Cut two iron meteorites, polish the faces, and etch them with acid, and they can look almost nothing alike. One shows broad metallic bands, wide enough to trace from across a room. Another shows a tight lattice that needs a loupe to resolve. Both are iron. Both formed in the core of an asteroid. The difference between them comes down to a single measurable feature — the width of one mineral phase — and that measurement is the basis of the entire octahedrite classification system.
What an Octahedrite Is
At the structural level, iron meteorites sort into three families. Hexahedrites, at roughly 4–6% nickel, are built from large single crystals of one mineral, kamacite, and show no interlocking pattern. Ataxites, above roughly 12% nickel, are so nickel-rich that they show no visible macroscopic structure at all. Between them sit the octahedrites — iron meteorites of about 6–12% nickel, and the only family that displays the Widmanstätten pattern: the interleaved geometry of low-nickel kamacite and high-nickel taenite that forms as the metal cools. The way that pattern forms is its own subject; this page takes it as given.
Within the octahedrites, every specimen carries the same two phases. What changes from one to the next is scale — specifically, how wide the kamacite bands are. That width has a name: kamacite bandwidth. It is the single variable the classification turns on.
The Octahedrite Classes
Vagn Buchwald’s 1975 Handbook of Iron Meteorites set the numerical boundaries the field still uses. The classes are defined by bandwidth alone, measured in millimetres:
| Class | Symbol | Approx. nickel | Kamacite bandwidth | Classic example |
|---|---|---|---|---|
| Coarsest octahedrite | Ogg | ~6% | greater than 3.3 mm | Sikhote-Alin |
| Coarse octahedrite | Og | ~7% | 1.3 – 3.3 mm | Canyon Diablo |
| Medium octahedrite | Om | ~8% | 0.5 – 1.3 mm | Cape York |
| Fine octahedrite | Of | ~9% | 0.2 – 0.5 mm | Gibeon |
| Finest octahedrite | Off | 9% and above | under 0.2 mm | Tazewell |
Read the table from top to bottom and one thing stands out: bandwidth narrows as nickel rises. That is not a coincidence, and it points to what the classification is really recording.
Why Bandwidth Differs
Kamacite bandwidth is not a surface effect or a finishing tolerance. It is a frozen record of time. As the molten metal at the centre of a parent asteroid cooled, kamacite grew outward from taenite over enormous spans — and the slower that cooling ran, the wider the kamacite bands grew. A coarse octahedrite is, in effect, a body that cooled more slowly; a fine one cooled faster. Nickel sets the chemistry, but the band width preserves the pace.
This is why the structural class is more than a label. It is a reading of an object’s thermal history — the rate, often measured in tens of degrees Celsius per million years, at which a core gave up its heat. The mechanism behind that figure is its own topic; here it is enough to note that bandwidth and cooling rate are two ways of describing the same event.
Where Aletai Sits — and Why the Answer Is Split
This is where a clean system meets a real specimen, and the two do not fully agree.
The Aletai meteorite is recorded in the Meteoritical Bulletin Database with a kamacite bandwidth of 0.89 ± 0.41 mm. Measurements across its individual masses cluster between roughly 0.89 and 1.2 mm, with petrographic summaries of some masses reaching about 1.4 mm. Set those numbers against Buchwald’s boundaries and the reading is plain: a mean below 1.3 mm places Aletai in the medium octahedrite range (0.5–1.3 mm), not the coarse one (1.3–3.3 mm).
And yet Aletai is very frequently described as a coarse octahedrite — in peer-reviewed papers, in encyclopedias, and in dealer listings. Both statements are documented. Both are, in their own context, defensible. The split is real, and it has a traceable cause.
The “coarse” label did not come from a bandwidth measurement. It came from a comparison. In 1984, when researchers reassigned the Armanty mass — later consolidated into Aletai — into the smaller IIIE chemical group, they described its kamacite bands as wider than those of related irons at the same nickel level. That relative description, coarser, attached itself to the specimen and was inherited by later papers, then copied into databases, even as the recorded bandwidth stayed below the coarse threshold. Buchwald’s system contains no provision that moves the boundaries for a chemically unusual iron. The numbers are the numbers.
The result is the situation seen today. Academic usage tends to preserve the inherited “coarse” term. Strict structural classifiers — including major auction houses and specialist dealers — list Aletai as a medium octahedrite, holding to the under-1.3 mm measurement. The wider community tolerates the disagreement for a straightforward reason: structural class is no longer the primary way iron meteorites are sorted. That role now belongs to chemical group, and Aletai’s chemical classification — Iron, IIIE-an (anomalous) — is not in dispute. The structural label is a description, not the meteorite’s identity, and on that description the record is honestly split.
What This Page Does Not Cover
The subject here is the structural classification system and where one meteorite falls within it. It is deliberately not a comparison of meteorites as collectible material — how Aletai measures against other irons across origin, rarity, or condition is a separate question, handled on its own page. Nor is it a buyer’s framework: judging whether a given piece is worth collecting follows different criteria, set out elsewhere. And the full account of Aletai’s composition and origin belongs to the material’s own profile. This page answers one thing — what the classes are, and how bandwidth assigns them.
Reading a Piece Honestly
Classification is not decoration. Knowing that a structural label can be inherited rather than measured, or that a single meteorite can sit honestly on a boundary, is part of reading any piece for what it is rather than what a listing claims. Movalor works with Aletai material — an iron documented in the Meteoritical Bulletin Database as IIIE-an — and treats that record, ambiguities included, as the thing worth being accurate about.
FAQ
What is an octahedrite? An octahedrite is an iron meteorite, generally containing about 6–12% nickel, that displays the Widmanstätten pattern — an interlocking structure of the minerals kamacite and taenite. It is one of three structural families of iron meteorite, alongside hexahedrites and ataxites.
How are octahedrites classified? They are classified by kamacite bandwidth — the width of the kamacite bands, measured in millimetres. Buchwald’s 1975 system defines coarsest (over 3.3 mm), coarse (1.3–3.3 mm), medium (0.5–1.3 mm), fine (0.2–0.5 mm), and finest (under 0.2 mm) classes.
What determines kamacite bandwidth? Cooling rate. The kamacite bands grew as the parent asteroid’s core cooled, and slower cooling produced wider bands. Bandwidth is effectively a record of how slowly that body lost its heat.
Is Aletai a coarse or medium octahedrite? Both labels appear in the record. Aletai’s measured kamacite bandwidth (0.89 ± 0.41 mm, clustering near 0.89–1.2 mm) falls within the medium range under Buchwald’s limits, and strict classifiers list it as medium. Much of the literature inherited a “coarse” description from a 1984 comparative reassignment. Its chemical classification, Iron IIIE-an (anomalous), is not disputed.
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The same structural detail, read on a polished face — expressed in wearable form.