IVB meteorites are a group of ataxite

iron meteorite Iron meteorites, also called 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 iron me ...

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 ...

(

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, ribbo ...

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

nickel Nickel is a chemical element; it has 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 slo ...

and refractory elements. Although most IVB meteorites are ataxites ("without structure"), they do show microscopic

Widmanstätten pattern Widmanstätten patterns (), also known as Thomson structures, are figures of long Phase (matter), phases of nickel–iron, found in the octahedrite shapes of iron meteorite crystals and some pallasites. Iron meteorites are very often formed ...

s. The lamellae are smaller than 20μm wide and lie in a matrix of plessite. The Tlacotepec meteorite is an

octahedrite Octahedrites are the most common Iron meteorite#Structural classification, 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 tae ...

, making a notable exception, as most IVBs are ataxites.

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 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; it has Chemical symbol, symbol Ga and atomic number 31. Discovered by the French chemist Paul-Émile Lecoq de Boisbaudran in 1875, elemental gallium is a soft, silvery metal at standard temperature and pressure. ...

germanium Germanium is a chemical element; it has Symbol (chemistry), symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid or a nonmetal in the carbon group that is chemically ...

and

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

). 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 AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...

s. Differentiation of the planet body into a

core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (laboratory), a highly specialized shared research resource * Core (manufacturing), used in casting and molding * Core (optical fiber ...

and mantle 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 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

Description

Classification

Parent body

Notable specimens

See also

References