Some Fundamentals of Common Chondrite Classification

by Ron Hartman
Department of Earth Sciences & Astronomy
Mt. San Antonio College

Some Fundamentals of Common Chondrite Classification

Third in a series of introductory topics for the new meteorite enthusiast!

In our last article we discussed the process of authenticating and naming meteorites.  I also suggested that we are experiencing an exciting era where meteorites are becoming commercially available more rapidly that they can be classified and have their names (numbers) approved by the Nomenclature Committee of the Meteoritical Society.  Dealers will advise buyers that a name (note a) is provisional, or accepted and pending.  Once approved it will appear in The Meteoritcal Bulletin.  Listings are updated periodically and May be seen at

http://groups.yahoo.com/group/MeteoriticalBulletin.
Archive listings (back to 1970) are found at:

http://www.uark.edu/campus-resources/metsoc/metbull.htm.

Along with listings, one will see, following the name, class and place of fall (coordinates), designations for shock (S), weathering grade (W) or (WG), and (Fa), fayalite (molecular %).  Sometimes these data will also be stated on the dealer’s label.  This is important information which tells a great deal about the meteorite and its history, and should by noted by the collector when selecting his choices in the marketplace.  We need to state first (but briefly) something about the composition of meteorites.  Meteorites are composed of nickel-iron and/or (silicate) stony materials.

There are five major groups of meteoritic minerals, each characterized by its general chemical formula (Leonard, 1954):

(a) The Nickel-Irons (kamacite, taenite and plessite (the latter being a mixture of the first two).

(b) The olivines [primarily fayalite or fosterite, usually a combination thereof: (Mg,Fe)₂SiO_4.

(c) The pyroxenes (including enstatite, bronzite and hypersthene) which are iron-magnesium silicates, and a few others.

(d) The plagioclase feldspars:  [a solid solution (Na,Ca)(AlSi₃O₈)]

(e) The silicas (SiO₂)

Here, we are interested in the olivines.  Olivine is not a specific mineral, but two that vary in their compositional makeup between Iron (Fe) and magnesium (Mg). The olivine varies between 100% (Mg₂)SiO₄ and 100% (Fe₂)SiO₄, the former being fosterite and the latter fayalite.  The mineral is said to be a “solid solution”, a term which means that in the crystalline lattice (arrangement) of the atoms making up its molecular structure, iron and magnesium can easily replace one another.

The fayalite number is the ratio of fayalite/fosterite in olivine.  It is especially useful in pairing meteorites that are found in close proximity to determine whether they are a part of the same fall. (note b).

Take a look at the following designations: NWA 516, a very rare Winonaite (Fa_1.1).  Compare it with NWA 515, an L6 with (fayalite _25.0).  While NWA 516 is not a chondrite, it is mentioned here for comparison because of its unusually low fayalite content.  In the examples above, the Winonaite has only 1.1% fayalite (98.9%  being fosterite); in NWA 515, 25% fayalite (75% being fosterite).

Readings are done on carbon-coated polished

thin sections using an electron microprobe. A number of readings are taken over the entire thin-section to average values.  The result then determines whether the meteorite is an H, L, or LL.  This is the only use of the microprobe in the classification process.  Petrologic types (article #2) as well as (S) and (W) values are determined under the microscope by the experienced researcher.

The H’s are characterized by  ~15-21% fayalite (these are sometimes referred to as olivine-bronzite chondrites although that is rather outdated usage), the L’s by ~22-26% fayalite (the olivine-hypersthene chondrites), the LL’s, > 28-32%.  The frequency of occurrence of fayalite numbers from the H’s through the L’s and LL’s is not uniform, but reaches a max. at Fa = 19 for H’s and 24 = for L’s, with very few around 20.  (DaG 591, for example, is classified as an H(L) with Fa = 20.0.   In verifying these statistics (citing several hundred meteorites listed in the Meteoritical Bulletin, 1982-83-84) I found no 21’s and none beyond 32%.

The question has been asked whether H’s can deteriorate into L or LL’s.  The answer is absolutely not.  Each of these groups has a completely different history of formation. For example, the H’s are considered to have formed nearest Earth but farther from the Sun than Earth, followed by the L’s, then the LL’s [at increasing solar distances] (Wasson, 1985), with the LL’s being most oxidized (Sears, 1978).  The H’s, L’s and LL’s are different in other ways, as well, in addition to their iron/magnesium ratios.

Shock is a measure of the degree of fracturing of the matrix, produced by accretion, such as two small bodies in space, or a meteoroid and larger asteroid.  Most probably, what we see is the result of impact of a small meteoroid into the basin of a crater on a much larger asteroid (Stoffler, 1991).  Smaller bodies colliding with one another would not have sufficiently great impact velocity to produce the pressures and temperatures required to produce shock effects, due to their lesser gravitational attraction for one another.  High instantaneous pressures, in excess of 5 GPa (1 Gpa = 10,000 atmospheres, or ‘bars’), are necessary to produce what is called shock metamorphism.  Brecciation, seen frequently in common chondrites, could not be the result of very small bodies accreting.  More probably, in brecciated chondrites we see the history of a small body having struck and mixed with the regolith material already present as a pre-existing mix on the surface of a larger asteroid.

