FT–IR microspectroscopy of extraterrestrial dust grains: Comparison of measurement techniques

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Planetary and Space Science 54 (2006) 599–611 www.elsevier.com/locate/pss

FT–IR microspectroscopy of extraterrestrial dust grains: Comparison of measurement techniques A. Morloka,b, M. Ko¨hlerc, J.E. Boweyd, Monica M. Gradya,e, a

Department Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK Department of Earth Sciences, Rokkodai-cho1-1, Nada-Ku, Kobe-City 657-8501, Hyogo, Japan c Institut fu¨r Planetologie, Wilhelm-Klemm-Strasse 10,48149 Mu¨nster, Germany d Department Astronomy, University College London, Gower Street, London, UK e Planetary Space Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, Bucks MK7 6AA, UK b

Received 8 May 2005; received in revised form 19 February 2006; accepted 27 February 2006 Available online 2 May 2006

Abstract Identification of astronomical dust composition rests on comparison of Infrared (IR) spectra with standard laboratory spectra; frequently, however, a single mineralogical composition is assumed for spectral matching. Advances in laboratory instrumentation have enabled very precise IR spectra to be measured on single grains and zones within grains; with a more complete set of spectral data for planetary dust, better compositional matches will be achieved for astronomical dust. We have compared several FT–IR spectroscopy techniques (open path transmission spectroscopy and diffuse reflectance spectroscopy of powders; microspectroscopy of single grains and powders and ATR spectroscopy of thin sections) to determine their utility for the direct measurement of the mid-IR spectra of small amounts of extraterrestrial grains. We have focussed our investigation on the spectra of the olivine series of silicates, (Mg,Fe)2SiO4, a species frequently identified as one of the major constituents of interstellar dust. The positions of three characteristic SiO4 stretching bands at 10.4, 11.3 and 12 mm were measured for comparison of the techniques. All methods gave satisfactory results, although care must be taken to guard against artefacts from sample thickness and orientation effects. Single grains hand-picked from meteorites can be analysed, but results are inaccurate if the grain size is too large (41–10 mm). Spectra for single grains also show variations that arise from sample orientation effects. Once the analytical artefacts are taken into account, we found that measurement of powder with a diamond compression cell is best suited for the analysis of small amounts of materials. r 2006 Elsevier Ltd. All rights reserved. Keywords: Infrared; Dust; Technique: Spectroscopic; Methods: laboratory

1. Introduction The ubiquity of dust (interstellar, circumstellar, interplanetary, cometary and asteroidal) is an indication of its importance in the astrophysical environment. Dust is one of the building blocks from which planets, stars and galaxies are constructed, and comprehension of its evolution from molecular clouds, through protoplanetary disks to planets is one of the outstanding challenges of Corresponding author. Planetary Space Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, Bucks MK7 6AA, UK. Tel.: +44 1908 659251. E-mail address: [email protected] (M.M. Grady).

0032-0633/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2006.02.002

contemporary astronomy. Any information about the composition and formation of dust gives us an insight into the very early stages of our Solar System and other planetary systems. Characterisation of dust has, historically, been undertaken by two communities of scientists, working independently. Planetary scientists use laboratorybased instrumentation to study the extraterrestrial dust that arrives from space. Material for study includes (i) meteorites (aggregates of dust, fragments broken from asteroids) within which presolar grains (i.e., grains formed by stellar processes beyond the Solar System) are buried and (ii) interplanetary dust grains (IDPs) that originated from comets and asteroids and were captured in the stratosphere. Astronomers observe dust by telescope, either

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prior to its aggregation into parent bodies (as interstellar molecules and circumstellar dust grains) or as a component of evolved planetary surfaces. In order to follow the evolution of dust from stellar production and processing to its final destination (planet, comet, asteroid), a common and consistent set of measured compositional parameters must be available. One of the most powerful tools that can be used to relate astronomical data to laboratory-based studies of dust is that of Infrared (IR) spectroscopy (transmission, emission and reflectance methods). Infrared spectroscopy is also used as a remote sensing tool to investigate the composition of planetary surfaces (e.g., Esposito et al., 2000; Harloff and Arnold, 2001; Schade et al., 2004). The main method employed is that of thermal emission and reflectance spectroscopy. Whilst this technique is an important method for planetary exploration, and is a related sub-discipline to astrophysical dust characterisation, it is not of direct relevance to the work described here, and so is not discussed in further detail. Astronomers use ground- and space-based instrumentation to observe dust in situ, e.g. the UK IR telescope on Hawai’i, the ISO and SPITZER orbiting observatories. IR spectroscopy and spectropolarimetry can give the composition, structure, grain size and opacity of the dust. Although many interpretations of IR spectra have assumed that the dust is mainly composed of amorphous silicates, the presence of crystalline silicates and oxides is now well-established (e.g., Henning, 2003 and references therein). Astrophysical silicates have been identified by comparison of their IR spectra with spectra from laboratory standards or natural terrestrial minerals (e.g., Bowey and Adamson, 2002); the standards have often been quenched, amorphous glasses (e.g. Ja¨ger et al., 1994), or extremely fine-grained smokes (Nuth, 1996), rather than actual minerals. Complementary grain size and morphology determinations for grains have been made (e.g., Colangeli et al., 2003), and although models to fit laboratory and observational data usually assume power law or exponential size distributions of spherical grains, there have been attempts to model the properties of ellipsoidal or irregularly shaped dust grains (Fogel and Leung, 1998; Fabian et al., 2001; Min et al., 2003). The interpretation of dust spectra in astrophysical environments frequently assumes that any silicates present are pure end-member forsteritic olivine (Mg2SiO4) and enstatite pyroxene (MgSiO3). This is despite the fact that pure end-member silicates are rarely present in either meteorites or IDPs. Laboratory-based IR spectroscopy of extraterrestrial materials focuses on IDPs and meteorites. A total of between 40 and 60,000 tonnes of cosmic dust accretes to the Earth each year, a significant fraction (by mass, if not by number density) of which is contributed by meteorites (e.g., Love and Brownlee, 1995). Chondritic meteorites are aggregates of interstellar silicates that have been altered during residence in the molecular cloud and again during accretion within a protoplanetary disk. Chondrites mainly consist of the silicates olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)2Si2O6, plus iron-nickel sulphides and metal. These

