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www.niger-meteorite-recon.de
Bassikounou Meteorite Fall
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Maps & Appendix
By Svend Buhl & Matthias Baermann
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Fragmentation of the meteorite
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Detail of fusion crust on a 495gm Bassikounou fragment (cat # 026). Note the bent
edge to the fragmented surface indicating a disrupture at the end of the hot flight
whith crust still in semi viscous state. This interpretation is backed
up by the fragmented surface which shows signs of minor heating
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Until present 489 individuals and fragments of
the shower producing fall have been recovered. The sizes
range from 2gm to 6120gm. Based on the few available
descriptions of the fireball which was perceived as a single object
by the eyewitnesses with azimuth towards the entry point the authors
assume that the meteorite entered the atmosphere as a single body.
In the Bassikounou area the fireball was perceived as
"three burning spheres" by 17 eye wittnesses. Two eyewitnesses
observed a terminal explosion which marked the beginning of
the dark flight. Three to four minutes after the terminal
explosion was observed meteorites impacted near and south of Oum Sdeira
area.
Although no visible signs of a fragmentation event previous to the
terminal flash were reported there is evidence, that at least
one fragmentation took place at an earlier stage of the hot
flight that divided the bolide in three parts.
In total 324 of the collected masses are individuals covered with thick
primary crust representing meteorites with a relatively long
history of individual atmospheric flight. This in mind it
is most probable that at least one of the three fireballs fragmented
before the terminal flash at the beginning of the hot flight.
It is mentionable that most of these individuals show partially pronounced
fissures indicating that these masses were exposed to a high ammount of
pressure stress either from a previous shock event on the mother
body or from cumulative stress obtained during the atmospheric passage.
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Flowlines on an oriented 1100gm Bassikounou individual. Photo courtesy of T. Jakubowski
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As 10% of the collected masses show a thin
secondary or a combination of both primary and secondary
fusion crust, it can be assumed that the stress fractures
have lead to further fragmentation of other individuals.
A few of the recovered masses display fractured surfaces
showing a minimal degree of heating. This provides further evidence
for a final break up at the last stage of the hot flight.
All findings together give evidence that single masses fragmented
successively along the path of the hot flight.
General characteristics of the Bassikounou meteorites
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Typical mass distribution pattern in a pile of 7.5kg
randomly harvested Bassikounou meteorites.
The three largest masses pictured are 2kg, 1.1kg and 0.56kg. Photo courtesy of T. Jakubowski
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Two thirds of the total mass recovered at present is
concentrated in the range below 3750gm weight. In total
22 masses with weights > 1kg have been recovered at present.
Most masses with a weight > 500gm
display rather angular shapes, with a single shield shaped mass
of 860gm being the most prominent exception. Most of the larger
masses show regmaglypts at least on one surface, only two of the
larger masses show distinct orientation features. Individual
masses with weights below 500 gm are predominantly of rounded organic shape.
The fusion texture is of a fine grained and velvet
like appearance and of dull black colour. On some meteorites,
particularly on those specimens > 500gm, the fusion
crust appears “washed” to a brighter ash grey.
A mentionable surface feature are the mm sized
white or bright grey inclusions which are abundant
on surfaces with primary crust on one third of these
masses. These are not adherent minerals of soil but
consist of different melt products in the same grain
size as the components of the other fusion crust
and are interwoven with and partly melted over by the latter.
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This 1304gm individual bears marks of its impact on sandstone rock
in the shape of a broad orange streak(cat # 011)
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About 25 percent of the recovered masses at present (June 2007) show no
signs of terrestrial oxidation while the rest displays
moderate signs of oxidation and some a slightly smoothened
fusion texture. The few rains in the Bassikounou area which
fell in December and January have provided enough humidity
to aid the oxidation of the meteorites.
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By comparing the soil types being found in the strewn field
with those adherent to the meteorites it is possible to
reconstruct relative find positions for some of the masses |
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While some of the larger masses show single surfaces at
least largely covered with contraction cracks there are
only very few masses with weights <500gm on which they
have formed.
Different from the contraction cracks are
the distinct fissures resulting from pressure stress
retained during the atmospheric passage or at a previous
shock event. These are often expanding over
several centimetres on the surface.
Adherent soil is abundant on two thirds of the
masses recovered. As mentioned earlier basically three types
of soil remnants were found: first, traces of a red to orange laterite
and sandstone soil. These occur in the shape of distinct orange to red streaks.
Second, a beige clay that is being found in the
shape of patches and broader streaks and sometimes contains plant fibres.
And finally a pale grey powder like clay that can be found as coating of complete surfaces
particularly in the contraction cracks and in the bottom other cavities.
One remarkable feature of some 10% of the
recovered masses are relatively large concretions
of NiFe protruding through the crust.
These are
surface melted but stick out up to several mm of
the fusion crust due to their higher melting point
than the surrounding material. They often show
flow lines and are of a chrome-blue colour.
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NiFe inclusion protruding through the fusion crust of a 499gm individual (cat # 024).
The melted surface of the concretion shows flow lines and is of bright chrome color
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Fresh 1044gm fragment showing delicate shock veining in a fresh ash grey matrix (cat # 050). Photo courtesy of T. Jakubowski
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The matrix of the meteorite is colored bright
grey with many anthracite colored shock veins
running through the meteorite in all three dimensions.
The angular appearance of many individuals is due
to planar fragmentation along these shock planes.
Chondrules cemented in the matrix are abundant in
sizes from 0.1 – 1.2 mm with an average size of
0.35mm. The chrome colored NiFe concretions
embedded can reach remarkable sizes and sometimes
appear in the shape of elongated needles with
lengths up to several centimeters. In some cases
they show clustering or parallel orientation.
On individuals collected in the first two weeks
after the fall there is typically no oxidation
visible in the matrix on fractured surfaces.
However some meteorites collected briefly
after the fall seem to have undergone inadequate
handling. These have developed some oxidation
while being transported or stored.
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Maps & Appendix
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