Following is
a summary of the stock stages developed and stated by D. Stoffler, K. Keil and
R. D. Scott:

S1: completely unshocked;
(up to 5 GPa)

S2: very weakly shocked
(5-10 GPa); uneven darkening of olivine as seen under polarized light; planar and irregular fractures, (breaks in other than a natural cleavage plane)

S3: weakly shocked
(15-20 GPa); weak fractures in olivine seen under polarized light; dark shock veins and some melt pockets

S4: moderately shocked
(30-35 GPa); weak planar fracturing of olivine under polarized light; some pockets of melted material, dark interconnected shock veins)

S5: strongly shocked
(45-55 GPa); very strong planar fracturing and deformation features in olivine; alteration of plagioclase into maskelynite; formation of dark melt veins

S6: very strongly shocked
(75-90 GPa); olivine recrystallizes, with local alteration to a mineral called ringwoodite (note c) and shock melting of plagioclase to a glass

Greater shock pressures will melt the rock, producing what is referred to as an “impact-melt”.  Seldom would these reach Earth (Stoffler, 1991).  True impact melts are very much sought after by collectors!

The fracturing of olivine crystals and other features must be observed under a microscope with shock effects observed under polarized light.  Larger structures, such as shock veins are visible to the eye.  Many of the shocked veins formed at the boundaries of polished surfaces of brecciated specimens exhibit beautiful spider-web-like structures.

‘Weathering’ is meant to imply chemical weathering, which takes place within the interior of the entire meteorite, as opposed to the cracking and deterioration we see near the surface due to proximity of the elements (wind, water, temperature, etc.)  Here, again, we have a weathering scale.

The weathering scale developed by F. Wlotzka (1993), and accepted for use by the Meteoritical Society, is summarized here:

W0:  no
apparent oxidation (rusting) of iron grains or troilite (iron-sulfide).

A meteorite usually exhibits this unique condition if recovered only very soon after falling, and before any of the elements can  do their “dirty work”.  You May have read of some dealers offering their new Bensour LL6 [working name, official name not yet assigned] which was seen to fall on both sides of the Morocco-Algerian border on Feb. 10, 2002, as having been recovered “before it rained”.

W1:  minor
oxide rims around metal grains and troilite, minor rusting (oxidation) in veins

W2:  moderate
oxidation (rusting) of metal (20-60% of grains)

W3:  heavy
oxidation of nickel-iron and troilite; 60-95% being replaced.

W4:  complete
oxidation of metal and troilite; silicates not yet altered.

W5:  beginning
alteration of mafic (iron/magnesium) silicates

W6:  massive
replacement of silicates by clay minerals and oxides.

Thus, a meteorite characterized as:  LV 001 +34°N +116°W,  L6, Fa 24.2 (S2)(W3) is a low iron (class L) of petrologic type 6, equilibrated ordinary chondrite), 24.3 % fayalite, shock stage=2; weathering grade=3, named LV 001 (Lucerne Valley 001), (latitude, longitude following name).  About as common as they come except that this one happens to be the first of the few I’ve ever found!

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Note a:  Hereafter in this series, a name will mean to refer to a number if a meteorite has been assigned a number instead of a name.

Note b:  For example, of the first four meteorites found in the Lucerne Valley (CA) strewnfield, LV 001 and LV 004, with fayalite values of 24.3 (+/-0.3) and 24.2 (+/-0.3) are considered to be related (paired), while LV 003 Fa=18.0(+/-0.4) found within minutes of LV 004 (but on the opposite side of the road) is not of the same fall.  [The finds, LV 001… numbered in the order they were found collectively are known as “Lucerne Valley”] (Met. Bull. 1999).

Note c:  Ringwoodite is an altered crystalline form of olivine.  Substances that are chemically the same but have different properties, such as crystal structure, density and hardness, are said to be dimorphous.

REFERENCES

Grossman, Jeffery N., The Meteoritical Bulletin, No. 83,
July, 1999, p.5;

also, No. 82, 83, 84
(random use of tables).

Kraus, Hunt and Ramsdell, Mineralogy (1951)
McGraw-Hill, New York.

Leonard, F.C., (1954); The Classification of the
Meteoritical Minerals and its Application to the Simplified Classification of
Meteorites; Meteoritics, V. 1, No. 2, pp. 160-161

[meteorite-list] (June 2, 2002) Weathering grades; note on:
Wlotzka, F., (1993) Weathering grades of meteorites; in Meteoritics, v.
28.  p. 460.

Norton, O. R., (2002) The Cambridge Encyclopedia of
Meteorites, Cambridge University Press, Cambridge, U.K.

Sears, D. W. (1978), The Nature and Origin of Meteorites,
Monographs on Astronomical Subjects: 5; Adam Hilger Ltd. p. 165.

Stoffler, D., Keil, K., and Scott, E. R. D., (1991); Shock
metamorphism of ordinary chondrites; Geochimica et Cosmochimica Acta, 55,
pp. 3845-3867.

Wahlstrom,E. E., Optical Crystallography, (1943)
John Wiley & Sons, New York

Wasson, John T., 1985; METEORITES: Their Record of Early
Solar-System History; W. H. Freeman , pp. 214-215.

Ron Hartman
Department of Earth Sciences & Astronomy
Mt. San Antonio College
Walnut, CA 91789 (U.S.)