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 FIRST
ROCKS
Chondrites are the most common meteorites that fall on Earth.
They are the most ancient rocks in the Solar System. Chondrites
come from asteroids that have never melted. Even though the
Earth was probably made from chondrite material, geological
activity has erased all traces of the original rocks that made
our planet. We can see back to the beginning of the Solar System
by studying chondritic meteorites.
| [pictured
above] This grey stone might not look very interesting
at first glance. But it actually tells a fascinating story
of how our solar system formed. The surface of this stone
of the Allende chondrite is covered with fusion crust that
formed when the stone fell through the Earth's atmosphere. |
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|  STONE
SOUP
If you look closely at the cut surface of a chondrite, you can
see that it is made up of all sorts of different particles, stuck
together in a dark matrix. The different particles formed separately,
then they collected together to form larger rocks that eventually
became asteroids.
 The
most common objects in chondrites are millimeter-sized beads of
rock, known as chondrules, which were formed in the solar nebula
disk. Chondrules were heated until they melted and then cooled
again very quickly, in just a few hours. Chondrules were described
over 100 years ago as "drops of fiery rain", by H. Sorby.
Scientists are not sure what heated chondrules. They might have
been heated when shock waves passed through the solar nebula,
or they might have been heated by sunlight when chondrules were
thrown above the disk by strong magnetic fields.
Chondrites also contain "refractory inclusions" which
look white on the broken surface of a chondrite. The most common
type is called "calcium-aluminum-rich inclusions," or
CAIs. Refractory inclusions condensed (grew) from the gas in the
solar nebula, in the same way that snowflakes grow from clouds.
Refractory inclusions are the oldest pieces of rock that have
ever been dated (see "Seeds of Planets"). Many of them
have unusual ratios of isotopes of some elements. From isotope
evidence we can tell that supernova explosions were taking place
at the same time as the Solar System was forming, and in the same
part of space.
 Many
chondrites also contain millimeter-sized grains of metal that
shine brightly on the surface. The metal is an alloy (mixture)
of iron and nickel. Grains of metal were floating freely in the
solar nebula along with the rocky dust. Iron metal is magnetic.
If a rock has small pieces of magnetic metal, there is a good
chance that it is a meteorite.
The dark matrix of a chondrite is made of tiny mineral grains
that are less than a micrometer in size. (There are 1000 micrometers
in a millimeter.) Most of the minerals are not unusual: they are
olivine, pyroxene, oxide and sulfide minerals. Presolar grains
(see "Stardust") are buried within the matrix. Matrix
also contains carbon which can be in the form of graphite or organic
molecules.
| [top
image] This cut surface of the Axtell carbonaceous
chondrite shows that it is made of many small beads of rock
(chondrules). The large white object in the center is a
refractory inclusion. The dark grey material that lies between
all the chondrules is the matrix.
[second
image] A chondrule from the Allende chondrite. This
image is taken with cross-polarized light in a light microscope.
The brightly colored bars are the mineral olivine. The black
area around the chondrule is the matrix of the chondrite.
The image is 1 mm across.
[third image] This cut surface
of the El Hammami ordinary chondrite shows that it is made
of chondrules and metal. The shiny flecks are metal grains.
[fourth
image ] Chondrules in the CO group of carbonaceous
chondrites are small, mostly less than 200 micrometers across.
It is hard to see them with the naked eye. This is an image,
taken in a light microscope, of the CO chondrite, Lancé.
The white and grey objects are chondrules and the black
material is a mixture of matrix and metal. The image is
2 mm across.
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  ASTEROID
GEOLOGY: WATER
Chondrites show us that water flowed on some asteroids.
Some chondrites contain minerals that can only form when water
is present. Therefore, water must have been present on the meteorite's
parent asteroid. Ice, stone and metal particles came from the
solar nebula when the asteroid formed (accreted). The original
minerals were changed when the ice melted and water began to flow.
Minerals such as olivine and pyroxene reacted with water to form
clay, iron metal rusted, and salts crystallized in cracks and
veins.