minerals chiefly occur in chondrules, the most volumetrically abundant component in primitive chondrites. Chondrules are millimetre-sized spherical objects whose precise formation mechanism is not clear, but they appear to have been produced by flash heating and quenching of interstellar and circumstellar dust during the early stages of aggregation from a protoplanetary disk. Turbulence in the disk allowed the chondrules to acquire rims of finegrained material; compound chondrules testify to more than one episode of heating, and relict grains inside chondrules indicate that not all chondrules were completely molten. Thus the epoch of chondrule formation was a period maybe of intermittent discharge heating of a fairly heterogeneous dust cloud. Chondrules, sulphides and metal droplets are embedded in fine-grained silicate matrix material. Buried within primitive meteorites, and locatable only after extensive demineralisation of the bulk of the sample, are populations of extra-solar grains, identified by their isotopic compositions, and derived from several sources, both interstellar and circumstellar (e.g., Zinner, 1997). Because presolar grains are generally identified in residues that have been produced by harsh demineralisation of whole rock meteorites, most presolar grains are carbonaceous (SiC, graphite, diamond); only very few interstellar silicates have been found in situ (Messenger et al., 2003; Nagashima et al., 2004). The mineralogies and chemistries of meteorites, IDPs and extra-solar grains can be characterised directly by laboratory-based instrumentation, including IR spectroscopy. Although some chondrites have experienced thermal and aqueous processing since accretion, the most primitive of chondrites still retain a record of the original primordial cloud. Interplanetary dust particles are also a valuable source of extra-solar silicates, particularly the components within them called glass with embedded metal and sulphides (GEMS). Recent comparisons between GEMS and interstellar grains have shown that there are good matches in the fine structure of their IR spectra, reinforcing the link between primitive asteroidal and cometary materials, and dust in astronomical environments (Bradley et al., 1999). Aggregates of grains of mixed composition, layered grains, hydrated grains and grains mantled by organic material could occur in the ISM, and can be modelled by comparison with similar mineral assemblages separated from meteorites. So IR spectra from minerals, separated from primitive meteorites are possibly better suited as standard material for comparison with IR spectra from astronomical sources than terrestrial or artificial standard minerals. Examination of meteorites in thin section reveals the complex nature of the chondrules, and the variety and textures and mineralogies that they exhibit. It is likely that interstellar and circumstellar dust also shows an equivalent variety of compositions. The purpose of this paper is to assess different laboratory-based IR spectroscopy techniques (open path transmission spectroscopy and diffuse reflectance spectroscopy of powders; microspectroscopy of single grains and

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powders and ATR spectroscopy of thin sections) for their utility in measurement of the IR spectra of astrophysically significant extraterrestrial grains. ‘Utility’ is within the context of a non-destructive, or non-invasive method vs. destructive methods, as well as measurement in situ vs. on separated grains. Such considerations are important for analysis of materials returned from space missions (e.g., STARDUST, future Mars sample return, etc.), when only minimal quantities of material will be available for analysis by a variety of complementary, sequential techniques. We consider parameters that affect the spectra, including mineralogical composition, grain size and grain orientation. One of the big differences between astronomical and laboratory determination of the IR spectra of dust grains is the fact that astronomical spectra are taken of the grains directly, in vacuo, whereas laboratory spectra have traditionally been acquired with grains held in a KBr matrix. This difference has led to much discussion about whether or not the KBr matrix affects spectral band position (e.g., Colangeli et al., 1995, Speck et al., 1999; Fabian et al., 2001; Bowey and Adamson, 2002). By measuring the IR spectrum of the same sample by different methods (both with and without a KBr matrix), we aim to determine whether or not there is a matrix effect, and if there is, then to assess its magnitude. We also describe determination of IR spectra of meteorite silicates in situ within meteorite thin sections. We compare these data with IR spectra determined by more conventional techniques, in order to see how much variation there is in spectrum (in terms of peak position, band intensity and band width) with method employed. The focus of the paper is on olivine, one of the main silicate components of meteorites and interstellar dust. We present initial mid-infrared data produced by IR-microspectroscopy from terrestrial standards (synthetic crystalline olivine powders) and two primitive chondrites, Parnallee and Allende, as well as from single grains separated from the differentiated meteorite, Admire. Parnallee and Allende are from different chondrite subgroups, and are known to contain olivines of heterogeneous composition, i.e., there is a variation in composition between and within olivine grains in each meteorite. In an effort to chart the variation of infrared signature within chondrules, we have taken spectra from discrete regions within individual chondrules. This gives us a ‘snapshot’ of how the spectra of interstellar and circumstellar dust might appear.

