OCTAHEDRITE
Iron meteorites are naturally dominated by iron (Fe)
metal. They also all include some metallic nickel (Ni) as well.
They represent metallic core samples from large, once-intact, differentiated
asteroids/dwarf planets.
Octahedrite is the most common type of Fe-Ni meteorite that falls to Earth.
Other Fe-Ni meteorite rock types include hexahedrite and ataxite. The
rock names octahedrite-hexahedrite-ataxite reflect an iron meteorite
classification based on the physical crystalline structure of the iron-nickel
minerals present. Meteoriticists have since augmented this structural
classification with information on trace element content.
Mineralogy: all octahedrites are dominated by two minerals having very similar
chemistries: kamacite (FeNi) and taenite (FeNi) Kamacite is
a silvery-colored iron-nickel metal alloy rich in iron, with about 5.5
weight-percent nickel. Taenite is a silvery-colored iron-nickel
metal alloy rich in nickel, with about 27-65 weight-percent nickel.
Octahedrites have much more kamacite than taenite. They also contain
minor amounts of other minerals (e.g., troilite (FeS), silicates, etc.).
Physical structure: the physical crystalline structure of octahedrites
is quite distinctive. It is best seen on cut, polished, and nitric
acid-etched surfaces (see below). Upon cooling from magma, the kamacite
& taenite crystallize as interlocking plates with octahedral (double
pyramid) geometries. Cut & etched surfaces show a distinctive
criss-crossing pattern of silvery-gray blades. This is called WidmanstŠtten
structure.
Octahedrite (1.8 x 1.8 cm) - a cut, polished, and nitric acid-ethed surface
showing WidmanstŠtten structure. Sample of the Gibeon Meteorite
from Great Namaqualand in southern Namibia. This is a group IVA iron with
~90% iron (Fe), ~8% nickel (Ni), and ~2% other minerals. Each thick blade
is kamacite. The thin whitish lines (best seen in the bottom half of the
photo) that bound the kamacite plates are taenite. (More info. on the
Gibeon Meteorite)
Octahedrite - a cut, polished, and nitric acid-etched surface showing
WidmanstŠtten structure. Sample of the Cape York Meteorite from
Greenland. The large, black, irregularly rounded structures are masses of
troilite - iron monosulfide (FeS). Each troilite nodule is
surrounded by a silvery rim of schreibersite - iron nickel cobalt
phosphide ((Fe,Ni,Co)3P). The criss-crossing blades of metal
are mostly kamacite, with some taenite. Collected 1982. Odessa
Meteor Crater Museum public display (Odessa, Texas, USA). (More info. on the
Cape York Meteorite)
Octahedrite - a cut, polished, and nitric acid-etched surface of criss-crossing
blades of kamacite with some taenite. Sample of the Carlton Meteorite
from northern Hamilton County, central Texas, USA (FMNH Me 879, Field Museum of
Natural History, Chicago, Illinois, USA).
(More info. on the
Carlton Meteorite)
Octahedrite - acid-etched surface of the Casas Grandes Meteorite from
Chihuahua State, northern Mexico (FMNH Me 881, Field Museum of Natural History,
Chicago, Illinois, USA).
(More info. on the
Casas Grandes Meteorite)
Campo del Cielo Meteorites (above & below; above left: 1.8 cm tall; above
right: 2.3 cm across; below left: 2.7 cm across; below right:
3.2 cm across) - the four rocks shown here are octahedrites from the Campo
del Cielo Meteorite (nicknamed "Campos"). They come from
the Gran Chaco Gualamba, south-central Chaco Province, northern
Argentina. The original Campo del Cielo fell during the mid-Holocene
(4000-6000 years ago). Published isotopic dating of Campos indicates that
they are 4.54 to 4.56 billion years old, representing the time of the solar
system's initial formation. (More info. on the
Campo del Cielo Meteorite)
Sikhote-Alin Meteorites (above & below; above: 6.5 cm across; below:
3.8 cm across) - the two octahedrites shown here are nice pieces of shrapnel
from the the Sikhote-Alin Meteorite. Sikhote-Alin is a group IIB
iron that impacted at 10:38 AM on 12 February 1947 near Novopokrovka, in the
Sikhote-Alin Mountains of Maritime Province, far-eastern Russia. No
photographs were taken, but an eyewitness to the Sikhote-Aline fireball made a painting
of what he'd seen. (More info. on the
Sikhote-Alin Meteorite)
Nantan Meteorite (2.0 cm across) - this octahedrite is a sample of the
Nantan Meteorite from northern Guangxi Province, southern China.
