Meteorite Classification

--- of a "black chondrite"

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Figure 1. Left: The available material, 1263.2 grams in all, including the 915.4-g main mass plus 7 smaller slices and wedges, each one 5 to 82 g in mass. The interior is dark, metal-flecked, and fresh in appearance, while the fusion crust (though not at all glassy in lustre) seems fairly unoxidized. Three polished thin sections were prepared, the one on the right appears shiny because of a removable carbon thin film applied for electron microprobe analysis. The total area of polished meteorite in the 3 sections is circa 16 cm2. Right: A close-up of some of the material.


"Rock of the Month #154, posted for April 2014" ---

Introduction to Meteorite Classification

The methodology, at least for common stony meteorites, is exemplified here by a study of an hitherto-unclassified North West Africa ("UNWA") meteorite, a striking "black chondrite". The meteorite is said to be the major part of a "find", that is, of a meteorite discovered on the ground an unknown time after its fall from space. The NWA meteorites, unlike the centrally-curated samples recovered by government-sponsored expeditions in Antarctica, are commonly found by the nomadic inhabitants of the north African desert countries and sold, via commercial meteorite dealers from Europe and North America, to collectors and researchers worldwide.

The different classes of meteorites require assorted complementary techniques for proper identification. Thus iron meteorites are classified on the basis of their metallographic (textural) properties and the bulk chemistry of gram-size samples (content of nickel, iridium, gallium and germanium, and as many as 8 other elements). Some of the achondrite meteorites are easy to identify as such, but much harder to classify with confidence. Some are best identified by a combination of petrographic study and oxygen isotope analysis. Mineralogy is key to the chondrites, a diverse clan representing some 95% of all known meteorites.

Classification criteria for chondrites include mineralogy, textures, electron microprobe data on mineral compositions, and magnetic susceptibility. The degree of shock metamorphism and terrestrial weathering may also be estimated for a more-complete characterization. Ideally, the minimum mineral-chemical information for chondrite classification will include an adequate analysis of olivine (including an estimate of the "Mg number", a ratio representing the proportions of magnesium and iron) plus either Ca-poor pyroxene (Mg number) or kamacite (Co content in the predominant metal phase).

The process begins with preparation of a polished thin section from a sawn slice of the meteorite. The slice, reduced with skill to a thickness of 0.03 mm, can be 1 to 10 cm2 in area, depending on the generosity of the initial sample. From this section, a petrographer (microscopist) can estimate the mode (proportions of various minerals) in the sample, and the type, abundance and degree of preservation of chondrules. These observations are generally sufficient to provide a petrologic grade, indicative of the degree of metamorphism of the meteorite in its parent body (Dodd, 1969, 1981; Brearley and Jones, 1998; Hutchison, 2004; Krot et al., 2005).

Shock and weathering criteria can also be estimated by visual examination. The class of a chondrite involves a number of factors. For the ordinary chondrites, the iron content is key, expressed as H, L and LL (high, low and very low Fe abundance). The proportions of Fe and Mg in rock-forming silicates like olivine can provide the necessary information. The bulk abundance of many chemical elements in a sample can also be determined quite precisely in a chemistry lab, usually with consumption of a milligram to gram quantity of the meteorite. Nowadays a good idea of the content of key elements like iron and magnesium can also be estimated quickly and non-destructively on a small (<1 cm2) area using one of the new class of portable x-ray fluorescence analysers.

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Figure 2. Photomicrograph of part of one of the 3 thin sections, in transmitted white light, using a stereomicroscope at 16X nominal magnification, showing chondrules in their granular matrix. Note localised darkening due to very fine-grained troilite in top left quadrant of image. See also Figure 5. Field of view is 8 mm wide.


The Classification Procedure for the Black Chondrite

1. General Observations: the sample is clearly a meteorite, with abundant disseminated grains of silvery metal and a dark matrix containing chondrules. These features are even more readily visible in sawn faces. The thin fusion crust is less oxidized that those seen on many desert stones, which develop a brown, waxy exterior.

