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 STONES
FROM THE SKY
At about 4 p.m. on February 18, 1948, hundreds of people witnessed
a brilliant fireball in the clear afternoon skies above Colorado,
Kansas, and Nebraska. Several people heard loud explosions,
followed by a roaring sound like the noise of a jet engine.
In the smoke train behind the fireball, puffs of smoke appeared
where the large meteorite that was streaking across the sky
broke apart in smaller pieces. In the following days and months,
hundreds of stones were recovered from a large area on the Kansas
/ Nebraska border, in Furnas County, Nebraska and Norton County,
Kansas. The meteorite became known as the Norton County meteorite.
[above left] Dr. Lincoln LaPaz,
founder of the Institute of Meteoritics, oversaw excavation
of the big Norton County stone from a corn field in 1948, and
its removal to Albuquerque.
[left] The largest piece of the
Norton County meteorite, in the Meteorite Museum at the University
of New Mexico.
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|  LARGEST
ACHONDRITE IN THE WORLD
The largest piece of the Norton County meteorite was not discovered
straight away. On July 3, 1948, two ranchers found a nearly circular
hole, about 2 m in diameter and 2 m deep, in a wheat field. When
they dug down, they struck something hard a further 30 cm below
the bottom of the hole. More digging uncovered a huge stone, about
a meter across, buried with its "nose" pointing downwards.
The bottom of the stone was 3 m below ground level. The recovery
team, led by Dr. Lincoln LaPaz, who was the Director of the Institute
of Meteoritics at the time, covered the stone in a thick coat
of plaster-of-Paris, lifted it out of the hole with a crane, and
transported it to Albuquerque.
The main mass of the Norton County meteorite has been housed
in UNM's Meteorite Museum since the Museum opened in 1974. This
stone is the largest single piece of an achondrite meteorite in
the world. It weighs approximately 1000 kg. Although many iron
meteorites are larger, it is rare for a large piece of a stony
meteorite to survive its journey through the Earth's atmosphere.
Norton County is a very crumbly rock, so it is amazing that such
a large piece survived. Only one chondrite is larger than Norton
County – the largest piece of the Jilin meteorite that fell
in China in 1976 weighed 1770 kg.
| [pictured
above] A Norton County stone, half buried in the ground
where it fell. |
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  RARE
MINERALS
Norton County is a very rare type of meteorite known as an enstatite
achondrite, or aubrite. Only one in a hundred observed falls are
of this kind. This makes the Norton County meteorite a true scientific
treasure.
Enstatite achondrites are mostly made up of the mineral enstatite,
which is a type of pyroxene that contains only magnesium, silicon
and oxygen. Most varieties of pyroxene contain iron and calcium
as well as magnesium. On the asteroid that Norton County came
from, all the iron turned into metal and separated into the asteroid's
metallic core. We say that the asteroid was very "reduced",
meaning that there was not very much oxygen available. Scientists
do not understand why some asteroids are so reduced.
Because enstatite achondrites are reduced, they contain a lot
of exotic minerals that are not found on Earth. Many of these
rare minerals are sulfide minerals. An example is oldhamite, a
calcium sulfide that is very rare on the Earth. Another rare sulfide
mineral is niningerite, named after Harvey Nininger, one of the
founders of modern meteorite science.
| [above
left] Enstatite achondrites, or aubrites, like Norton
County are mostly made of a single mineral, enstatite, which
is a magnesium pyroxene. Many of the enstatite grains have
inclusions of a calcium pyroxene, the colored mineral in
this image. This is a light microscope image taken in cross-polarized
light, and it is 2 mm across.
[above right]
Aubrites contain small amounts of other minerals, in addition
to enstatite. In particular they contain a lot of rare sulfide
minerals like oldhamite, a calcium sulfide, the red mineral
in this photograph. This stone is 3 cm across. |
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