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Pallasite
The pallasites are a class of stony–iron meteorite. Structure and composition It consists of centimetre-sized olivine crystals of peridot quality in an iron-nickel matrix. Coarser metal areas develop Widmanstätten patterns upon etching. Minor constituents are schreibersite, troilite, chromite, pyroxenes, and phosphates (whitlockite, stanfieldite, farringtonite, and merrillite). Classification and subgroups Using the oxygen isotopic composition, meteoric iron composition and silicate composition pallasites are divided into 4 subgroups:O. Richard Norton. ''The Cambridge encyclopedia of meteorites''. Cambridge, Cambridge University Press, 2002. . * Main group pallasites (PMG): Almost all pallasites * Eagle Station group (PES): 5 specimens known. They are related to IIF irons. * Pyroxene Pallasite grouplet (PPX): Counts only Vermillion and Yamato 8451. They take their name from the high orthopyroxene content (about 5%). Metal matrix shows a fine octahedrite Widmanstätten pat ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hexahedrite
Hexahedrites are a structural class of iron meteorite. They are composed almost exclusively of the nickel–iron alloy kamacite and are lower in nickel content than the octahedrites. The nickel concentration in hexahedrites is always below 5.8% and only rarely below 5.3%. The name comes from the cubic (i.e. ''hexahedron'') structure of the kamacite crystal. After etching, hexahedrites do not display a Widmanstätten pattern, but they often do show Neumann lines: parallel lines that cross each other at various angles, and are indicative of impact shock on the parent body. These lines are named after Johann G. Neumann, who discovered them in 1848. Chemical classification Concentrations of trace elements (germanium, gallium and iridium) are used to separate the iron meteorites into chemical classes, which correspond to separate asteroid parent bodies. Chemical classes that include hexahedrites are: * IIAB meteorites (includes also some octahedrites) * IIG meteorites See also * Glos ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