2. Mineralogy and petrology: the thin sections provide key textural and mineralogical observations.

  • 1. Abundant chondrules in a fine-grained matrix affirm that this is indeed a chondrite.
  • 2. Chondrules are diverse and cover a range of sizes, with the largest 1 mm or more in maximum dimension, e.g., a) barred olivine chondrules (1.0 mm), b) radiating pyroxene chondrules (1.4 mm), c) very fine-grained granular chondrules (2.4 mm), and d) porphyritic olivine chondrules (1.8 mm, except for one exceptional 4.8 mm example).
  • 3. Many chondrules are broken. Most are fairly well defined against the enclosing matrix (Fig. 4). Smaller chondrules (0.2-0.8 mm) are often intact, compared with the largest examples.
  • 4. There is a range of grain size in the matrix, including some relatively coarse (1 mm) pyroxene and olivine, but not feldspar (which, if present, is therefore very fine-grained).
  • 5. There is abundant metal (kamacite) and iron sulphide (troilite), which both occur as jagged, anhedral grains (Fig. 5). There is also a trace of chrome spinel (chromite). Some sulphide grains are up to 1 mm in size, but clouds of tiny sulphide blebs are dispersed throughout the groundmass (Fig. 5).

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Figure 3. A scanned image of the polished thin sections in transmitted white light, showing chondrules and angular, opaque grains of metal and/or sulphide. Each section (glass backing slide) is 47x26 mm in area. The section on the right (with a large metal mass at the bottom) was selected for microprobe analysis.


3. Mineral chemistry and chondrite class refinement: the general procedure is to determine the composition of olivine (Mason, 1963, 1967), low-Ca pyroxene (generally orthopyroxene) and, according to preference, any other minerals (my third choice is kamacite). Results of wavelength-dispersive electron microprobe analysis of 28 points are summarized below. These analyses, employing reference materials, were part of a larger "bundle" of work that was devoted largely to meteorites and fulgurites. The carbon-coated thin section was analysed on a Cameca SX-50 (University of Toronto Dept. of Earth Sciences, with thanks to lab manager Dr Yanan Liu).

  • Olivine (n=5), Fa24.78 ± 0.65 (2σ), FeO/MnO 48.1. Apparently quite homogeneous, even more so than the opx (see below).
  • Orthopyroxene (n=11), Of21.34 ± 1.30 (2σ), FeO/MnO 28.7. This composition is bronzite. The estimated mean CaO contents of olivine (0.10 wt.%) and of bronzite (1.07 wt.%) are in the expected ranges for these minerals.
  • Kamacite (n=9) averages 10.6 wt.% Ni (very variable, 7.4-14.3%) and 0.592 ± 0.085 wt.% (2σ) Co (tightly clustered, range 0.54-0.67%). Trace of P noted in the metal, but no detectable S nor Cr.
  • Troilite (n=2) contains trace Cr, which is not unusual for this meteoritic sulphide.
  • Goethite (n=1) rich in Ni (6.8 wt.%) and Co (0.3%), clearly derived from metal by terrestrial weathering.

4. Shock state: the overall dark appearance, mosaic extinction of olivine, and irregular extinction in coarse (1.0 mm) olivine (Fig. 4) are consistent with appreciable shock in the source region of this meteorite's parent body. The darkening of the rock is due in large part to dispersion of minute (0.001-0.05 mm) blebs of troilite through the groundmass, and into the coarser silicates (Fig. 5). A proportion of carbonaceous matter might also be present, but the troilite seems to be the cause of the darkening (whence "black chondrite"), and organic matter content is probably low. Opaque shock veins and large melt pockets were not noted, but the common clouds of micron-scale sulphide droplets are consistent with incipient melting and a moderate, S4 shock state (Stoffler et al., 1991). Note one limitation of this analysis, which in a sense is luxurious, with access to 3 thin sections. A great many stony meteorites are breccias, composed of clasts (fragments) surrounded by a very variable proportion of fine-grained matrix. A slice through another part of the UNWA might reveal more in the way of melt veins and pockets, and then the classification might grade into S5.

5. Weathering grade: the material is quite fresh, and this plus the lack of well-developed "desert varnish" on the fusion crust may indicate a relatively short terrestrial residence time. Some iron oxyhydroxide (goethite) is however present on margins of metal grains and a weathering grade of W1 is assigned (where W0 is pristine and W6 is thoroughly altered: Wlotzka, 1993).

6. Bulk magnetic susceptibility: three faces of the sample were measured 4 times each using an SM-30 meter with a 50-mm coil. The measurements were made on the main mass, which is circa 9.0 x 8.5 x 6.0 cm in size. The mean of 12 readings is 166x10-3 SI units, which with an assumed density of 3.3 g/cc yields a corrected log value of 4.92 by the methodology of Gattacceca et al. (2004; see also Rochette et al., 2003, 2008, 2009a,b; Macke et al., 2010, 2011a,b), consistent with an L chondrite, or one of the more magnetic classes of carbonaceous chondrite. However, basic petrographic observations quickly rule out CI, CK, CB, CR and CO classes (see, e.g., Hutchison, 2004; Norton, 2002; Norton and Chitwood, 2008).

