Stony-Iron Meteorites
Dating back to the early days of meteoritics, the class of stony-iron meteorites represented a somewhat anachronistic category. It comprises several chemically and genetically unrelated classes of meteorites that have just one thing in common - they are composed of approximately equal parts of nickel-iron metal and different types of stony components. Several groups of chondrites and achondrites would fit neatly into this definition, e.g. the "bencubbinites" or the "lodranites", and several silicated irons could be regarded as true stony-irons also. However, modern meteoritics assigns just two groups to this heterogeneous class, the Pallasites and the Mesosiderites, both are detailed below.
Stony-iron meteorites are less abundant than their stony and iron cousins. Taken together, all pallasites and mesosiderites comprise a total known weight of only 10 tons, representing approximately 1.8% of the entire mass of all meteorites known worldwide. This low abundance is also reflected by their fall-ratio; when compared to the other major types of meteorites, stony-irons are exceptionally rare, representing just 1.5% of all witnessed falls.
Pallasites
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 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 are now named after him, and the type specimen of the pallasite group, Krasnojarsk, became known as the "Pallas Iron".
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 to Universities, Collectors, and research centers. Cut and polished pallasite slices are highly coveted among meteorite collectors and bring a tremendous demand.
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 patterns. 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: (1) the main group pallasites, (2) the Eagle Station grouplet, and (3) the pyroxene grouplet.
Main Group Pallasites
Comprising about 40 members, the main group pallasites represent the most abundant class. They 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 patterns upon etching. Boundary regions between metal and olivine often contain other rare minerals such as troilite, schreibersite, and chromite. 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. 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
This grouplet is named for a pallasite that was found near Eagle Station, Kentucky, in 1880. It consists of just three members - Eagle Station, Cold Bay and Itzawisis. They all 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 main group members, other minerals are present in the form of troilite, schreibersite, and chromite. The elemental and oxygen isotopic compositions of Eagle Station 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.
Pyroxene Pallasites
This is another small grouplet, consisting of just two members - the pyroxene-rich, Antarctic pallasite, Yamato 8451, and Vermillion, an unusual pallasite that was found in Kansas, USA, in 1991. Both pallasites 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 Yamato 8451 and Vermillion represent a third parent body on which pallasites were formed. Comparisons made to the groups of iron meteorites yielded no match, inferring that the pyroxene pallasites represent a previously unsampled asteroid
Ungrouped Pallasites
A number of pallasites are so unique that they can't be accommodated in any of the established groups or grouplets. A renowned example is the beautiful Springwater pallasite, found in Saskatchewan, Canada, in 1931. It shows abundant, small, rounded olivine crystals in an ungrouped nickel-iron matrix, suggesting a formation on a distinct, previously unsampled parent body. Another famous ungrouped member is the gorgeous Glorieta Mountain pallasite. Many individuals of this olivine-poor pallasite have been found since 1884 near Canonçito, New Mexico, USA. Glorieta Mountain displays elemental and isotopic compositions similar to those found in the group IIICD irons, suggesting a possible common parent body for Glorieta Mountain and the IIICD members
Additional Information is available on the Pallasite group of Meteorites through universities, and research centers worldwide as well as through the internet.