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pellet generates a transmission spectrum (TM) that can readily be converted into an absorbance spectrum. Powdered samples can also be finely dispersed onto a support, and IR measured as it reflects back off the grains; this gives a diffuse reflectance spectrum (DR) that can be transformed into absorbance units if the grain size and thickness of the powder are known. In this paper, TM and DR are grouped together as ‘open path spectroscopy’. Single grains separated from specimens can be analysed either as a powder or as individual grains by first focussing the IR beam through a microscope objective, allowing the IR spectrum of a specific grain, or zone within a grain to be determined. The grains are supported on either a 2 mmthick KBr disc (TMMicro) or within a diamond window (DCC). These techniques are grouped as ‘grain microspectroscopy’. Both open path and microspectroscopy techniques involved analysis of separated grains or of powders. Spectra can also be taken in situ from a thin section, either by transmission (TMSec) or using attenuated total

2. Techniques and sample preparation IR spectra of silicates can be obtained by several different methods, and each method has its advantages in specific cases. Material can be analysed as either a powder, as discrete grains, or in situ in a petrographic thin section. The classical technique of IR spectroscopy is to powder a specimen, and compress it with potassium bromide (KBr), into a transparent pellet. An IR beam passing through the

Fig. 1. Schematic illustrating the main features of the IR microscope, and showing the beam path for different measurement methods: (a) options for beam path (open or microscope); (b) beam geometry of the TMMicro technique and (c) beam geometry of the ATR technique.

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reflectance through a germanium crystal (ATR); these are grouped as ‘in situ microspectroscopy’. The FT–IR system employed for the study comprises a Perkin Elmer Spectrum One IR workbench coupled to a Perkin Elmer AutoImage microscope fitted with IR transparent CsI windows. Wavelength range is 2.5–25 mm for open path spectroscopy (spectral resolution of 1 cm 1; 0.01 mm at mid-range) and 2.5–16.7 mm for microspectroscopy (spectral resolution of 4 cm 1; 0.05 mm at mid range). The system was freshly aligned and calibrated prior to the start of the period of data acquisition. A schematic diagram of the system is given in Fig. 1(a).

windows. Material was placed between these windows; by tightening two screws, the windows pressed against each other, and the sample was crushed and compressed. This resulted in a fine, spread powder with grain size o1 mm (DCCpowder). For the analysis of continuous layers (DCCLayer), a larger amount of material was compressed at least 30 times (or until a sufficient absorption was achieved, see below) with the cells. Thus a very thin layer of material was created. For DCC analysis, the diamond window of the cell was used for background measurements.

2.1. Open path spectroscopy (TM and DR)

Polished thin sections with a thickness of 20 mm were used for transmission analyses (TMSec). A solvent-soluble resin (Crystal BondTM) was used to attach the samples to the glass slides; this allowed the sections to be demounted from the slide after preparation. During analyses, the de-mounted thin section was placed on a KBr disc, from which background spectra were taken before analysis. Aperture size was 20  20 mm (Fig. 1(b)). The ATR method involves focussing the IR beam through a germanium crystal (70 mm across) that touched the sample. However, because it is very difficult to centre the Ge crystal exactly, we used an aperture size of 100  100 mm, slightly greater than the 70 mm diameter of the crystal, to be certain that the correct region was analysed. In the case of relatively heterogeneous areas analysed (e.g. some rim or matrix areas) a difference between the actual source area of the IR-signal and the composition presented has to be taken into account. For IR microscope spectra, an average of 20 scans was taken. Fig. 1(c) is a schematic illustration of the microscope set-up with the ATR accessory in place.

For classic transmission spectroscopy (TM), samples were ground in an agate pestle with Aristar Grade KBr to grains o1 mm in size, then compressed in an evacuated steel pellet die to 1 cm diameter pellets under a weight equivalent to 10 tonnes for 30 s. To prevent contamination by water vapour, the KBr was kept in a desiccator and all equipment dried at 100 1C prior to use. Once fabricated, the KBr pellets were examined under a binocular microscope for optical continuity and transparency, to ensure that they were homogeneous. For diffuse reflectance spectroscopy (DR), powdered material was dispersed onto a metalcoated diamond abrasive pad. The spectrum of a blank abrasive pad was taken prior to each measurement and subtracted from the spectrum of the sample dispersed on the pad. In this way, any features present in the spectrum from the abrasive pad were removed, and hence no correction for the abrasive pad was required. Spectra were taken across the wavelength range 8–16.7 mm; sixteen scans were taken to produce each spectrum.

2.3. In situ microspectroscopy (TMSec and ATR)

2.4. Meteorite samples 2.2. Grain microspectroscopy (TMMicro and DCC) Spectra from single grains and powders were acquired over a wavelength range from 8 to 16.7 mm; 20 scans were taken to produce each spectrum. The maximum beam aperture was 100  100 mm. For whole grain analyses, meteorite samples were lightly crushed in an agate mortar, and single grains picked from the debris. Selected grains were washed with deionised water and alcohol, then dried, before being placed with a tungsten needle on a 2 mm-thick KBr disc (TMMicro); the background of the KBr disc was measured several times in each session. To study potential orientation effects in bigger single mineral grains, olivine grains 30–60 mm in size were mounted on a steel needle and rotated through 1801 or 3601, with absorption spectra taken at several steps. The process was repeated with the same grain in different orientations on the needle, thus ensuring that spectra were taken parallel to all three of the crystallographic axes. Single grains and standard materials were crushed or flattened in a commercially available diamond compression cell (DCC), which consists of two steel bars with diamond

IR microspectroscopy has previously been used in the study of extraterrestrial materials (meteorites and interplanetary dust particles), both in reflectance (Bukovanska´ et al., 1998; Cooney et al., 1998; Maras et al., 2001), and transmission/absorption analysis (Raynal et al., 2000; Keller et al., 2000, 2002; Morlok et al., 2003, 2004). We analysed olivine grains in situ in polished thin sections of two meteorites, the primitive chondrites Allende and Parnallee. These meteorites were selected for study because they have not melted or suffered extensive alteration by heating, aqueous fluids or shock since they formed some 4567 million years ago from the primordial dust of our protoplanetary disk. The two meteorites are from different parent bodies that sampled dust from separate regions of the solar nebula, and both contain abundant olivine grains up to about 500 mm in size. The olivine compositions of the two meteorites were measured by scanning electron microscope (SEM) prior to IR analysis (see Section 2.6), and are heterogeneous (i.e., there is a variation in composition between and within olivine grains in each meteorite). Single olivine grains were hand-picked from a

