Ataxite (Santiago Papasquiero Meteorite)

Ataxites (from Greek meaning "without structure") are a structural class 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 met ...

s with a high

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 to ...

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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 ribbons ...

s upon etching.

Characteristics

Ataxites are composed mainly 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 iro ...

, a native metal found in meteorites that consists of the mineral

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" ...

with minor amounts 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 tMindat ...

troilite Troilite is a rare iron sulfide mineral with the simple formula of FeS. It is the iron-rich endmember of the pyrrhotite group. Pyrrhotite has the formula Fe(1-x)S (x = 0 to 0.2) which is iron deficient. As troilite lacks the iron deficiency whic ...

, and microscopic lamellae 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:ni ...

. Ataxites are the most nickel-rich meteorites known; they usually contain over 18% nickel. The high nickel content is the reason that they do not develop a Widmanstätten structure, because in this case

can be exsolved from

only at such a low temperature (below about 600°C) where diffusion is already too slow.F. Heide, F. Wlotzka, Meteorites, Messengers from Space, Springer-Verlag 1995, page 143

Classification

Most ataxites belong to the IVB group or are classified as "Iron, ungrouped" because they do not fit in any of the groups currently recognized by

meteorite classification In meteoritics, a meteorite classification system attempts to group similar meteorite, meteorites and allows scientists to communicate with a standardized terminology when discussing them. Meteorites are classified according to a variety of chara ...

. Some ataxites belong to the IAB group and fall into the sHL (high-Au, low-Ni subgroup), sLH (low-Au, high-Ni subgroup), sHH (high-Au, high-Ni subgroup) and "ung" subgroup (does not fit any subgroup of IAB). Only a couple of ataxites have been classified into the IAB complex and the IIF, IVA, IIAB, IIIAB groups.

Abundance

They are a rare class, with none of the about 50 observed iron meteorite falls being an ataxite, however, the largest meteorite ever, the

Hoba meteorite The Hoba ( ) meteorite, short for Hoba West, is a meteorite that lies on the farm of the same name, not far from Grootfontein, in the Otjozondjupa Region of Namibia. It has been uncovered, but because of its large mass, has never been moved from w ...

(found 1920 in Namibia, weight 60 tons), belongs to this class. A Tibetan Buddhist statue, the ''Iron Man'', was likely carved from an ataxite meteorite. It might even be made from a fragment of the

Chinga meteorite The Chinga meteorite is an iron meteorite. It is structurally an ataxite with very rare kamacite lamella. The meteoric iron is a part of the lamella taenite. The total chemical composition is 82.8% iron, 16.6% nickel, and the rest mostly cobalt an ...

. Other examples of ataxites are the

Dronino meteorite The Dronino meteorite is a iron meteorite that was found in the Ryazan Oblast of Russia in July 2000. It is classified as an ataxite. Mineralogy The Dronino meteorite is a classified as an ataxite (iron meteorite). Most of the meteoric iron is ...

and pieces of the Gebel Kamil.

References

{{Meteorites

Characteristics

Classification

Abundance

See also

References