7. Conclusions: visual inspection ([1], above, and Figs. 1-3) indicated a chondrite of some kind. The mineralogy [2] and degree of preservation of the chondrules suggest a petrologic grade of 4. The composition of olivine and orthopyroxene [3] indicate an L chondrite. Mineralogy and the relative homogeneity of the rock-forming silicates, and absence of coarse recrystallized feldspar, are consistent with an L4 stone, absent features strongly supportive of a rarer carbonaceous chondrite (see [2], also [6]). Further support for an L stone comes from metal chemistry [3], specifically the Co content* (Rubin, 1990). Shock [4] and weathering [5] states are estimated at S4 and W1, given the mosaic texture and planar fractures in olivine, clouds of minute dispersed troilite blebs in the groundmass, and the very limited occurrence of goethite on kamacite margins. The bulk magnetism of the material [6] is consistent with an L chondrite.

In conclusion, the meteorite is classified as an ordinary chondrite, L4 (S4, W1). The TKW (total known weight) of this meteorite (this batch of NWA material) is the 1263.2 grams seen in Figure 1. However a small amount, perhaps between 25 and 100 grams, was dispersed prior to the sequential purchase of the material described here.

Given the lack of documentation, it is of interest to see whether any closely similar NWA chondrites are listed in the Meteoritical Bulletin database, in other words, whether this is part of a known find or is a similar meteorite, possibly a "paired" find that may be part of the same fall. As of 31 March 2014, the Meteoritical Bulletin lists a total of 7,648 NWA meteorite names, of which 5,120 have been validated by adequate research and documentation. No less than 1,663 of the validated NWA are L chondrites, which is not surprising, given that L chondrites comprise some 35% of all known meteorites, second in abundance only to the H chondrites.

This classification (first posted 01 April 2014, no joke intended!) will likely be upgraded and reported here in due course.

* Footnote: At the same time a few analyses were made of the H4 (S1, W3-4) chondrite NWA 5425, previously featured here. The results were as follows:

  • Olivine (n=5), Fa18.8 ± 4.1 (2σ).
  • Kamacite (n=8) averages 6.67 wt.% Ni (uniform, 6.61-6.74%) and 0.408 ± 0.025 wt.% (2σ) Co (tightly clustered, range 0.395-0.425%).
The lower Fa (fayalite, iron) content in the olivine falls perfectly in the H chondrite range, and lies within error of the published value for this meteorite. The lower Co content also supports the classification of NWA 5425, though the estimated Co contents of kamacite in the UNWA and NWA 5425 are circa 25% and 10% below the averages for the assigned classes, respectively (Rubin, 1990). This may be accurate, or at least partly due to imperfect calibration for cobalt. The UNWA Ni contents of kamacite vary widely, so the result is not due to one or more analyses sampling a grain of an alloy with higher Ni content (taenite or tetrataenite).

A brief scanning electron microscope examination of uncoated polished thin section 2010-1, courtesy of Dr Giovanni di Prisco, provided some confirmation of the petrographic and microprobe results (Fig. 6). The backscattered electron imagery and energy spectra revealed the following: olivine and bronzite, kamacite and troilite, and an aluminosilicate glass (with minor Na, Mg, K, Ca, Fe) between silicates in both barred olivine and radiating pyroxene chondrules. No taenite was noted in the kamacite. The metal oxidizes in situ into a nickeliferous goethite. Chondrules may be mantled by troilite. Chromite was noted, up to 60 microns in diameter. A 430x45-micron Ca phosphate grain (apatite or whitlockite?) was plated on the margin of a barred olivine chondrule.

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Figure 4. Two photomicrographs in cross-polarized transmitted light. Left: a porphyritic olivine chondrule with minor orthopyroxene. Nominal magnification 50X, long-axis field of view 1.7 mm. Right: a large olivine crystal, showing planar fractures and undulose extinction indicative of significant shock-induced deformation. Note the set of conjugate fractures that do not seem to be simple cleavage planes (at first I took this to be bronzite). Nominal magnification 100X, field of view 0.9 mm.

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Figure 5. Two photomicrographs in plane-polarized reflected light. Left: a meandering veinlet of relatively coarse troilite cuts through silicate host, which also contains traces of kamacite (white) and chromite (medium grey). Nominal magnification 100X, long-axis field of view 0.9 mm. Right: a zone of incipient shock melt darkened by very fine-grained troilite, on the margin of a porphyritic pyroxene-olivine chondrule (which contains very little sulphide or metal). Nominal magnification 200X, field of view 0.45 mm.

