Stony-Iron Meteorites

Dating back to the early days of meteoritics, the class of stony-iron meteorites represents 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 too. However, modern meteoritics assigns just two groups to this heterogeneous class, the pallasites and the mesosiderites, both detailed below.

Stony-iron meteorites are less abundant than their stony and iron cousins are. Taken together, all pallasites and mesosiderites comprise a total known weight of about 10 tons, representing approximately 1.8% of the entire mass of all meteorites known. 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 were named for 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, 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: (1) the main group pallasites, (2) the Eagle Station grouplet, and (3) the pyroxene grouplet. >> top...

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 figures upon etching. Boundary regions between metal and olivine often contain accessory 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. >> top...

Eagle Station Pallasites

This grouplet is named for a pallasite that was found near Eagle Station, Kentucky, in 1880, and 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, accessories 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. >> top...

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. >> top...

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. >> top...

Mesosiderites

The mesosiderites are named for the Greek words mesos for "middle" or "half", and sideros for "iron", meaning "half iron". In fact, they are typical stony-iron meteorites, consisting of approximately equal portions of nickel-iron metal and silicates. Excluding all probable pairings, the mesosiderite group comprises about 50 distinct members, while seven members represent witnessed falls.
Texturally, mesosiderites are a complex mixture of a nickel-iron metal portion and a heavily brecciated silicate portion, consisting of mostly pyroxene and plagioclase. Strangely, the silicates are obviously evolved igneous rocks, representing the crust of an achondritic parent body. They are quite similar to eucrites, diogenites, and other members of the HED group, even plotting on the same oxygen isotope fractionation line. However, the metal in mesosiderites is similar to group IIIAB irons, obviously representing the core of a distinct, differentiated asteroid, genetically unrelated to the precursor of the eucritic and diogenitic portion. This suggests a complex formation history for the mesosiderites and their parent body. One theory has them formed by the collision of two differentiated asteroids, allowing the still liquid core of one asteroid to mix with the solidified crust of the other. This scenario includes the collisional disruption and gravitational reassembly of at least one of the asteroids - the one that later became the parent body of the mesosiderites. It is still heavily debated whether the HED parent body, 4 Vesta, actually represents one of these asteroids.
Based on textural and mineralogical differences, the mesosiderites have been divided into four distinct groups that were further divided into subgroups. These groups are designated 1A, 1B, 2A, 2B, 2C, 3A, 3B, 4A, and 4B. However, there seems to be no scientific consensus about this classification scheme, as it has been differently interpreted by different researchers. To avoid any confusion, we won't elaborate on this matter. Famous members of the mesosiderite group are the witnessed falls of Estherville, Iowa, USA, in 1879, and Lowicz, Poland, in 1935. Another renowned member is Vaca Muerta, a find from Chile. Several hundred individuals of this well-preserved mesosiderite have been recovered from its strewn field in the Atacama Desert, making it the most common mesosiderite in private and public collections. >> top...