ATAXITE
Ataxites
are rare iron meteorites that lack the Widmansttten Structure seen in
octahedrites and the Neumann Lines seen in hexahedrites. Ataxites are
said to be internally structureless. Cut & polished & nitric
acid-etched surfaces of ataxites show a finely crystalline texture that would
be described as aphanitic if it were a terrestrial igneous rock.
Compositionally, ataxites can be relatively rich in
the mineral taenite (nickel-rich Fe-Ni alloy, with 27-65 wt.% nickel), or
they can be finely crystalline mixes of taenite and kamacite (iron-rich Fe-Ni
alloy, with ~5.5 wt.% nickel).
Ataxite
(field of view ~2.5 cm across) - a cut, polished, nitric acid-etched surface of
the Santiago Papasquiero Meteorite, found in 1958 in Durango, Mexico. It
consists of a finely crystalline mix of kamacite & taenite, plus other
minor minerals. Santiago Papasquiero is a strange ataxite that appears to
be a completely metamorphosed and recrystallized octahedrite. Most
recrystallized octahedrites still retain vague hints of the original
Widmansttten structure. This meteorite doesn't have any, so it isn't an
octahedrite - it's an ataxite.
Published chemical info. indicates that Santiago
Papasquiero has 7.5% nickel content overall. The kamacite component has
6.8% Ni. The taenite component has 30% Ni.
(More info. on the
Santiago Papasquiero Meteorite)
Ataxite
- weathered individual of the taenite-rich Chinga Ataxite from the Tannu Ola
Mountains, Siberia, Russia. The nickel content of this meteorite is
16.7%. An associated impact crater is not yet known for this
meteorite. Chinga Meteorites have been exposed to Earth surface conditions
for >2000 years.
(public display at meteorite museum at Odessa Impact
Crater, Texas, USA)
(More info. on the
Chinga Ataxite)
Some info. from:
Buchwald (1975) - Handbook of Iron Meteorites, Volume
3: 1080-1083.
Grokhovsky et al. (2000) - Mssbauer spectroscopy of
the Chinga Meteorite. 63rd Annual Meteoritical Society Meeting
Abstracts.