Ol-06A (Tut) consists mostly of flattened discs containing tiny ( + 50 nm) crystals embedded in a noncrystalline material (FS, Fig. 2). Spot spectra suggest that some of the embedded crystallites are iron sulfides. Pyrrhotite, however, cannot be the only phase for sulfur, since this particle, like Zodiac and Chaugnar, exhibited (S/Fe) 2 2 in some locations (Table 4). Polycrystal SAED indicates the presence of pyroxene and magnetite in the fine-grained material. It would be instructive to see if such crystallites would grow in the matrix of Zodiac and Chaugnar on heating. One coated. angular grain (FFSW, Fig. 4b; EDX analysis l-F20) large enough for single crystal SAED was identified as clinoenstatite. The apparent composition of this crystal (Fig. 1). and the IDP as a whole (Table 3). show a silicon excess. It is not known if this is from silicon idigenous to the IDP. or from silicone oil silicon which has survived the process of cleaning. Presently, the excess silicon should be regarded with suspicion. Particle 14-03-09A (Bounce) is a very large aggregate, most of which was lost in a mishap during the cleaning process. The portion mounted for TEM work evidenced matrix cracks upon crushing, and appears to consist largely of 0.1 pm ‘spotted’ crystals, coated with and connected by a low-Z material. Broad ( -0.02 reciprocal A) magnetite rings dominate SAED patterns, suggesting that the ‘spots’ on the crystals are small (e.g. 50nm) magnetite crystals. SAED from one ‘spotted’ crystal (Fig. 6) which contained sufficient silicon to be an iron-rich silicate revealed no silicate spots at all, but a pattern corresponding to single crystal magnetite. The darkfield image in Fig. 6 using one of the magnetite spots shows that the ‘spots’ are apparently magnetite precipitates aligned with respect to one another in (or on) the silicate. Creation of magnetite seems to be a byproduct whenever material with chondritic abundances is ablated in the atmosphere (cf. BROWNLEEef al.. 1975). Severe heating on atmospheric entry, more likely because of the large initial size of this particle (cf. FRAUNDORF.198Ob). may also be responsible for the fractured matrix and the relative depletion of sulfur observed in Bounce. Particle 14-06-09A (Lost f Found) is a coarse-grained aggregate which was very difficult to handle because of severe electrostatic charging effects. Upon crushing, the aggregate proved to be mostly crystalline. Only two crys-

tals were examined closely, and both proved to be olivines (e.g. Fig. 2c, FRAUNDORF et al., 1980a). Particle 14-07-2A5) (Tex) has not yet been examined in any detail. It is a mixture of 0.1 pm crystals and finegrained material. Crystals in some of the aggregate are characterized by a curious precipitate-like texture (Fig. 7). possibly magnetite decoration. Only magnetite could be identified from the polycrystal analysis to date {Table 7). although the crystal in Fig. 7 appears to be an olivine on the basis of EDX data (EDX analysis lA-F6). Thus the spotted texture shown in the micrograph may be a mild form of the severe magnetite decoration observed in Bounce. 4.2 Similarities and differences

Emphasis in the past literature has been on similarities between the particles. In summary, earlier work combined with the results of this study show the following similarities: (i) most of the ‘chondritic’ IDPs have relative major element abundances for Z > 11 within a factor of 2 of the chondritic average; (ii) in the SEM they often shown reentrant structures constructed from rounded nodules in the 0.1 to 1 pm size range; (iii) in the TEM they frequently show submicron pyroxene, olivine, and pyrrhotite crystals coated by or embedded in a low-Z amorphous matrix; and (iv) X-ray and electron powder diffraction often indicate the presence of magnetite. In spite of these similarities, significant differences exist with regard to composition. internal morphology, and mineralogy. Five of the eleven particles examined contain ubiquitous fine-grained magnetite crystals, either as oriented precipitates in (or on) ironrich silicate grains (Bounce, Tex). or as part of a finegrained mix of silicates, sulfides, and low-Z materials (Sam-i, Mash, Tut). Since magnetite is known to result from the heating of chondrite material during atmospheric entry (BROWNLEEet al., 1975), it is likely that these are more altered particles. Four of the particles contain some large ( > 0.5 pm) clear crystals, and might be considered less ‘dis-

