Editing Octahedrite

{{Short description|Structural class of iron meteorites}}
{{Use American English|date=January 2019}}
{{Use mdy dates|date=January 2019}}
{{Infobox meteorite subdivision
|Subdivision               = Structural class
|Name                      = Octahedrite
|Alternative_names         = 
|Image                     = TolucaMeteorite.jpg
|Image_caption             = Octahedrite from [[Toluca]]
|Image_alt_text            = 
|Compositional_type        = Iron
|Type                      = 
|Class                     = 
|Clan                      = 
|Group                     = 
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|Subgroups                 = {{flatlist|
*
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|Parent_body               = 
|Composition               = 
|Petrologic_type           = 
|Number_of_specimens       = 
|TKW                       = <!-- {{Convert||kg}} -->
|Image2                    = Meteoric iron phase diagram taenite kamacite octaehedrite.svg
|Image2_caption            = A phase diagram showing the link between structural and chemical classification.
|Image2_alt_text           = 
}}
'''Octahedrites''' are the most common [[Iron meteorite#Structural classification|structural class]] of [[iron meteorite]]s. The structures occur because the [[meteoric iron]] has a certain [[nickel]] concentration that leads to the [[exsolution]] of [[kamacite]] out of [[taenite]] while cooling.

==Structure==
{{see also|Widmanstätten pattern#Lamellæ formation mechanism|Widmanstätten pattern#Shape and orientation}}
Octahedrites derive their name from the crystal structure paralleling an [[octahedron]]. Opposite faces are parallel so, although an octahedron has 8 faces, there are only 4 sets of [[kamacite]] plates.

Due to a long cooling time in the interior of the parent [[asteroid]]s, these alloys have crystallized into intermixed millimeter-sized bands (from about 0.2&nbsp;mm to 5&nbsp;cm).<ref name=CdE2009>{{cite journal |title=Iron meteorites: Crystallization, thermal history, parent bodies, and origin |journal=Chemie der Erde – Geochemistry |volume=69 |issue=4 |pages=293–325 |doi=10.1016/j.chemer.2009.01.002 |year=2009 |last1=Goldstein|first1=J.I |last2=Scott|first2=E.R.D |last3=Chabot|first3=N.L |bibcode=2009ChEG...69..293G }}</ref> When polished and acid etched the classic [[Widmanstätten pattern]]s of intersecting lines of lamellar kamacite, are visible.

In gaps between the kamacite and taenite lamellae, a fine-grained mixture called [[plessite]] is often found.  An iron nickel phosphide, [[schreibersite]], is present in most nickel-iron meteorites, as well as an iron-nickel-[[cobalt]] [[carbide]], [[cohenite]]. [[Graphite]] and [[troilite]] occur in rounded nodules up to several cm in size.<ref name=him1975>Vagn F. Buchwald: Handbook of Iron Meteorites. University of California Press, 1975.</ref>

==Subgroups==
[[File:ZacatecasMeteoriteGDL cropped.JPG|thumb|280px|The Zacatecas Meteorite found in 1782 in Zacatecas Mexico, weighing 780kg.]]
{{see also|Iron meteorite#Structural classification}}
Octahedrites can be grouped by the dimensions of kamacite lamellae in the [[Widmanstätten pattern]], which are related to the [[nickel]] content:<ref>James H. Shirley,Rhodes Whitmore Fairbridge, ''Encyclopedia of planetary sciences'', Springer, 1997. {{ISBN|978-0-412-06951-2}}</ref>
* Coarsest octahedrites, lamellae width >3.3&nbsp;mm, 5–9% Ni, symbol Ogg
* Coarse octahedrites, lamellae 1.3–3.3&nbsp;mm, 6.5–8.5% Ni, symbol Og
* Medium octahedrites, lamellae 0.5–1.3&nbsp;mm, 7–13% Ni, symbol Om
* Fine octahedrites, lamellae 0.2–0.5&nbsp;mm, 7.5–13% Ni, symbol Of
* Finest octahedrites, lamellae <0.2&nbsp;mm, 17–18% Ni, symbol Off
* Plessitic octahedrites, kamacite spindles, a transitional structure between octahedrites and [[ataxite]]s,<ref>Geochimica et Cosmochimica Acta, Volume 45, Ed. 9–12</ref> 9–18% Ni, symbol Opl

==Mineral==
Octahedrite is an obsolete synonym for [[anatase]], one of the three known [[titanium]] dioxide [[mineral]]s.{{cn|date=September 2023}}

==See also==
* [[Glossary of meteoritics]]
 
==References==
{{Reflist}}

==External links==
*[http://webmineral.com/data/Kamacite.shtml Webmineral]
*[http://www.amonline.net.au/geoscience/meteors/types.htm Meteorites Australia]

{{Meteorites}}