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third meteorite, the Admire pallasite, for the determination of any difference in IR spectrum with grain size; again, olivine composition was determined by SEM. Admire was selected because it is a stony-iron meteorite, an approximately 50:50 mix of iron-nickel metal with silicate grains (almost all olivine). The olivines are several mm across, and are compositionally homogeneous, thus providing a source of known olivine composition available over a range of grain sizes. 2.5. Data management All IR spectra were converted to transmission spectra and presented in arbitrary absorbance units (A) with wavelengths in microns (mm). This removes artificial differences that would occur through comparison of diffuse reflectance with transmission spectra. For greater clarity in the figures, the spectra are offset from each other. Several spectra (16 or 20 scans, depending on method, see above) were taken for each sample. If not stated otherwise, statistical evaluation of data is based on the mean of band positions obtained from each spectrum, and not from a single spectrum produced by averaging individual spectra. Although band positions taken from averaged spectra can differ when compared with averaged band positions from the individual spectra, the differences are usually below the spectral resolution (o0.01 mm) for powdered material, and thus negligible. However, the effect becomes more significant when considering data acquired from individual single grains, where several spectra were added together and averaged to ‘synthesize’ a final spectrum. This treatment was favoured because there were much bigger differences between the single grain spectra, where not only peak position, but also intensity often varies strongly owing to grain size, morphology and grain orientation (see below). 2.6. Mineralogical composition The olivine group of minerals is composed of Si, O, Mg and Fe with general formula (MgxFex 1)2SiO4; minor (usually o1 wt%) of Ca, Al, Cr and others can replace Mg and Fe in the lattice. Olivine forms a solid solution between pure Mg2SiO4 (forsterite, Fo100Fa0) and Fe2SiO4 (fayalite, Fo0Fa100) end-members. End-member composition is given by atomic % Mg/(Fe+Mg). Olivine in most meteorite groups tends towards the forsteritic end of the solid solution, and variation in olivine composition is a marker for thermal effects and oxidation conditions within meteorite parent bodies (e.g., Van Schmus and Wood, 1967; Rubin, 1997). The composition and size of standard olivine powders, the Admire olivine separates, as well as grains within the Parnallee and Allende thin sections, were determined by SEM, using a JEOL 9500 electron microscope fitted with energy-dispersive X-ray microanalysis. Typical parameters for compositional determination are a working distance of 10 mm between sample and detector,

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electron beam current of 2 nA and accelerating voltage 20 kV. Olivine composition is determined to 70.1%Fo or better. 3. Results and discussion 3.1. The effect of olivine composition IR spectra of olivine exhibit three characteristic features in the 8–16.7 mm (1250–600 cm 1) region (e.g., Burns and Huggins, 1972). On the basis of single crystal measurements (Hofmeister 1987, 1997; Hofmeister et al., 1989), these have been identified as characteristic SiO4 stretching bands. The strongest band, that we refer to as the main band (MB), occurs around 11.3 mm; two slightly weaker features occur 12.0 and 10.4 mm and we refer to these as SB1 and SB2, respectively, for subsidiary bands. It is wellknown that band position shifts systematically with olivine composition (e.g., Salisbury et al., 1992; Ja¨ger et al., 1998; Koike et al., 2003). Most measurements of this effect have been carried out using either classical open path transmission spectroscopy (KBr pellet technique) or microspectroscopy of synthetic olivine glasses. In order to verify that our experiments produced results that could be compared directly with those from other analysts, we measured the IR spectra by open path transmission spectroscopy of a suite of synthetic olivines of known specific forsterite composition (Fo0, Fo20, Fo50, Fo80 and Fo100; Table 1). The olivines are crystalline, not glassy, and were produced by a method similar to that described by Redfern et al., (2000). They were powdered to a grain size ofo1 mm. Fig. 2 shows how positions of the MB, SB1 and SB2 bands vary both with forsterite composition and measurement technique. For all three stretching bands, the positions of the maxima shift to lower wavelength with increasing forsterite percentage (i.e., increasing magnesium content) for each measurement technique. Taking a mean value of the band positions measured, then the shift in position between Fo00 and Fo100 is 0.22 mm for MB, 0.16 mm for SB1 and 0.44 mm for SB2 (Table 1). It is clear that band positions in olivines depend on forsterite contents, in line with the results of earlier studies (e.g. Salisbury et al., 1992; Ja¨ger et al., 1998; Koike et al., 2003), and the bands shift systematically, from higher to lower wavelengths with increasing forsterite content (Fig. 2). We can also compare our data with those reported by other analysts using a similar variety of olivine compositions (Table 2). The literature data were mainly obtained by TM (i.e., KBr pellet method). The band positions in Hofmeister (1997) and Salisbury et al. (1992) were reported as transmission or absorption, while Ja¨ger et al. (1998) and Koike et al. (2003) used mass absorption coefficients. Our TM data (column 2 in Table 1) agree well with earlier studies. The biggest spreads observed in the literature values were 0.07 mm for the MB at Fo50, 0.06 mm for SB1 at Fo20 and 0.14 mm for SB2 at

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Table 1 Band positions in microns (mm) for the three strongest olivine bands for different forsterite (Fo) compositions and techniques Olivine composition (%Fo)

TM (mm)

DR (mm)

TM (micro, mm)