turbed’. Two olivines were identified in a particle (Lost + Found) which appears to be mostly crystalline. A second particle (Gemini) contained many radially crystallized pyroxene crystals reminiscent of some meteoritic chondrules, although much smaller in size. The other two particles with occasional ‘large’ crystals are quite different in that they consist mostly of a noncrystalline ‘chondritic’ material. These two particles (Zodiac, Chaugnar) also evidence high (S/Fe) ratios not explainable in terms of the most common sulfide identified in the JDPs: pyrrhotite. The presence of occasional healthy crystals makes it unlikely that the material in these particles was rendered amorphous by heavy radiation damage subsequent to assembly of the aggregate. The remaining two particles contain the targest amounts of low-Z ‘amorphous’ material, and they both exploded upon crushing into hundreds of tiny pieces. These facts, and the distinctness of constituents in the two aggregates, make them least likely to have been altered by atmospheric entry heating. The

Fig. 3. Grain

containing

olivine

and an unidentified iron phase (probably (Blowup). Darktield illumination.

magnetite1 from

Ii-OX-l6

A

E E

f

9.35

.-

,

.I .(._ --

Y37

Interplanetary dust in the transmission electron microscope chemically more reduced of these two particles (Oz) contains carbide-coated low-Ni taenite droplets, clusters of crystalline silicate droplets, and pyrrhotite crystals, all typically between 30 and 200 nm in size and embedded in a ‘fluffy’ low-Z matrix. In contrast. the other (Blowup) contains magnetite-decorated nonstoichiometric olivine and pyroxene grains, iron-sultide grains. and one grain with only tin detectable by EDX. Almost all of these grains have diameters between 50 and 400nm, thick low-Z coatings, and disturbed crystal structures. The wide range of grain chemistries and the thick low-Z coatings argue against significant compositional alterations subsequent to assembly of the aggregates.

5. DISCUSSION

The major result of this study is the demonstration that in spite of their similarities. ‘chondritic’ IDPs exhibit significant differences in composition, internal morphology, and mineralogy. The observations suggest that the particles contain fine debris from diverse processes in the early solar system. Specific findings which point the way toward future studies include: (i) the existence of two types of noncrystalline material: low-Z and ‘chondritic’: (ii) the existence of oriented magnetite crystals in (or on) some IDP silicates. and the existence of magnetite decoration in other silicates; (iii) the existence of two particles composed of grain types predicted in interstellar dust: one with highly reduced droplets and the other with coated. unequilibrated silicates; and (iv) two possible thermometers of the atmospheric entry heating process: magnetite precipitation and the absence of solar flare tracks. The absence of tracks is discussed in detail in FRALNDORF etd. (1980a). 5.2 Comp~irison to meteorites Brownlee and coworkers have shown that the average detectable element abundances in IDPs deserve the appellation ‘chondritic’. It is therefore appropriate to examine IDP data in terms of the metamorphic and compositional criteria established for chondritic meteorites. There are ten criteria for metamorphic level, according to VAN SCHMUS and WORD (1967). The following evidence shows that the IDPs consist of relatively unmetamorphosed materials: (i) The composition of mafic silicates within a chondritic IDP often varies considerably. BR~WNLEE(1978a) has cited examples with olivines. and 13-08-16 (Blowup) represents a case with variable pyroxene compositions. (ii) SAED data on pyroxenes from three particles indicate monoclinic as opposed to orthorhombic crystal structure. (iii) The fact that secondary feldspar has not been identified as a major phase in the IDPs further suggests unmetamorphosed material. (iv) Although no well-defined glasses have been identified, noncrystalline ‘chondritic’ material was a dominant feature of