It's a group IIICD iron with ~92% Fe, ~7% Ni, and trace amounts of other
minerals. The Nantan fireball was witnessed and documented by the Chinese
in May or June of 1516 A.D., but samples were first recovered & recognized
much later in the mid-20th century. (More info. on the
Nantan Meteorite)
Odessa Meteorite (3.1 cm across) - weathered, unprepared
octahedrite. The Odessa Meteorite is a group IAB iron octahedrite that
impacted the western Texas plains ~64,000 years ago during the Late
Pleistocene. The impact site is about 9-10 miles southwest of the town of
Odessa, in Ector County, Texas, USA. As seen from the ground, the main
crater is subtle. Fragments of Odessa Meteorite have been collected for
decades. (More
info. on the Odessa Meteorite)
Barringer Crater (aka "Meteor Crater", northeast-central
Arizona, USA) as it looked in the 1940s. Alexis McKinney photo on public
display at the Colorado School of Mines Geology Museum (Golden, Colorado, USA).
The crater shown above in the Arizona desert was formed by the impact of an octahedrite ~49,000 years ago during the Late Pleistocene. The rocks shown below are fragments of the impactor, the Canyon Diablo Meteorite. Such fragments have been collected for decades from the desert surrounding the crater.
Canyon Diablo is composed of ~90% kamacite, ~1-4% taenite, and up to 8.5% troilite-graphite nodules (FeS & C). The original mass has been estimated to be 100 feet across & about 60,000 tons. Canyon Diablo rocks are well dated to 4.55 billion years.
Canyon Diablo Meteorite - individual on public display at the Field Museum of
Natural History, Chicago, Illinois, USA.
Canyon Diablo Meteorite - 67 pound individual collected by Harvey Nininger
(CSM # 12270, Colorado School of Mines Geology Museum, Golden, Colorado, USA).
Canyon Diablo Meteorite - large cut-polished-nitric acid etched slice (above)
that fantastically displays the WidmanstŠtten structure. Field Museum of
Natural History public display (Chicago, Illinois, USA).
Canyon Diablo Meteorite - another cut & polished & acid-etched slice
with its component minerals & inclusions labeled. Kamacite
& taenite are metallic Fe-Ni alloy minerals; troilite is iron
monosulfide (FeS); graphite is elemental carbon (C); plessite is
not a mineral, but rather a mix of finely crystalline kamacite &
taenite. Field Museum of Natural History public display (Chicago,
Illinois, USA).
Canyon Diablo Meteorite - acid-etched surface showing WidmanstŠtten
structure.
(More info. on the
Canyon Diablo Meteorite)
Octahedrite - Weekeroo Station Meteorite, a type IIE iron found at Mannahill,
South Australia in 1924 (FMNH Me 2118, Field Museum of Natural History,
Chicago, Illinois, USA).
(More info. on the
Weekeroo Station Meteorite)
Murnpeowie Meteorite - this spectacular, 2520 pound iron meteorite was
found in the South Australian Outback in 1909. The mass has a
well-preserved, dark-colored, outer surface with nice regmaglypts (surface
cavities). Iron meteorites exposed at Earth's surface oxidize & rust
relatively quickly. Given the fresh nature of Murnpeowie, it's been
estimated that it fell to Earth within five years of it being found.
This rock is an octahedrite, dominated by kamacite
& taenite. Sample material cut away for analysis (see dark
orangish-colored area at upper left of meteorite) has shown that the
WidmanstŠtten structure commonly seen in octahedrites is not well preserved in
Murnpeowie. This has been inferred to be evidence for a significant
heating event at some time in the rock's history.
(SAM Met 30A, South Australian Museum, Adelaide,
Australia)
Location:
South Autralian Outback between Lake Callabonna and Lake Blanche, NNE of Mt.
Hopeless, near the Strzelecki Track, eastern South Australia.
(More info. on the
Murnpeowie Meteorite)
Ainsworth Meteorite - a group IIB iron meteorite that was found in 1907
in Brown County, northern Nebraska, USA. It is texturally classified as a
coarsest octahedrite (aka granular hexahedrite; aka kamacite
octahedrite), meaning it has the largest size of metal crystals known in iron
meteorites, and lacks thin, criss-crossing blades. (Me 1059, FMNH public
display, Field Museum of Natural History, Chicago, Illinois, USA)
(More info. on the
Ainsworth Meteorite)
Willamette Meteorite (above & below) - a recrystallized octahedrite, a
type IIIAB iron meteorite, found at Willamette, Oregon, USA in 1902 (FMNH Me
592, Field Museum of Natural History, Chicago, Illinois, USA). A
significant heating event caused recrystallization of this rock - so WidmanstŠtten
structure is not easily seen in samples.
(More info. on the
Willamette Meteorite)
