Meteorites: A primer

Dr Kendal Martyn

Meteorites have long held fascination for me – that is, they aren’t from this planet. Added “glamour” has come from recent suggestions that at least one meteorite impact on earth could be responsible for mass-extinction events, the largest “smoking gun” in evolutionary selection. Also, meteorites are the only realistic chance we’ll have to get-to-grips with material from places other than our own planet.

Observed falls (associated with fireballs, rocks bouncing on the sofa after piling through the roof and so on) show meteorites come from some distance at some speed. Glassy, black fusion crust (Fig. 2) is the result of friction heating as the meteorite travels through the atmosphere, a texture distinguishing meteorites from other rock types.

Fig. 2. Ordinary chondrite with fusion crust.

Geochemical dating shows that most meteorites are 500 million years older than the oldest dated earth rocks. Therefore, they are direct samples of different ages, stages of development and places in the solar system. They have received more scientific attention than many other rock types. Cheaper than manned or unmanned space flights, meteoritics (the study of meteorites) provides bountiful geochemical and mineralogical observations – observations that have formed (and tested thoroughly) many theories about the birth and formation of the solar system. Meteoritics is where geology, mineralogy, geochemistry, astronomy, astrophysics, biology and palaeontology collide.

Meteorites are broadly divided into three main categories: iron, stony-iron and stony. Divisions relate to the relative abundance of iron metal and silicate minerals (for example, olivine and pyroxene). Superimposed upon this broad division are more subtle, yet significant chemical and textural/mineralogical divisions.

Why is this important?

All classification schemes should be more than just descriptions: “genetic” links or natural divisions reflecting natural processes are much more informative than it “looks like a potato” (“Looks like a potato” does not supply much useful information unless your classification scheme is trying to distinguish root vegetables from tubers). Therefore, the classifications (and descriptions) outlined below assign meteorites into meaningful, insightful groups that reflect the source region they came from and the processes altering meteorites after the formation of the solar system.

(1) Iron meteorites

These are readily available in shops and on the internet, and are reasonably familiar to the layperson (Fig. 4).

Fig. 4. Complete piece of Sikhote-alin iron meteorite.

Divisions based on mixtures of metallic alloys of iron (Fe) and nickel (Ni) are based on variations in texture, particularly the intergrowth of different Fe-Ni alloys. Etching with 10% nitric acid produces interlocking patterns (Fig. 5).

It also allows division into hexahedrites (HEX), cctohedrites (O) and ataxites (ATAX) (Table 1).

Within the octohedrites, a five-fold division based on how wide the bands are: fine through to coarsest (Table 2).

A separate chemical classification runs in parallel to the textural one outlined here. Iron meteorites are most likely to represent the metallic core of a disintegrated proto-planet and represent 6% of known meteorites.

(2) Stony-iron meteorites

Theseare extremely distinctive and divided into two groups: pallasites and mesosiderites. Pallasitescontain crystals of olivine, which is an iron-magnesium silicate that makes up 60% to 90% of the Earth’s mantle, the solid part of the earth between liquid outer core and the thin outer skin of aluminium and silicon rich crust. This is suspended in Fe-Ni matrix (Fig. 6) and probably represent the partial separation (or remixing) of iron during core formation within a proto-planet or asteroid.

Mesosiderites are breccia mixtures of iron and silicate rich rock clasts (Fig. 7), and probably represent the result of an iron meteorite/asteroid collision. (Breccia is a rock composed of angular, broken rock fragments (clasts) cemented by a ﬁner matrix.) Stony-irons represent only 1% of known meteorites.

Fig. 7. Esterville mesosiderite (picture courtesy Allan Lang).

(3) Stony meteorites

Theserepresent the most varied group with broad division into Chondrites (undifferentiated, primitive meteorites hardly changed from the primordial solar system) and achondrities (differentiated, evolved meteorites often showing igneous textures). Chondrites are so called as they exhibit a distinctive texture made up of chondrules, which are spherical masses of silicate minerals a few millimetres across, thought to have formed during the very first “clumping together” of material in the Solar System (Figs. 8 and 9).

Fig. 9. Chondrite thin section showing chondrules.

Variable amounts of chondrule recrystalisation (through thermal metamorphism) is seen, related to how close the meteorite was to the sun. Signs of chondrules are usually preserved except where heating has passed the 600⁰C level to produce some Enstatite chondrites (E), named after the abundance of enstatite, a low calcium type of pyroxene.

Ordinary chondrites (OC) are so called as they are the most abundant clan. Carbonaceous chondrites (C) are a rare and extremely varied group with low free iron but abundant carbon and water (Table 3).

Carbonaceous chondrites are interesting to biologists as some contain measurable amounts of amino-acids – the building blocks of DNA.

Finally, a petrologic type (Table 4) is assigned to describe the amount of recrystalisation. Recrystalisation in space is closely related to distance from the sun: too close and any water boils away (for example, types 4 to 6, with heat causing recrystalisation); too far and everything remains frozen (comet-like material). A narrow band between frozen and boiled-off extremes is where water is available to interact with and re-crystallise minerals.

Table 4: Petrologic (textural) classification of chondrites.

Most carbonaceous chondrites are types 1 to 3. Ordinary chondrites are mainly types 4 to 6. Chondrites account for 86% of all known meteorites and are thought to have avoided planet and large asteroid formation, the best insight into early solar system material (particularly type 3). Differentiated stony meteorites are generally called achondrites for their lack of chondrules (they also have little or no free iron). Texture, mineralogy and chemistry strongly suggest that achondritescome from magma similar to that producing igneous rocks on Earth.

Achondrites are meteorite samples of a most interesting type: rocks of other large solar system bodies such as Mars, the Moon and asteroids. Does this mean that other meteorites (yet to be recognised?) are samples of other planets such as Mercury or Venus? They represent 7% of all meteorites and several related types are seen, forming four main groups:

Angrites, eucrites and howardites: angrites (ANG) are igneous rocks with cumulate textures (cumulate is an igneous rock produced by accumulation of crystals separated from magma by physical process); eucrites (EUC) are igneous rocks with textures typical of surface or near surface volcanic rocks (Fig. 10); and howardites (HOW) are impact-related breccias of angrite/eucrite rock fragments. Spectral-reflectance measurements are consistent with 4 Vesta (a cratered proto-planet/asteroid) being the source for ANG-EUC-HOW meteorites.
Aubrites, diogenites and urelites are calcium poor equivalents of the ANG-EUC- HOW series, also likely to be from another large asteroid, probably Asteroid 3103.
Shergotites, nakhlites and chassingites (SNCs): sherogitites are volcanic, basaltic rocks; and nakhlites and chassingites have igneous- cumulate textures, rich in olivine and pyroxene. Several lines of evidence strongly suggest that SNCs are from Mars. SNCs also have relatively young ages, suggesting Mars was volcanically active until quite recently.
Lunar breccias and basalts (LUN): a group of about twenty meteorites thought to come from the moon. They are texturally, chemically and mineralogically identical to material returned through the Apollo moon missions.

Fig. 10. Millbillillie eucrite slice (picture courtesy Allan Lang).

Meteorites provide a vital, direct sample of solar system material that isn’t from Earth. Samples that give information that tests fundamental assumptions about how the solar system, life and the planet we live on came to be; truly “Big Science”.

Meteorites: A primer