Y3Y

two particles. (v) With regard to the nickel content of metallic minerals, atomic Ni concentrations of majorelement-iron grains in 2 IDPs are well under 20”,. In one of these, the grains were identified as low-nickel metallic iron droplets with the high temperature taenite structure ‘frozen-in’. This indicates a faster cooling rate for iron than is seen in meteorites. (vi) The average nickel content in iron sulfides is >0.5”.. another indicator of low metamorphism in chondrites. SAED data from pyrrhotites in one particle indicate the absence of large superlattice structures common in terrestrial pyrrhotites. (vii) Although meteoritic chondrules are larger than individual IDPs. two IDPs contained submicron silicate or metal droplets. Figure 3 shows one of the metallic ‘micro-chondrules’. Not only is the separation between droplet and ‘amorphous’ matrix well-defined. but there is a distinct crystalline rim of iron carbide around the droplet. (viii) The matrix material of these IDPs is usually dark and extremely fine-grained, and many of them are chemically heterogeneous on a finer scale than, for example, the Cl chondrites (cf. FS; FRAUNDORF, 1980a). (ix) The particles contain percent levels of carbon (BROWNLEEet a/., 1974). and this fact is supported by the observation of low-Z material as a coating and matrix in the fragile, reentrant structures of these IDPs (FS), (x) Finally, the bulk water content of the IDPs is unknown. Hydrated silicates, in particular. are not a dominant phase in the examined IDPs. Even though indigenous hydrated silicates may have been altered by exposure in space or by heating on atmospheric entry. the particles do not appear to represent matrix material from Cl and C2 chondrites, as might be expected from the status of Cl and C2 chondrites as ‘least metamorphosed’ meteorites (cf. WILKENING, 1978). A distinction between examined IDPs and primitive carbonaceous chondritic matrix material is also suggested by the chemistry of iron in the particles. The chemical grouping of chondrite types is shown by a graph of reduced (metal + sulfide) Fe/‘Si versus oxidized Fe/Si (Fig. 8). The observations on the IDPs indicate that sulfur in some of the IDPs is in the form of iron sulfides which contain approximately as much iron as sulfur. The oxidation state of non-sulfide, nonsilicate iron ranges from complete for 14-03-09A (Bounce), whose silicates appear to have metamorphosed into magnetite, to quite low in 13-10-03 (02) for which all matrix inclusions appear highly reduced. If we use the low-iron silicates to calculate an upper limit, and the sulfur abundance to calculate a lower limit on the amount of iron in reduced form. then some IDP compositions probably lie within the trapezoid shown in Fig. 8. In other words. some IDPs have oxidation states one might expect for unmetamorphosed versions of the more reduced chondrites. 5.3 Working hypotheses The observations of this study indicate: (i) that portions of some chondritic IDPs have not been signifi-

940

P. FHAUNWRP

0

.5 Oxidized Fe /Si

Fig. 8. The grouping of chondrite types based on reduced (metal + sulfide) versus oxidized Fe/Si atomic abundance ratios. See Wasson (1974). The letters E, H. L, LL and C denote clusters associated with the five major compositional groups of chondritic meteorites. Although meteorites from all groups except C appear to show signs of considerable metamorphism, some relatively unmetamorphosed IDPs appear to fall in the shaded area.

cantly altered by thermal or radiation processes (including atmospheric deceleration) since their assembly, and are consistent with (ii) the hypotheses that chondritic IDPS are of cometary origin; and (iii) the dust parent materials consist of relatively unaltered debris from a wide range of processes which occurred early in the history of our solar system. The evidence, and prospects of further verification. for each of these hypotheses will be taken up in turn. Alrerution. Severe thermal alteration of some IDPs is unlikely in light of their fragile reentrant structures and high measured abundances of volatile elements (BROWNLEE,1978; GANAPATHYand BROWNLEE,1979). The well-defined droplet structure and reduced chemistry of inclusions in one IDP of this study, and the coated, unequiliberated grain structures in another. are even stronger evidence against severe thermal alteration of the crystalline components of the IDPs subsequent to assembly of the particles. The presence of clear, well-defined silicate crystals in many particles rules out post-assembly radiation damage sufficient to make silicates amorphous, although the presence of fewer than 10s detectable iron-group nuclei tracks per square centimeter in examined silicates may indicate some annealing of the expected solar flare track record during atmosphere entry (FRAUNDORF er al.. 1980a). The noncrystalline components of the particles, both low-Z material and high S/Fe ‘chondritic’ material, are much less well characterized than the crystalline components. The low-Z material may represent residue from the evaporation of cometary ices, and thus be a result of low-temperature thermal alteration. Alteration of the particles by other processes is

