Octahedrite

<h2 id="structure">Structure</h2>
<p class="note">See also: <a href="/facts/Widmanst%25C3%25A4tten_pattern/K7JCbTpb">Widmanstätten pattern § Lamellæ formation mechanism</a>, and <a href="/facts/Widmanst%25C3%25A4tten_pattern/K7JCbTpb">Widmanstätten pattern § Shape and orientation</a></p>
<p>Octahedrites derive their name from the crystal structure paralleling an <a href="/facts/Octahedron/G38q92xW">octahedron</a>. Opposite faces are parallel so, although an octahedron has 8 faces, there are only 4 sets of <a href="/facts/Kamacite/9JHfxXsL">kamacite</a> plates.
</p><p>Due to a long cooling time in the interior of the parent <a href="/facts/Asteroid/g7Ulr1VB">asteroids</a>, these alloys have crystallized into intermixed millimeter-sized bands (from about 0.2 mm to 5 cm).<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> When polished and acid etched the classic <a href="/facts/Widmanst%25C3%25A4tten_pattern/K7JCbTpb">Widmanstätten patterns</a> of intersecting lines of lamellar kamacite, are visible.
</p><p>In gaps between the kamacite and taenite lamellae, a fine-grained mixture called <a href="/facts/Plessite/fFFNrKjg">plessite</a> is often found.  An iron nickel phosphide, <a href="/facts/Schreibersite/EsYNkM5E">schreibersite</a>, is present in most nickel-iron meteorites, as well as an iron-nickel-<a href="/facts/Cobalt/gqhScEw4">cobalt</a> <a href="/facts/Carbide/xfCMTjYJ">carbide</a>, <a href="/facts/Cohenite/YjBoj0hl">cohenite</a>. <a href="/facts/Graphite/sSf3KMXr">Graphite</a> and <a href="/facts/Troilite/tWP2rSj1">troilite</a> occur in rounded nodules up to several cm in size.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>
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<h2 id="subgroups">Subgroups</h2>

<p class="note">See also: <a href="/facts/Iron_meteorite/rjgT7fec">Iron meteorite § Structural classification</a></p>
<p>Octahedrites can be grouped by the dimensions of kamacite lamellae in the <a href="/facts/Widmanst%25C3%25A4tten_pattern/K7JCbTpb">Widmanstätten pattern</a>, which are related to the <a href="/facts/Nickel/KwMCHYRP">nickel</a> content:<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a>
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<ul><li>Coarsest octahedrites, lamellae width >3.3 mm, 5-9% Ni, symbol Ogg</li>
<li>Coarse octahedrites, lamellae 1.3-3.3 mm, 6.5-8.5% Ni, symbol Og</li>
<li>Medium octahedrites, lamellae 0.5-1.3 mm, 7-13% Ni, symbol Om</li>
<li>Fine octahedrites, lamellae 0.2-0.5 mm, 7.5-13% Ni, symbol Of</li>
<li>Finest octahedrites, lamellae <0.2 mm, 17-18% Ni, symbol Off</li>
<li>Plessitic octahedrites, kamacite spindles, a transitional structure between octahedrites and <a href="/facts/Ataxite/CQ5Qd17Y">ataxites</a>,<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> 9-18% Ni, symbol Opl</li></ul>
<h2 id="mineral">Mineral</h2>
<p>Octahedrite is an obsolete synonym for <a href="/facts/Anatase/wD932gCW">anatase</a>, one of the three known <a href="/facts/Titanium/53ArnDoy">titanium</a> dioxide <a href="/facts/Mineral/LkUFbNmv">minerals</a>.
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<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Glossary_of_meteoritics/TbTPV1Ut">Glossary of meteoritics</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://webmineral.com/data/Kamacite.shtml">Webmineral</a></li>
<li><a href="http://www.amonline.net.au/geoscience/meteors/types.htm">Meteorites Australia</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Goldstein, J.I; Scott, E.R.D; Chabot, N.L (2009). "Iron meteorites: Crystallization, thermal history, parent bodies, and origin". Chemie der Erde – Geochemistry. 69 (4): 293–325. Bibcode:2009ChEG...69..293G. doi:10.1016/j.chemer.2009.01.002. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Vagn F. Buchwald: Handbook of Iron Meteorites. University of California Press, 1975. <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>James H. Shirley,Rhodes Whitmore Fairbridge, Encyclopedia of planetary sciences, Springer, 1997. ISBN 978-0-412-06951-2 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Geochimica et Cosmochimica Acta, Volume 45, Ed. 9-12 <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
</ol>

Octahedrite open-in-new

Octahedrite