IVB meteorites are a group of

ataxite Ataxites (from Greek meaning "without structure") are a structural class of iron meteorites with a high nickel content and show no Widmanstätten patterns upon etching. Characteristics Ataxites are composed mainly of meteoric iron, a native metal ...

iron meteorite Iron meteorites, also known as siderites, or ferrous meteorites, are a type of meteorite that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases: kamacite and taenite. Most i ...

s classified as

achondrites An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystalli ...

. The IVB group has the most extreme chemical compositions of all

s, meaning that examples of the group are depleted in volatile elements and enriched in refractory elements compared to other

Description

The IVB meteorites are composed of

meteoric iron Meteoric iron, sometimes meteoritic iron, is a native metal and early-universe protoplanetary-disk remnant found in meteorites and made from the elements iron and nickel, mainly in the form of the mineral phases kamacite and taenite. Meteoric i ...

(

kamacite Kamacite is an alloy of iron and nickel, which is found on Earth only in meteorites. According to the International Mineralogical Association (IMA) it is considered a proper nickel-rich variety of the mineral native iron. The proportion iron:ni ...

taenite Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with a chemical formula of and nickel proportions of 20% up to 65%. The name is derived from the Greek ταινία for "band, ribbon" ...

and

tetrataenite Tetrataenite is a native metal alloy composed of chemically-ordered L10-type FeNi, recognized as a mineral in 1980. The mineral is named after its tetragonal crystal structure and its relation to the iron-nickel alloy, taenite. It is one of the mi ...

). The chemical composition is low in volatile elements and high in

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

and refractory elements. Although most IVB meteorites are

s ("without structure"), they do show microscopic

Widmanstätten pattern Widmanstätten patterns, also known as Thomson structures, are figures of long nickel–iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbon ...

s. The lamellae are smaller than 20µm wide and lie in a matrix of

plessite Plessite is a meteorite texture consisting of a fine-grained mixture of the minerals kamacite and taenite found in the octahedrite iron meteorites. It occurs in gaps (its name is derived from the Greek "plythos" meaning "filling"according tMinda ...

. The

Tlacotepec meteorite The Tlacotepec meteorite is an iron meteorite classified as an IVB meteorite. It is the only meteorite of the IVB group that is an octahedrite. The meteorite is named after Tlacotepec, Guerrero (Mexico). See also * Glossary of meteoritics Thi ...

is an

octahedrite Octahedrites are the most common structural class of iron meteorites. The structures occur because the meteoric iron has a certain nickel concentration that leads to the exsolution of kamacite out of taenite while cooling. Structure Octahedr ...

, making a notable exception, as most IVBs are

Classification

Iron meteorites were originally divided into four groups designated by Roman numerals (I, II, III, IV). When more chemical data became available some groups were split. Group IV was split into

IVA Iva or IVA may refer to: Organizations * Independent Voters Association, a North Dakota U.S. political organization * Informationsvidenskabelige Akademi, a Danish university * Royal Swedish Academy of Engineering Sciences, (Swedish: ''Kungliga Ing ...

and IVB meteorites. The chemical classification is based on diagrams that plot

content against different trace elements (e.g.

gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group ( alum ...

germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbo ...

and

iridium Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of ...

). The different iron meteorite groups appear as data point clusters.

Parent body

IVB meteorites formed the core of a parent body that was later destroyed, some of the fragments falling on Earth as meteorites. Modeling the IVB parent body has to take into account the extreme chemical composition, especially the depletion of volatile elements (gallium, germanium) and the enrichment in refractory elements (iridium) compared to other

s. The history of the parent body has been reconstructed in detail. The IVB parent body will have formed from material that condensed at the highest temperatures while the solar nebula cooled off. The enrichment in refractory elements was caused by less than 10% of the condensible material going into the parent body. Thermal models suggest that the IVB parent body formed 0.3millionyears after the formation of the calcium-aluminium-rich inclusions, and at a distance from the sun of 0.9

Astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbi ...

s. Differentiation of the planet body into a core and

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

was most likely driven by the heat produced by the decay of ²⁶Al and ⁶⁰Fe. The high nickel concentrations were caused by oxidizing physical conditions. The chemical variation of IVB specimens can be explained as different stages of the

fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of the ...

of the convecting core of the parent body. The exact size of the parent body is still debated. Modelling of cooling rates suggest that it had a 140±30km radius with a 70±15km radius core. The fast cooling rates are explained by a grazing-shot collision of the parent body with a larger asteroid. This removed the mantle from the parent body, leaving the shattered iron core behind to rapidly cool.

Notable specimens

As of December 2012, 14 specimens of IVB meteorites are known. A notable specimen is the Hoba meteorite, the largest known intact meteorite. There has never been an observed fall of an IVB meteorite.

References

{{meteorites