New England Meteoritical
Services"Albion, A New
Iron Meteorite" as published in the November 1995 issue of the
quarterly publication METEORITE!
Albion, A New Iron Meteorite
by Russell Kempton, New England Meteoritical Services
"My tomato field burned last night. The ground is scorched."
relates a farmer calling from North Dakota. "it must have been a meteor" he
says. Patiently, I explain a basic truth: "meteors" do not land in a ball of
flame, and suggest that he look for other explanations. Another caller tells of
finding many "perfectly round little meteorites" in a river stream. I ask if
they attract a magnet. "Well, no," he says, "but they look like meteorites!"
Again, I explain a basic truth: meteorites are not found as perfectly shaped
spheres and all will attract a magnet to some degree. A caller from South
Carolina tells of finding a 220 gram "iron meteorite." He describes "holes" and
little pockets of "bubbles" in the interior. "No," I explain, "it's probably a
piece of slag." A basic truth of iron meteorites is that they do not have
"holes" or vacuoles throughout their interior. Well, we can't say that any
longer. Unbelievably, an iron meteorite with vacuoles has been found!
Whitman County, Washington, USA
During the winter of 1966 - 1967, a single 12.28 kg iron
mass was found in a wheat field adjacent to the Palouse River in Albion,
Washington. The mass was in the possession of its finder, Kenneth Oliphant,
until sometime in 1991 when it was sectioned to determine if it was a
meteorite. Based upon the initial classification by Dr. John Wasson of UCLA as
a Fine Octahedrite (IVA), it was recently submitted to the Meteoritical Society
with the proposed name of Albion, Washington.
The interior of Albion is remarkable! Occurring
homogeneously throughout Albion are irregular shaped vacuoles -- holes ranging
in diameter from 4mm to 9mm. In one 620 gram slice (174 mm x 134 mm x 6 mm), 9
vacuoles were observed. The interior of these voids is lined with what appears
to be solid spherical blebs (bubbles) covered with well developed, highly
intergrown cubic crystals of almost pure iron. Of the more than 800 known iron
meteorites, none have exhibited any form of vugs or "holes" within their Ni/Fe
structure. To find them in an iron meteorite appears to "rock the boat" on the
way we think of asteroid cores -- the parent bodies of iron meteorites.
It is generally accepted that the solar nebula was formed by
the gravitational collapse of an extended mass of interstellar gas. Accretion
led to the creation of bodies of differing composition and to their location at
varying distances from the early Sun. The decay of radioactive elements within
these bodies was the dominant source of heat that led to differentiation -- the
separation into layers. As temperatures rose to around 1500° C,
large quantities of material melted. Most of the dense elements, i.e.
nickel and iron, gravitated toward the center of the core where pressures in
the smaller asteroidal bodies increased to levels around 1 to 2 kilobars -- one
to two thousand atmospheres. The unmelted outer areas formed a mantle of rock
and surface crust that was to act over time as an insulative blanket, thus
allowing for the extraordinary slow cooling rates of the Ni/Fe core.
There is substantial evidence that iron meteorites are the
shattered remains of differentiated asteroidal cores 10 to 800 km in diameter
violently disrupted through impact. Most appear to have gone through a liquid
state about 4.5 billion years ago, slowly cooling at the rate of 0.4° C to
500° C per million years, to the region in which the Widmanstatten Pattern
forms through diffusion at 700° to 450° C. Dr. John Wood of the
Smithsonian Astrophysical Observatory estimated that an asteroidal body larger
than 850 km would not cool to below 500° C at the center within
the age of our solar system. Consequently, any proposed parent body for iron
meteorites seems to be restricted to asteroidal-sized bodies rather than
planetary size. The high temperature and intense pressures of one to two
thousand atmospheres within the core during their molten core phase should
preclude the existence of any type of voids or vacuoles that we see in Albion.
Therefore, it seems very unlikely that a void could form in molten nickel-iron
under such high pressures. Perhaps we should look further for an explanation of
Our solar system is bombarded from the outside by cosmic
rays from galactic sources (109 electron volts per nucleon) and from within by
solar radiation (107 electron volts). The iron cores of asteroids are shielded
from these high energy cosmic rays prior to breakup. However when the parent
asteroid is disintegrated by one or more violent collisions, the broken core
fragments become exposed to this intense radiation. The period of time that a
meteoroid is exposed to these particles is referred to as their cosmic ray
The cosmic ray exposure ages of iron meteorites indicate
that most fragmented as solidified, cool bodies about 500 million years ago --
approximately 4 billion years after their initial molten state. Plenty of time
to cool below 450° C. In Albion, microscopic examination of the
Widmanstatten Pattern reveals no distortion of the kamacite and taenite
lamellae indicating normal cooling throughout this period. However, the
granular structure of the solid blebs seems to indicate rapid cooling after an
intense melting event. This implies that the vacuoles in Albion are secondary
Unlike the primary structures in iron meteorites i.e.:
kamacite, taenite, etc. that formed through a process of solid state diffusion,
secondary structures are, geologically speaking, metamorphic. They are the
product of shock and heating events. Buchwald in the HANDBOOK OF IRON
METEORITES defines four main categories of secondary structures:
shock-induced plastic deformation, shock-induced solid state transformation,
shock melting, and thermal annealing. Of the four, shock melting seems the most
likely candidate for the structures in Albion.
Diamond, troilite, kamacite, cohenite, and schreibersite are
all shock indicators in iron meteorites. Of these, the mineral troilite (FeS)
is a particularly sensitive mineral to shock and will undoubtedly play a major
role in the research of Albion. For now, while research is ongoing, let's
speculate on the origins of these vacuoles using the following shock-melting
After being repeatedly hit by other celestial bodies, a
fractured 100 km diameter asteroid breaks apart; its iron core exposed.
Ultimately, a particularly violent "hypervelocity" impact shatters the core.
The impacting projectile is completely vaporized while thousands of fragments
from the impacted asteroid survive, some of them hundreds of meters in
diameter. Created is a range of meteoroids, shock-altered to varying degrees
according to their distance from the impact point.
One of these meteoroids, with silicate (olivine) and
graphite inclusions throughout its nickel-iron structure, reacted to the
temperature rise of the hypervelocity impact by vaporizing its inclusions. As
the internal pressure of the core decreased from breakup, the released oxygen
from the vaporized inclusions resulted in the smelting of iron from the
silicates (olivine) and, through a vapor phase, deposited the iron back upon
the spherical blebs that we now see several hundred million years later.
Smelting is the melting process that separates pure metal from extraneous
substances. Ferrous silicates i.e. olivine-rich inclusions, are vulnerable to
smelting in the presence of graphite. This smelting is suppressed at high
pressures but promoted as pressure falls. The entire process probably occurred
within minutes, leaving the voids that we now see in Albion.
Again, this is only speculation, but think how fortunate we
are to find mysteries that keep us searching for new answers. "Holes" in the
internal structure of iron meteorites have never before been seen. They were
not thought to be possible. But then, my next phone call may be a verified
report of a scorched vegetable garden littered with Ureilites.
Acknowledgments - The author expresses his sincere thanks to
Dr. Carl Francis, Curator, Mineralogical Museum, Harvard University and Dr.
Timothy Grove, Department of Earth, Atmospheric, and Planetary Sciences,
Massachusetts Institute of Technology, for their valuable discussions.
Russell W. Kempton is the Director of New England
Meteoritical Services based in Mendon, Massachusetts, USA.
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