DCC (powder, mm)

DCC (layer, mm)

Range (mm)

Mean (mm)

Subsidiary band 1 (SB1) 0 20 50 80 100 Spread in mean band position

12.08 12.05 12.01 11.96 11.92

12.09 12.06 12.03 11.96 11.92

12.1470.03 12.0770.02 12.0470.01 11.9870.02 11.9670.01

12.0770.02 12.0270.01 11.9970.02 11.9170.01 11.9070.02

12.04 12.02 11.97 11.91 11.88

0.10 0.05 0.07 0.07 0.08

12.08 12.05 12.01 11.94 11.92 0.16

Main band (MB) 0 20 50 80 100 Spread in mean band position

11.42 11.40 11.35 11.29 11.26

11.46 11.49 11.40 11.32 11.22

0.19 0.19 0.13 0.13 0.14

11.42 11.39 11.34 11.26 11.20 0.22

Subsidiary band 2 (SB2) 0 20 50 80 100 Spread in Mean band position

10.58 10.51 10.38 10.23 10.14

10.60 10.54 10.43 10.26 10.15

0.04 0.04 0.04 0.04 0.03

10.58 10.52 10.40 10.24 10.14 0.44

(1) (1) (1) (1) (1)

11.5270.19 11.4370.10 11.3770.00 11.2870.04 11.2070.06

(3) (9) (3) (6) (3)

11.3870.03 11.3570.03 11.3270.03 11.2470.03 11.2070.03

10.5970.00 10.5370.01 10.4270.01 10.2670.01 10.1570.00

(25) (25) (24) (25) (25)

11.3370.01 11.3070.03 11.2770.03 11.1970.02 11.1270.02

10.5870.01 10.5070.01 10.4070.01 10.2270.02 10.15

10.56 10.50 10.39 10.23 10.12

(3) (3) (3) (3) (3)

Figures in brackets are the number of replicate analyses, for which 71s errors are given.

Table 2 Comparison of TM spectroscopy data (i.e., KBr pellet technique) from this study with published data This study %Fo Subsidiary 0 20 50 80 100

mm

Hofmeister (1997)

Ja¨ger et al. (1998)

Koike et al. (2003)

%Fo

%Fo

%Fo

%Fo

band 1 (SB1) 12.08 1 12.05 18 12.01 51 11.96 11.92 92

Main band (MB) 0 11.42 20 11.40 50 11.35 80 11.29 100 11.26 Subsidiary 0 20 50 80 100

Salisbury et al. (1992) mm

12.08 12.06 12.00

0

11.90

100

1 18 51

11.44 11.44 11.37

0

92

11.24

100

10.58 10.52 10.39

0

10.46

100

band 2 (SB2) 10.58 1 10.51 18 10.38 51 10.23 10.14 92

Fo100. These are similar to the range of values that we obtained using microscope powder methods. The differences between the averages of the microscope powder data in our study and the average literature powder data are p0.1 mm (Table 2).

mm

12.06

mm

0

12.10

55

12.00

11.88

100

11.90

11.43

0

11.40

55

11.30

11.22

100

11.20

10.57

0

10.60

55

10.40

100

10.20

10.14

0 21.8 77 100 0 21.8 77 100 0 21.8 77 100

mm

12.10 12.00 11.90 11.90 11.40 11.39 11.29 11.24 10.57 10.49 10.16 10.06

3.2. The effect of measurement technique Potential differences between spectra of grains and powders obtained by open path spectroscopy (the two methods abbreviated as TM and DR) and grain

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microspectroscopy (three methods abbreviated as TMMicro, DCCpowder and DCCLayer) were assessed from the measurements of the crystalline olivine standards (Table 1 and Figs. 2 and 3). Since much literature data exist for spectra obtained by open path transmission spectroscopy using the KBr pellet method (TM), and this has been the technique of choice for many years, we took data from this method as a baseline against which to compare the results from other techniques. We found that, generally, the spectra of standard olivines in all five of the techniques listed above were similar to each other and to literature data (Table 2): no matter which one of the five techniques we tested was used, all spectra exhibited the same three stretching bands. Table 1 and Fig. 2 show quite clearly that measurement technique does not, in fact, have much influence on results from standard olivine grains. Although there are differences in peak positions (the biggest range for positions of the main stretching band (MB) for all the five techniques for a given olivine composition is 0.19 mm), the offset is not systematic with either technique or olivine composition. Only the DCCLayer technique tends to have positions consistently at the lowest wavelength (Fig. 2). If we continue to take the TM technique as a benchmark, then

Fig. 2. Positions of three major features in the mid-IR spectra of synthetic crystalline olivine with different forsterite contents obtained with a variety of techniques. It is clear that all three features are shifted to lower wavelengths with increasing Fo composition (increasing Mg/Fe ratio). The size of each symbol represents 0.05 mm, the effective resolution of the spectrometer in mid-wavelength range. Errors in wavelength are, therefore, about the size of the symbol, and are less than the spread in wavelength between techniques.

