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Concept
Pallasite
Summary
The pallasites are a class of stony–iron meteorite. They are relatively rare, and can be distinguished by the presence of large olivine crystal inclusions in the ferro-nickel matrix.
These crystals represent mantle and core material from differentiated planetesimals, which were destroyed by violent collisions during the early formation of the solar system.
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).
Using the oxygen isotopic composition, meteoric iron composition and silicate composition pallasites are divided into 4 subgroups:
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 pattern.
Pallasite ungrouped (P-ung): Specimens that don't fit into any groups or grouplets
Pallasites were once thought to originate at the core-mantle boundary of differentiated asteroids that were subsequently shattered through impacts. An alternative recent hypothesis is that they are impact-generated mixtures of core and mantle materials. This hypothesis is supported by isotopic analysis indicating admixture of material from multiple planetesimals.
A common error is to associate their name with the asteroid 2 Pallas but their actual name is after the German naturalist Peter Pallas (1741–1811), who studied in 1772 a specimen found earlier near Krasnoyarsk in the mountains of Siberia that had a mass of . The Krasnoyarsk mass described by Pallas in 1776 was one of the examples used by E.F.F. Chladni in the 1790s to demonstrate the reality of meteorite falls on the Earth, which most scientists at his time considered as fairytales.
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