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Figure 6. Three BSE images of select features. Left: A fine-grained, subtly radiating silicate-dominated chondrule with (medium grey) troilite and outlying metal (white). Field of view 2.0 mm. Centre: Kamacite (white) with minor oxidation to goethite, troilite (medium grey), and an equant chromite grain (darker) in a (dark grey) silicate matrix. Field of view 0.65 mm. Right: Close-up showing equant chromite with adjacent kamacite and troilite, and small dark olivine grains. Field of view 0.20 mm.

References

Brearley,AJ and Jones,RH (1998) Chondritic meteorites. In `Planetary Materials' (Papike,JJ editor), Min.Soc.Amer. Reviews in Mineralogy 36, chapter 3, 398pp.

Dodd,RT (1969) Metamorphism of the ordinary chondrites: a review. Geochim.Cosmochim.Acta 33, 161-203.

Dodd,RT (1981) Meteorites, a Petrologic-Chemical Synthesis. Cambridge University Press, 368pp.

Gattacceca,J, Eisenlohr,P and Rochette,P (2004) Calibration of in situ magnetic susceptibility measurements. Geophys.J.Int. 158, 42-49.

Hutchison,R (2004) Meteorites: a Petrologic, Chemical and Isotopic Synthesis. Cambridge University Press, 506pp.

Krot,AN, Keil,K, Goodrich,CA, Scott,ERD and Weisberg,MK (2005) Classification of meteorites. In `Meteorites, Comets, and Planets' (Davis,AM editor). Treatise on Geochemistry volume 1 (Holland,HD and Turekian,KK editors), Elsevier- Pergamon, Oxford, 737pp., 83-128.

Macke,RJ, Britt,DT and Consolmagno,GJ (2011a) Density, porosity, and magnetic susceptibility of achondritic meteorites. Meteoritics & Planetary Science 46, 311-326.

Macke,RJ, Consolmagno,GJ and Britt,DT (2011b) Density, porosity, and magnetic susceptibility of carbonaceous chondrites. Meteoritics & Planetary Science 46, 1842-1862.

Macke,RJ, Consolmagno,GJ, Britt,DT and Hutson,ML (2010) Enstatite chondrite density, magnetic susceptibility, and porosity. Meteoritics & Planetary Science 45, 1513-1526.

Mason,B (1963) Olivine composition in chondrites. Geochim.Cosmochim.Acta 27, 1011-1023.

Mason,B (1967) Olivine composition in chondrites - a supplement. Geochim.Cosmochim.Acta 31, 1100-1103.

Norton,OR (2002) The Cambridge Encyclopedia of Meteorites. Cambridge University Press, New York, xx+354pp.

Norton,OR and Chitwood,LA (2008) Field Guide to Meteors and Meteorites. Springer-Verlag London Limited, 287pp.

Rochette,P, Weiss,BP and Gattacceca,J (2009b) Magnetism of extraterrestrial materials. Elements 5 no.4, 223-228.

Rochette,P, Gattacceca,J, Bonal,L, Bourot-Denise,M, Chevrier,V, Clerc,J-P, Consolmagno,G, Folco,L, Gounelle,M, Kohout,T, Pesonen,L, Quirico,E, Sagnotti,L and Skripnik,A (2008) Magnetic classification of stony meteorites: 2. Non-ordinary chondrites. Meteoritics & Planetary Science 43, 959-980.

Rochette,P, Gattacceca,J, Bourot-Denise,M, Consolmagno,G, Folco,L, Kohout,T, Pesonen,L and Sagnotti,L (2009a) Magnetic classification of stony meteorites: 3. Achondrites. Meteoritics & Planetary Science 44, 405-427.

Rochette,P, Sagnotti,L, Bourot-Denise,M, Consolmagno,G, Folco,L, Gattacceca,J, Osete,ML and Pesonen,L (2003) Magnetic classification of stony meteorites: 1. Ordinary chondrites. Meteoritics & Planetary Science 38, 251-268.

Rubin,AE (1990) Kamacite and olivine in ordinary chondrites: intergroup and intragroup relationships. Geochim.Cosmochim.Acta 54, 1217-1232.

Stoffler,D, Keil,K and Scott,ERD (1991) Shock metamorphism of ordinary chondrites. Geochim.Cosmochim.Acta 55, 3845-3867 (1991).

Wlotzka,F (1993) A weathering scale for the ordinary chondrites. Meteoritics 28, 460.

Graham Wilson, 26-28 February 2014, 18,30-31 March, 01-04,11,15 April 2014.

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