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Fig. 3. Mid-IR absorption spectra (arbitrary units) of terrestrial standard olivine (Fo80) obtained with different techniques; the results are offset vertically for clarity. The spectra labelled TM (obtained by the KBr pellet technique) and DR (diffuse reflectance) were obtained by open path spectroscopy of bulk powders using a Perkin–Elmer SpectrumOne workbench. The other three spectra were obtained using a Perkin–Elmer AutoImage microscope; TMMicro-transmission via the microscope objective on to grains supported on a KBr disc; DCC–samples pressed within a diamond compression cell.

the overall biggest shifts in band position are for the MB in the DCCLayer spectrum, where peak position moves by 0.08–0.13 mm. All the other techniques show differences that are of the order of the spectrometer resolution. Apart from peak positions, there are some differences in the general shape of the spectra (Fig. 2): TM and DCC show very sharp, clear peaks, while those in the TMMicro and DR techniques are rounded and broadened. Also the intensity of SB1 at 12 mm varies: it is highest for the DR and TMMicro methods, and lowest for the DCC. Because there is no systematic offset in peak position for a given technique for a specific olivine composition, it is more likely that variations among the spectra result from experimental inadequacies rather than analysis technique. There are several potential artefacts, especially for the grain microspectroscopy methods. Hofmeister et al. (2000a, b) found that uneven distribution of material in the aperture leads to light leakage and irregular grain size and heterogeneous sample thickness result in higher absorbances for discrete mineral grains than for homogeneous thin layers. These effects are also a potential source for differences between analyses using the same method. The olivine samples that we used were ground to grain sizes p1 mm, so irregular grain size should not have been a problem. However, the DR, TMMicro and DCCpowder methods all relied on grains being sprinkled onto a substrate, possibly leading to uneven material distribution and thus light leakage. The DCCLayer method took many grains and ground them to a continuous layer, potentially producing a layer of uneven thickness. Although each of the three grain microspectroscopy techniques has its individual drawbacks, they all produce IR spectra that are adequate for the purpose of analysis of

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small amounts of planetary materials. One of the aims of this study was to investigate whether or not there was an effect resulting from analysis of material held within a KBr matrix. The DCCpowder method (in which grains were compressed between two diamond plates, without KBr) had shown itself to be the most reliable of the microspectroscopy technique, thus we analysed the crystalline olivine standards repeatedly by this method to compare results with those from TM analysis of KBr pellets. We made 25 replicate measurements at each olivine composition (Table 1); results were highly reproducible spectra with averaged band positions differing from the KBr pellet data by p0.05 mm (i.e., of the order of the spectrometer resolution). It seems safe to infer from this observation that a correction for a KBr matrix effect is not required, as long as homogeneous KBr pellets are produced. This is in agreement with the results of Colangeli et al. (1995) for a variety of silicates and Speck et al. (1999) for silicon carbide. 3.3. The effect of grain size on IR spectra The standard olivines were homogeneous sub-micron powders, and so were not suitable for assessment of possible grain size effects on IR spectra. In order to estimate potential effects of grain size, we separated olivine grains (Fo88) from the stony-iron meteorite, Admire. The grains were gently crushed, then hand-picked on the basis of grain size from 100 mm down to 51 mm. Because the grains were not sieved, the length of the longest dimension is taken as the size of the fraction, and only broad size range limits are given. IR spectra of the different size fractions were taken with the TMMicro technique

Table 3 TMMicro measurements of grain size fractions of olivine (Fo88) from the admire pallasite Size fraction (mm)

SB1

MB

SB2

4100 450 50 o50 o10 51

11.92 11.96 11.98 12.00 11.96 11.94

10.95 11.28

10.21 10.20 10.20 10.20 10.20 10.20

11.31 11.34 11.27

(Fig. 4 and Table 3). For analyses of grains410 mm, 10 spectra were taken, and band positions derived from the mean of the spectra. Several effects on IR spectra were observed for fractions of different grain sizes. The most obvious was the asymmetric broadening of the main band at 11.3 mm. As grain size increased from 51 to 100 mm, the band gradually became wider. The high absorbance of grains with thicknesses 410 mm resulted in spectra with a broad and flat feature between 10.5 and 11.5 mm, to which no maximum could be assigned. SB1 and SB2 showed only small shifts for all size fractions, although the relative intensity of SB1 and SB2 changed systematically with grain size (Fig. 4). Effects similar to the broadening of the main feature between 10 and 12 mm in olivines with increasing grain size, and the accompanying change in the intensity of subsidiary bands have also been observed by other analysts, e.g., Raynal et al. (2000). Hofmeister et al. (2000a, b) investigated the effect in quartz samples of different thickness and sample density, and interpreted the broadening as a result of light leakage through cracks, and incomplete coverage of the aperture by the sample. It is likely, given that the aperture size of the microscope was 100  100 mm, that the peak broadening observed in this study was the combined result of the high absorption and scattering within thick samples, leakage and incomplete aperture coverage. It was not possible, given the sizes and shape of the grains, to fill completely the aperture with an even layer of sample material. 3.4. Orientation effects

Fig. 4. Comparison of mid-IR spectra, obtained by TMMicro, of grains hand-picked from the Admire pallasite. Grain size is the length of the longest grain dimension. While the peak for the finest fraction is sharp, the spectra of coarser fractions (450 mm) show a broad feature with a flat top, that allows no exact determination of peak position. The features become sharper and the relative intensities of SB1 and SB2 decrease with decreasing grain size. A shoulder is visible at 9.5 mm in the coarser grain fractions (compare with Fig. 5).

