If you have an interest in meteorites, Namibia has a couple of attractions
that you won't want to miss. In the north, not far from
Grootfontein is the
Hoba Meteorite which is the largest knownof metal meteorite in the world, and beautifully displayed right in the
middle of Windhoek are a selection
of the Gibeon Meteorites, which are the largest known shower of
extra terrestrial bodies ever to land on Earth.
The exact number of
the Meteorites discovered is unknown as many have been smuggled
out of the country over the years. To date a total of 120 pieces have
been recorded as having an almost identical composition
and are thought to have initially formed a body weighing in excess of 15
ton.
Gibeon Meteorite Display
Situated in the Post Street Mall consists of 33 pieces
weighing between 195kg and 555kg and numbers the larger specimens that
presently remain in Namibia. It offers you a unique opportunity to view
some of the oldest and rarest known of material to man. Radiometric
dating has aged them at 4 billion years old.
It is thought that the original body that comprised the Gibeon Meteorites would have been
about 4 x 4 x 1.5 m and would have fragmented shortly after entering the
earth's atmosphere at a low trajectory from a north-westerly direction
and that the pieces experienced burn-out causing their surface structure
to alter. On the final stage of their
journey while burning through the
earth's atmosphere the meteorites were thermally altered. The
indentations on the surface can clearly be seen.
One of the
Gibeon Meteorites has been sliced in two parts in order that the interior
of the body can be seen. The metal is resistant to corrosion owing
to the percentage of Nickel.
- Astronomical Origin:
Thought to have been from an exploding supernova over 4 billion
years ago.
- Classification - Octahedrite:
The most common form of Iron-Alloy meteorites. The composition
is:
- 87% iron, 8% nickel ( Taenite, Gamm-fe with 8,5%:
Kamacite, Alpha-fe with 5,5% nickel - see Widmanstatten Pattern below), 0,5%
cobalt, 0,04% phosphorus with small amounts of carbon, sulphur, chromium. Traces
of zinc, gallium, germanium and iridium are present.
- Meteor Trajectory:
Approximately 20 degrees from North-Westerly direction
- Collision with Earth:
Unknown, estimated between 200 - 220 million years ago
- Location Of Landing:
Elliptical area 275 x 100km (171 x 62 miles) Gibeon Area.
- Weights of Meteorites:
range from a few grams to in excess of 1 ton.
Gibeon Meteorites - The 'Discovery': The first European to make
record of the unusual 'metal
rocks' that became known as the Gibeon Meteorites was the British explorer Sir James
Edward Alexander in 1838 when travelling north of the Bethany mission
station. His enquiries of the local Nama people established that they
valued the metal pieces and when found, had been smelting them for use
as spear and arrow-heads for some generations.
He sent samples of the metallic objects to London
for the attention of the respected chemist John Herschel, where it was
established that they were of meteoritic origin.
In 1853 John Gibbs transported by
ox-wagon an 81kg sample sample of the Gibeon
Meteorites to Cape Town, from where it was shipped to London.
The meteorite was purchased by Prof. John Tennant a mineralogist
and to America.
Between 1911-1913 the Chief Geologist for the
Colonial Administration of German South West Africa prospected the
area and collected the know of remaining Gibeon Meteorites
specimens which were brought to
Windhoek, from where a number were donated to foreign museums and
prominent collectors. Meteorites are
now protected by strict laws in Namibia. It is an offence to even
move one from its resting place
Widmanstatten Pattern:
Named
after Count Alois Von Beckh Widmanstatten (1753 to 1849) an Austrian
printer and scientist who noted that when the machined surface of an iron -
nickel meteorite was etched with acid a distinct cross-hatched pattern
appeared on the flat metal face. This crystalline structure is unique to metal
|
|
bodies,
such as meteorites, that have
formed in space, and happens when a molten metal meteorite having a composition
of about a 90% percent iron and 10% nickel begins to cool. Calculations indicate
that this cooling process, which takes place under zero gravity conditions, is
extremely slow, being approximately 2°c per million years. As the temperature of
the metal alloy meteorite slowly lowers to about 700°c the meteorite would still
be in a liquid form, but within this approximate temperature range, bands of
crystals of the two associated metals kamacite and taenite would begin to be formed in a process know as
diffusion. (kamacite being formed in the low nickel phase while
taenite is formed during the high nickel phase). At this temperature
the composition of the kamacite would be about 4% nickel. Over a
period of some further 200 million years the meteor's temperature
would have cooled to about 600°c during which period the migration
of nickel atoms within the cooling molten metal mass would have
increased the nickel composition of the bands of kamacite to about
6% , Whereas the bands of taenite would have risen to have a
nickel content of approximately 19%. At approximately 500°c the
migration of the atoms ceases. The resultant crystalline structure
of the metal mass is of a cross-hatched formation now named as the Widmanstatten Pattern. This condition is not found on any
metal bodies originating on planet earth.
Acknowledgements and further reading: G1, G2, H12, P1
Of Interest: There is a further collection of 9 of the Gibeon
Meteorites on display in the Museum of the
Ministry of Mines and Energy - Department Of
Geological Survey Museum. plus
1 in the museum at Rehoboth
|
|
