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Pallasites

 

General: The meteorites of this group are named for the German naturalist Peter Simon Pallas. In the late 18th century, he was invited by the Russian emperor, Catharina the Great, to explore the vast areas of Siberia. In 1772 during one of his travels, he studied a large iron mass that had earlier been found in the mountains near Krasnojarsk. This unusual mass contained large olivine crystals set in an iron matrix, strange enough to catch Peter Pallas’ attention. He thoroughly described the unusual find in one of his reports, not knowing that it was a genuine rock from space. Some decades later, in the early days of meteoritics, it became obvious that Pallas had discovered a new type of meteorite. Thereafter, all similar stony-irons were named for him, and the type specimen of the pallasite group, Krasnojarsk, became known as the “Pallas Iron”.

 

Description: Modern meteoriticists use the term “pallasite” to describe a certain structural class of stony-iron meteorites that contains abundant silicate inclusions in a nickel-iron matrix. Usually, the silicates are large olivine crystals, often of gem quality. These peridots make the pallasites some of the most attractive meteorites known, and cut and polished pallasite slices are highly coveted among meteorite collectors. Sometimes pallasites and pallasitic peridots are used in jewelry, making them the only genuine cosmic gemstones on Earth.

Based on their origin and formation history, the pallasites are regarded as samples of core/mantle boundary material from differentiated asteroids, inferring that a close relationship exists to the iron meteorites. Upon etching, larger metal portions of polished slices display typical Widmanstätten figures. In addition to this, pallasites display chemical, elemental, and isotopic trends that link them to specific chemical groups of iron meteorites, linking their origin to a common parent body. Hence, they are classified into three distinct groups or grouplets, similar to the chemical groups of the iron meteorites: the main group pallasites (MGP), the Eagle Station grouplet (ESP), and the pyroxene grouplet (PXP).

 

 

Main Group Pallasites (MGP)

 

Mineralogy: MGPs contain varying amounts of magnesium-rich olivine crystals set in a nickel-iron matrix, usually displaying an olivine-to-metal volume ratio of about 2 to 1. The olivine crystals have typical diameters of 0.5 to 2 cm, and the nickel-iron matrix displays medium Widmanstätten figures upon etching. Boundary regions between metal and olivine often contain accessory minerals such as troilite, schreibersite, and chromite.

 

Formation history & Origin: The elemental and oxygen isotopic compositions of the nickel-iron metal are similar to the values determined for group IIIAB irons, suggesting a common parent body for both groups. Both probably formed on the same parent body, a differentiated asteroid that was disrupted by a single impact event. The IIIAB iron meteorites represent fragments of the core, while the MGPs represent samples of the core/mantle boundary of this common parent body.

 

Members: Comprising about 42 members, the main group pallasites represent the most abundant class. Famous main group pallasites include Krasnojarsk, Brenham, Brahin, Imilac, and the most beautiful, Esquel. The main group also comprises the only three witnessed pallasite falls, one of which is the renowned and visually attractive Marjalahti, a meteorite that fell in the Karelian Republic, Russia, in 1902.

 

 


 

Eagle Station Pallasites (ESP)

 

Mineralogy: ESPs contain highly fragmented olivines, intermixed with small, irregular olivine splinters, in a nickel-iron matrix. The olivine is extraordinarily iron-rich, and the metal consists of higher nickel content than any other pallasites. As in the MGPs, accessories are present in the form of troilite, schreibersite, and chromite.

Formation history & Origin: The elemental and oxygen isotopic compositions of ESP nickel-iron are similar to that of IIF irons, and both groups probably share a common parent body. Another very interesting isotopic link exists between the Eagle Station trio and the carbonaceous chondrites of the CO/CV clan. This data suggests that the IIF/Eagle Station parent body may have originated in the same nebular region in which the CV chondrite parent body formed – perhaps even inside of this very asteroid.

Members: This small grouplet is named for a pallasite that was found near Eagle Station, Kentucky, in 1880, and it consists of just three members – Eagle Station, Cold Bay and Itzawisis.

 

Pyroxene Pallasites (PXP)

 

Mineralogy: PXPs contain minor clinopyroxenes, which occur as inclusions in the olivine crystals, as large grains in the nickel-iron matrix, and as grains bordering the olivines. They share similar elemental and isotopic compositions distinct from the main group and Eagle Station pallasites, indicating that the PXPs represent a third parent body on which pallasites were formed.

Formation history & Origin: Comparisons made to the groups of iron meteorites yielded no match, inferring that the pyroxene pallasites represent a previously unsampled asteroid.

Members: This is another small grouplet, consisting of just four members – the pyroxene-rich, Antarctic pallasite, Yamato 8451, Vermillion, an unusual pallasite that was found in Kansas, USA, in 1991, NWA 1911, and a more recent find from Africa named Zinder. 


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