also likely. The presence of solar-wind He. Ne and AI in the particles is evidence that they were exposed for at least tens of years as small particles in the vacuum of space (RAJAN et ~(1.. 1977; HUDSON c’f J., 1980). Half of the particles have probably cxpcrienccd z-second temperature maxima on atmospheric dcceleration of greater than 550 C (FRACNDUH~ it d.. 1980a; FRAUNDORF. 1980b). Further cxpzrimcntal work to identify thermometers of atmospheric entry heating and the effects of space exposure is important so that we might understand the most recent history of these particles. Cometury oriyin. There is Indirect evidcncc that many of the collected IDPs are from comets. Based on astrophysical observations, comets apprar to br the major supplier of particles for the interplanetary dust cloud (cf. BROWNLEE, 1979). The ‘cosmic. elcmentat abundances. the fragile aggregate structures. and the low metamorphism represent primitive characteristics likely for cometary material which must have been stored at low temperatures in its parent body. Finally, the reentrant structure is consistent with prior presence of the volatile ices which are known to be present in comets, Unless the voids were tilled in with such a solid. survival of the fragile reentrant structures at depth in a gravitationally bound body in itself seems unlikely. Direct proof of a cometary origin may be more difficult. Perhaps some clues to the association of IDPs with volatile ices reside in the poorly character. ized noncrystalline materials in the particles. Fourier transform spectroscopy on individual particles (cf. FRAUNWRF et (II., 1980b) or electron energy loss spectroscopy might allow better characterization of the noncrystalline materials, and hence provide constraints on the prior presence of volatiles. Auothcr alternative is to collect particles associated with it known stream of cometary meteoroids. This IS complicated by the relatively high entry c&cities for stream particles, by the fact that cometary streams arc likely to be depleted in IOitm and smaller particles because of radiation pressure effects on particle orbits (cf. BROWNLEE, 1979). and by the small area-time factor and low altitude of the current collection effort. The porous-aggregate nature of collected 1DPs is consistent with the low-density dustball \truzture\ inferred from radio meteor observations (cl’. tlt’C;Hts. 1978). Of course, a comet rendezvous mission equipped to do electron microscopy on collected particles might provide the most direct evidence of all. Primitive oriyins. The abundances of both major elements (cf. Table 3; BROWNLEEet ul., 1977: BROWNLEE, 1978) and trace elements t U: FFSW; Ir. SC. C’o. Zn: GANAPATHY and BROWNLEE. 1979) in the chondritic IDPs suggest that the particles formed from materials very similar to those believed to have been present in the early solar nebula. Because much of the trapped noble gases in meteorites is associated with low-Z carbonaceous material (cf. FRICK er i/i., 1979: LEWIS et ul.. 1977), one expects high concentrations of

