IIAB meteorites are a group of
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 iron ...
s. Their structural classification ranges from
hexahedrite
Hexahedrites are a structural class of iron meteorite. They are composed almost exclusively of the nickel–iron alloy kamacite and are lower in nickel content than the octahedrites. The nickel concentration in hexahedrites is always below 5.8% a ...
s to
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 ...
s.
IIABs have the lowest concentration of
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 ...
of all iron meteorite groups.
Most
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 iron ...
s are derived from the metallic
planetary cores of their respective
parent bodies, but in the case of the IIABs the metallic magma separated to form not only this meteorite group but also the
IIG group.
Naming
Iron meteorite groups are designated with a
Roman numeral
Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, ea ...
and one or two letters. Classification is based on diagrams in which the
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 ...
content 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 ir ...
is plotted against certain trace elements (e.g.
gallium,
germanium 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 ...
). Clusters in these diagrams are assigned a row (Roman numeral) and a letter in alphabetical order. The first two cluster of the second row, IIA and IIB, were merged when additional measurements connected the two clusters into one, the IIAB group.
Description
All
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 iron ...
s are made of a
native metal called
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 ir ...
. The concentration of
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 ...
has an influence on the mineralogy of the meteoric iron. During cooling
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: ...
is
exsolved from
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" ...
. The lower the concentration of
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 ...
, the more
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: ...
is formed.
IIABs have some of the lowest nickel concentrations of all iron meteorites. They are in the range of 5.3 to 6.6%. For this reason they mostly consist of
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: ...
with minor amounts of
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" ...
.
The two groups that were merged into the IIAB group had different nickel concentrations and therefore different structural classifications. The IIA group has lower nickel concentrations and forms
hexahedrite
Hexahedrites are a structural class of iron meteorite. They are composed almost exclusively of the nickel–iron alloy kamacite and are lower in nickel content than the octahedrites. The nickel concentration in hexahedrites is always below 5.8% a ...
s, the IIB has higher nickel concentrations and forms
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 ...
s.
Parent body
The IIAB meteorites formed the metallic core of their
parent body
In meteoritics, a parent body is the celestial body from which originates a meteorite or a class of meteorites.
Identification
The easiest way to correlate a meteorite with a parent body is when the parent body still exists. This is the case fo ...
before it was destroyed, and some of the fragments reached earth as
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 iron ...
s.
The
planetary core of the IIABs was rich in
sulfur and
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
. This special chemical composition caused the
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
to split into two separate liquids while cooling. The concentration of
sulfur is estimated to have been about 5%. For this reason the metallic
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
reached the
liquidus curve (a point where solids coexist with a liquid) of the iron + liquid field. This led to the crystallization of the IIAB meteorites. The remaining liquid was trapped in cavities of the IIABs and crystallized once the temperature reached the
eutectic point. At this temperature the remaining
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
crystallized
schreibersite
Schreibersite is generally a rare iron nickel phosphide mineral, , though common in iron-nickel meteorites. It has been found on Disko Island in Greenland and Illinois.
Another name used for the mineral is rhabdite. It forms tetragonal crystals ...
and
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
, thereby forming the
IIG meteorites.
Notable specimens
There are currently 117 meteorites classified as IIAB and 1 as IIAB-anomalous. Of these only three were observed falls.
Seven IIAB meteorites weigh more than 1000 kg.
The
Sikhote-Alin meteorite
An iron meteorite fell on the Sikhote-Alin Mountains, in southeastern Russia, in 1947. Large iron meteorite falls have been witnessed and fragments recovered but never before, in recorded history, a fall of this magnitude. An estimated 23 tonnes ...
is the heaviest of these and was an observed fall,
while the
Old Woman meteorite is, at and originally, the largest meteorite found in
California
California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
and the second largest found in the United States.
References
{{meteorites