A major problem in the analysis of both separated single grains and grains in thin section is the preferred orientation of the grains as a result of their crystallographic structure. Non-symmetric crystals will demonstrate different IR spectra (transmission, emission or absorbance) depending on orientation of the grains relative to the incident radiation. Olivine (of all compositions in the solid solution series Fo0 to Fo100) is orthorhombic, and would therefore be expected to produce different spectra from a single grain orientated parallel to one of the x-, y- or z-axes. In powder analyses, where IR spectra are obtained from a large number of small, randomly orientated particles, the effect

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is nulled. However, should grains be arranged with a specific orientation, then the effect becomes significant. So, for example, orientation effects might become an important consideration in certain astrophysical environments, such as where dust is entrained in the outflow from a newly forming star resulting in orientation of the grains with a specific axis parallel with flow direction. In our experiments, recognition of potential orientation effects is important for analyses involving measurements from individual grains (e.g., in thin section). The magnitude of a potential orientation effect for olivine grains can either be calculated, or it can be determined empirically through measurement of single crystals. Spectra were acquired by TMMicro from a grain, mounted on a steel needle (Fig. 5). Several spectra were taken of the same grain over a number of random orientations, and then summed, in order to produce something approximating a powder spectrum. Fig. 5 shows the representative averaged spectra for a single grain 60 mm long  30 mm wide and 30 mm thick. The results for several different olivine grains were very similar. The purpose of this experiment was not to determine individual spectra with the grain orientated relative to specific axes (a study that has been undertaken by other authors, e.g., Hofmeister, 1987, 1997; Fabian et al., 2001), but rather to see how many randomly orientated single spectra were required in order to obtain a ‘powder’ spectrum. As Fig. 5 shows, the spectrum from a single grain orientation is of low quality, with a broad feature between 10 and 14 mm that encompasses all the main band positions. We found

Fig. 5. TMMicro measurements of a single olivine grain (30–60 mm in size) extracted from the Parnallee chondrite and mounted on a needle. Measurements of 1, 5 and 10 spectra were taken of the grain at several positions as it was rotated; the spectra were then summed together. The combination of only a few spectra results in a relatively stable spectrum that approximates to the powder spectrum. The vertical lines mark the band positions of SB1 (12.0 mm), MB (11.3 mm) and SB2 (10.3 mm) calculated from the standard olivine data in Fig. 1 for a TMMicro spectrum of olivine with Fo75 (a representative olivine composition for Parnallee). The origin of the features around 9 and 14.5 mm is not known, but might be an artefact.

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that the spectra converged after 5 individual spectra at random orientations had been taken, and that discrete bands (albeit of low intensity) could possibly be distinguished within the broad absorption. The SB2 band at 10.2 mm is the most clearly recognisable of these features. Two additional features appeared at 9.3 and 14.5 mm; these do not appear in the size-fraction data (Fig. 4), except for a weak shoulder for grain sizes 410 mm. It is possible that the features are an artefact of grain size, rather than orientation.

Fig. 6. Calculated spectra for analyses along the three crystallographic axes (x, y, z) of Fo95, converted from reflectance data into absorbance units. The measured band positions of SB1, MB and SB2 in Fo100 are shown as vertical lines. The calculated bands are shifted by 0.5 mm towards lower wavelengths compared with peak positions of standard Fo100.

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To clarify the source of any new or altered features, we compared the grain spectra with those that we calculated for the three crystallographic axes (Jx, y, z) of 1 mm-sized olivine grains (Fo95) using Mie calculations (Fig. 6). This theory describes optical properties (absorption and scattering) of spherically symmetrical bodies, based on Maxwell equations. The calculations were performed with a code by Bohren and Huffman (1983), using data obtained by Fabian et al. (2001). It is clear from Fig. 6 that there is only a poor fit between the summed calculated spectra (given as an ‘average’ in the figure), and the measured Fo100 TM spectrum. We can allow for the 5% discrepancy in forsterite composition by calculating band positions for Fo95 from the data in Fig. 2: this moves the vertical lines in Fig. 6 by a further 0.1 mm to higher wavelengths, so increasing the difference between calculated and measured spectra. Comparison of the spectra generated from theoretical models with the powder spectrum (Fig. 3) also demonstrates that model spectra cannot be used alone to determine best fits to observational data. This finding also reinforces the danger of selecting a single compositional parameter with which to model astronomical dust. 3.5. Microspectroscopy of polished thin sections from Allende and Parnallee The advantage of looking at grains in situ within meteorite sections is that it is possible to look at the context of minerals and the relationship exhibited with surrounding matrix. Two techniques were employed: ATR microspectroscopy and transmission microspectroscopy (TMSec). As discussed in the previous section, in order to prevent potential orientation problems and to obtain a more representative spectrum, we took six spectra of each grain at random orientations of the thin sections. In both methods, clearly defined spectra could be obtained and matched with spectra from the synthetic crystalline olivine powders. The averaged spectra are given in the left hand side of Figs. 7 and 8, keyed to the relevant grains shown in

Fig. 7. ATR IR spectra of olivine grains (Fo96 to Fo99) within a chondrule in a thin section of the Allende chondrite. The size of the aperture (100  100 mm) is shown by the grey squares; the area of the ATR crystal tip (70 mm across) is shown by the white circles.

Fig. 8. IR spectra obtained by TMSec analysis of olivine grains (average Fo73) in a thin section of the Parnallee chondrite. The size of the aperture (20  20 mm) is shown by the grey squares. Because of the high absorbance, the spectra are broadened compared to powder analyses. The main band is also shifted towards lower wavelengths in comparison to Fo80 standard analyses, or ‘buried’ as shoulder in the broad central feature. SB1 and SB2 at 12 and 10.2 mm are clearly present.