Interplanetary

dust in the transmission

‘planetary’ noble gas in other primitive objects containing carbonaceous material (e.g. FS). Data on the elemental abundance of rare gases in a suite of chondritic IDPs (HUDSON rt ul., 1980) support this expectation. The measured xenon seems to have contributions from ‘planetary’ sources, and to be present in concentrations higher than those in any bulk meteorite. Thus there is SOUUY direct evidence of a primitive origin. Very similar conditions of assembly for chondritic IDPs are suggested by the fragile structures seen in the SEM and the role of low-Z carbonaceous material as a matrix and coating. At the same time, the wide range of mineralogical and textural differences between IDP constituents suggests that the chondritic IDPs consist of materials compositionally as diverse as. and in some cases less ‘metamorphosed’ than. material available in our meteorite collections. This case is most strongly made on the basis of particle 13-10-03 (0~). which may contain unmetamorphosed material formed in a highly reducing environment. That such environments did exist in the early solar nebula is argued by the oxidation state of the more reduced. and in particular the enstatite. chondrites (cf. KFIL. 1968). Crystalline inclusions in other IDPs do not show pristine euhedral or droplet-shaped morphologies. but instead have the appearance of broken fragments in various states of weathering. These materials resemble debris from the collisional evolution of solid bodies in the solar system rather than primary condensates. Hydrated silicates, common in the matrix of thermally unmetamorphosed Cl and C2 chondrites, can be ruled out as a major phase in many of the examined IDPs. On the other hand. the texture of the ‘microchondrule’ fragments in 13-05-03C (Gemini; e.g. FFSW. Fig. 5) is more akin to the textures of meteoritic chondrules than are the ‘pristine’ droplet textures of 13-10-03 (0~). Thus the chondritic IDPs apparently sample tine-grained material from a wide range of processes in the early solar system. This hypothesis might be further tested in two ways. First. sufficient numbers of the chondritic IDPs should be examined by the techniques of analytical electron microscopy to better define the range of differences between particles, and to establish a taxonomy of particle types. Fourier transform spectroscopy on individual particles may aid in this task, and at the same time provide important insight into astrophysical observations of cosmic dust. For example, astronomical observations of reproducible absorption features in interstellar dust seem to indicate that an amorphous silicate capable of withstanding temperatures above 1000 K without recrystallization is common (cf. MILLAR and DULEY, 1980). However, a silicate with the required features is not known in the laboratory. Information on the optical properties and thermal stability of the noncrystalline ‘chondritic’ material in particles 13-06-05A (Zodiac) and 13-08-09 (Chaugnar) might be very relevant to this issue. Also. the study here shows that the chondritic

electron

94 I

microscope

IDPs can legitimately be treated individually in the same way that macroscopic meteorites are. The improvement of particle handling techniques so that each IDP can be divided up among a variety of experiments and investigators is thus in itself an important area of research. Secondly, isotopic tests of the primitive origin hypothesis should be sought. Measurement of the isotopic composition of the xenon is a promising objective, but unfortunately a sufficient number of particles for this experiment is presently not available. A test for 160 enrichment in the more oxidized IDPs is another obvious goal (cf. WASWN and WEATHERILL. 1979; too

CLAYTON rf (I/.. 1976), much

material.

but

Examination

again

may

of individual

require par-

or by direct loading mass spectrometry (ESAT et ul.. 1979) presently provide the most promising access to isotopic clues.

ticles

by ion microprobe

Acl;no,vledyrrne)~rs~ The need for a cooperative effort seems to increase as the available amount of sample decreases. The following individuals deserve special thanks: D. BROWNLEE. for generously providing IDPs for this study; G. J. FLYNN. for critical discussions of the results: G. KEEF~,. for assistance in interpreting the characterization data; J. SHIR(.K. for the sample curation and mounting techniques; R. M. WALKER. for a continuing interest in IDPs: and E. ZINNER. for critical reading of the manuscript. Thanks go to D. BROU.N. M. L. HAGER, S. TAYLOR and S. W. WHIT~ON of the Southern llhnois University at Edwardsville School of Dental Medicine for providing opportunity and assistance in use of the Philips 300G TEM. and to R. M. SMITH and L. JARR~TT of the Washington University Medical School Pathology Department for providing opportunity and assistance in use of the JEOL 1OOCX. which operates under NIH Grant AM 20097. Helpful comments were provided by D. J. BARBER. D. BROWNLEE. D. S. BURNETT. P. R. BUSE~I(. R. Cowalc, P. GIBBONS. J. KERRIDGE. M. E. LIPS(.H~ITZ. M. MAI:RCTTI:.. G. L. NORD, JR, and P. SWAN. This work was supported m part by NASA Grant NGL !6-008-062.

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