Octahedrite (Seymchan Meteorite) (Magadan District, Russia) 6 : 無料・フリー素材/写真
Octahedrite (Seymchan Meteorite) (Magadan District, Russia) 6 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Octahedrite from the Asteroid Belt between Mars and Jupiter. (cut-polished-nitric acid etched surface; 2.8 centimeters across along the top)Meteorites are rocks from space. Three basic categories exist: 1) stony meteorites; 2) iron meteorites; and 3) stony-iron meteorites. As the name suggests, iron meteorites are dominated by iron metal (elemental Fe). They also include some metallic nickel (Ni), plus minor minerals. They represent metallic core samples from differentiated asteroids/dwarf planets in the Solar System that have been disrupted by one or more large impact events.Iron meteorites come in three textural varieties: octahedrites, hexahedrites, and ataxites. Octahedrites are the most common type of Fe-Ni meteorites. The textural classification of iron meteorites has been augmented with information on trace element content.Mineralogically, all octahedrites are dominated by two minerals having very similar chemistries: kamacite (FeNi) and taenite (FeNi). Kamacite is a silvery-colored iron-nickel metal alloy rich in iron, with about 5.5 weight-percent nickel. Taenite is a silvery-colored iron-nickel metal alloy rich in nickel, with about 27-65 weight-percent nickel. Octahedrites have much more kamacite than taenite. They also contain minor amounts of troilite (FeS - iron monosulfide), silicate minerals, and others.The physical crystalline structure of octahedrites is distinctive. On cut, polished, and nitric acid-etched surfaces (see example here), a criss-crossing pattern of silvery-gray blades is evident. This is called Widmanstätten structure. It formed when kamacite and taenite crystallized from cooling magma. The two minerals formed interlocking plates with octahedral (double pyramid) geometries.The octahedrite seen here is a cut slice from the Seymchan Meteorite. Two large specimens were found in 1967 in the Magadan District of far-eastern Siberia, Russia. Additional specimens were recovered in the 2000s and 2010s. The total known weight of recovered samples is ~500 or more kilograms. The first two rocks were both identified as octahedrites. Additional specimens include some with an olivine component. Seymchan has since been classified as a pallasite, one of two groups of stony-iron meteorites. Lithologically, it is improper to refer to olivine-free samples as pallasite. The Seymchan Meteorite grades from a coarse octahedrite to a pallasite. Actually, Seymchan lithologies can be somewhat difficult to characterize (see the 2nd-from-the-bottom illustration at: www.psrd.hawaii.edu/June10/pallasites-origin.html). The term "strongly heterogeneous pallasite" has been applied to Seymchan. After kamacite, taenite, and olivine, Seymchan Meteorite samples have also been reported to contain schreibersite (an iron-nickel-cobalt phosphide mineral: (Fe,Ni,Co)3P) and chromite (iron chromium oxide: FeCr2O4).------------------------------See locality info. at:www.mindat.org/loc-122386.html |
| 撮影日 | 2019-01-22 18:14:31 |
| 撮影者 | James St. John |
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