an SEM image of the sections. Using the regression relationship defined in Fig. 2, we then hoped to match the spectra with those obtained from the crystalline olivine powders, and thus determine the olivine composition of the grains. As a check on the procedure, the composition of the grains was also determined by SEM analysis of the sections subsequent to the FTIR analyses. ATR microspectroscopy was carried out on a polished thin section (30 mm thick) of the Allende carbonaceous chondrite (Fig. 7); the section was mounted on a glass slide, and the Ge crystal of the ATR probe positioned over the relevant region. We analysed four olivine grains in a 1 mm chondrule; each grain was about 100 mm across. The resulting spectra keyed to the specific areas analysed are shown in Fig. 7. It is apparent that the spectra are poor, and exhibit peak broadening. Since only a very thin surface layer (few nm) is analysed by the ATR technique, ATR spectra should not be broadened. The presence of impurities (such as cations in addition to Mg or Fe in the olivine lattice, or sub-micron crystallites of other minerals) within grains would degrade an IR spectrum, but the olivine grains in Allende, as measured by analytical SEM, were stoichiometric, implying that any influence from impurities was low. There are, however, several other reasons that might explain the disappointing results. One of the problems with this analytical technique was the very low absorbance, only a few % of that obtained for the TMMicro method. When this is coupled with the observation that the areas analysed in the thin section are not complete crystals, but are visibly cut by cracks, it is perhaps not surprising that poor spectra were obtained. It is possible that, as for grain microspectroscopy techniques, spectral data obtained by ATR from mounted thin sections suffer from radiation leakage. In this case, though, the leakage is not a result of incomplete aperture coverage, but occurs through cracks within grains in the sections. Transmission microspectroscopy (TMSec) was carried out on a 100 mm chondrule in a 20 mm-thick section of the

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Parnallee chondrite (Fig. 8). In this case, the section was demounted from its glass slide, and was instead supported on a KBr disc. Again, the main problem encountered was absorbance: because 20 mm is too thick for efficient IR transmission, absorbance, was very high, about 70% higher than obtained in the grain microspectroscopy techniques that involved finely crushed particles (with grain sizeso1 mm). The resulting spectra showed broadened main bands similar to those obtained for the larger grain size-fractions (Fig. 4). As for the grain size-fraction analyses, it is likely that the broadened peaks were a consequence of light scattering and stray light. We found, then, that there were only poor matches between olivine composition, as determined by SEM, and that measured in thin sections and calculated from the standard data presented in Fig. 2. In both ATR and TMSec techniques, the measured bands were shifted to higher wavelengths by some 0.3 mm or so. For Parnallee, where a 20 mm-thick wafer of rock was analysed, the discrepancies arise from the thickness of the sample. For Allende (measured by ATR), the 100  100 mm aperture area covered regions within the section that were cracked, resulting in averaged spectra that included bands from neighbouring species. It is clear from this study that there are problems in using transmission techniques to acquire IR spectra of grains in situ within meteorites. The ability to prepare rock wafers of thicknesso10 mm by the usual polishing techniques is problematic. Although much thinner wafers can be produced by the use of ultramicrotome or ion beam thinning, these techniques are expensive and time-consuming. Reflectance microspectroscopy is, therefore, likely to be the most appropriate technique for characterisation of grains in situ. 4. Summary and conclusions The use of IR spectroscopy to characterise relatively large amounts of powdered mineral grains is well known and accepted; the analysis of powders and grains by microscope IR techniques is also firmly established in the mineralogical field. However, these methods require either relatively large amounts of material (of the order of 1 mg) or individual grains separated from a host or matrix. Here, we have demonstrated the potential of several IR techniques. We have started a comparison of IR spectra from primitive meteorites with spectra from standard materials. To achieve this, in addition to the IR methods commonly used for characterisation of minerals (i.e., transmission through KBr pellets), we also analysed larger samples or individual grains in situ with IR microscopy. The advantage of IR microscopy is that it allows measurement of very small amounts of material, in a field where often only a few grains are available. Also of interest is the ability to analyse minerals in their petrological context, or to get exact information on specific small objects, such as single mineral grains.

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Using a set of synthetic crystalline olivine grains of specific and known composition, we have tested several different techniques for the measurement of the IR spectrum of olivine, and found that, for grain microspectroscopy, the spectra are not sensitive to the method of measurement employed. Neither are they sensitive to the presence of KBr matrix, thus no matrix correction is required. We have shown, in agreement with other authors, that the positions of maxima in the IR spectrum of olivine shift with olivine composition. Although the maxima also shift with grain size, olivine composition is a more critical parameter than grain size, shape or grain orientation. Notwithstanding this observation, spectra calculated from first principles using Mie scattering theory do not fit well with measured data, suggesting that spectra generated from theoretical models cannot be used alone to determine best fits to observational data. This finding also reinforces the danger of selecting a single compositional parameter with which to model astronomical dust. Having noted these variations, it then becomes more problematic as to how the IR spectra of astronomical silicates should be modelled. So, for instance, a recent paper by Okamoto et al. (2004) modelled the dusty disk around the protostar b-Pic in terms of olivine with different grain sizes. The same data might have been modelled assuming a range in olivine compositions, or aggregates of grains or grains with coatings. Thus, when modelling the IR signature of astronomical dust, spectral data from a range of silicate compositions should be considered, as well as potential variations in grain size and morphology. Data acquired by in situ microspectroscopy of meteorite thin sections show that it is possible to demonstrate a finescale variation in IR spectral characteristics within single chondrules from primitive meteorites, even though an exact match between IR spectra and olivine composition is hard to achieve using standard petrological sections. The use of FT–IR microscopy for the analysis of small amounts of planetary materials offers a wide scope of opportunities. However, one has to be aware of a variety of potential technical problems. In situ methods such as ATR or transmission analyses of thin sections are prone to artefacts including peak broadening and peak shifting. Whole grain analyses show some promise, but are subject to grain-size and grain-orientation effects. The analysis of powdered material is more fruitful, but here sample preparation plays an important role. For the analysis of small amounts of material with the FT–IR microscope, the DCCPowder technique is the most appropriate, as it has sufficient accuracy (especially when compared with earlier works) and is easy to use. Olivine spectra obtained by the DCCPowder method do not require any corrections for postulated matrix effects from the use of KBr. In cases where grains are too thick for effective transmission of IR radiation, such as study of grains in situ within meteorites, reflectance microspectroscopy is likely to be the most appropriate characterisation technique.

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