Asteroids did not have rivers, lakes or oceans of water. When
asteroids formed, the rocks were very porous, with a lot of space
between grains. Water flowed through these spaces, like in aquifers
on the Earth. This took place in the first few million years after
the asteroid formed.
| [pictured
above left] The Murchison carbonaceous chondrite
was altered when water flowed through the asteroid it came
from, 4.5 billion years ago. In this light microscope image
of a thin section, the green material is clay that formed
when olivine and pyroxene grains reacted with water.
[pictured
above right] At very high magnifications, clay minerals
look like string beans. This is a TEM image of the green
mineral in the light microscope image above. The scale bar
is 30 nanometers long. |
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|  ASTEROID
GEOLOGY: HEAT
Chondrites show us that many asteroids were heated soon after
they formed. Heating changes the structure of a chondrite and
new minerals grow. These changes are described as "metamorphism."
 Minerals
in chondrites tell us that rare radioactive aluminum (the aluminum-26
isotope, 26Al) was present when chondrites formed. Radioactive
aluminum was part of the stardust that the Solar System was made
from. Energy from decay of radioactive aluminum probably provided
the heat that cooked asteroids. Some extra heating probably came
from collisions with other asteroids.
An asteroid that was "cooked" by radioactive elements
would have been hottest in the middle and cooler towards the surface.
The middle of a chondritic asteroid probably reached temperatures
around 950°C. The asteroid took tens of millions of years
to cool down again.
| [pictured
top] In a chondrite that has never been heated, the
chondrules stand out very clearly, like in this image of
the Semarkona ordinary chondrite. Chondrules contain brightly
colored grains of olivine and pyroxene, and matrix is black.
The image is 2 millimeters across and is taken with cross-polarized
light in a light microscope.
[pictured
above] In a chondrite that has been heated and metamorphosed,
the chondrules become less obvious. In the Kilabo (LL6)
ordinary chondrite, it is still possible to make out chondrules,
but they have become mixed in with the matrix. The image
is 2 millimeters across and is taken with cross-polarized
light in a light microscope. |
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CHONDRITE
FAMILIES
There are three main classes of chondrites: ordinary (O),
enstatite (E) and carbonaceous (C) chondrites. Each of these can
be divided into different groups. Each group probably comes from
a different type of asteroid.
We give chondrites classification labels that tell us which of
the groups they belong to, as well as whether they have been changed
by water or heat. A chondrite is assigned a number (a "petrologic
type") that gives information abut how it was altered on
its parent asteroid: type 3 means that it has not been greatly
altered, types 2 and 1 have been altered by increasing amounts
of water, and types 4, 5, and 6 have been altered by increasing
amounts of heat (metamorphism).
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|  CARBONACEOUS
CHONDRITES
Carbonaceous chondrites come from dark-colored asteroids far
out in the asteroid belt.
Carbonaceous chondrites formed from ice, rock, metal, and carbon-rich
(organic) materials. Most carbonaceous chondrites have a lot of
matrix. Many carbonaceous chondrites have been affected by water,
which must have flowed through the asteroids soon after they formed.
The largest asteroid, Ceres, is probably made up of material similar
to carbonaceous chondrites.
In a carbonaceous chondrite, carbon usually makes up about 1%
of the rock. It is found in the fine-grained matrix of the chondrite.
Carbon compounds include amorphous carbon, graphite, and organic
material. Many different organic compounds are present in carbonaceous
chondrites, including hydrocarbon chains, rings and amino acids.
| [pictured
top] Carbonaceous chondrite Murray (CM2). Many carbonaceous
chondrites have a lot of matrix, the grey material in the
photograph. Matrix contains a small amount of carbon, including
organic compounds.
[pictured
above] Carbonaceous chondrites usually contain a
lot of refractory inclusions. This is a refractory inclusion
called a calcium-aluminum-rich inclusion (CAI) that was
removed from the Allende CV3 chondrite. The inclusion contains
unusual minerals like melilite and fassaite (both light
grey) as well as spinel (darker grey). This is a back-scattered
electron SEM image and it is 2 mm across. |
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|  ORDINARY
CHONDRITES
Ordinary chondrites are the most common meteorites that fall on
the Earth. They come from the middle of the asteroid belt.
Ordinary chondrites contain finely-mixed rock and metal. H and
L ordinary chondrites contain high and low amounts of iron, and
LL chondrites contain low amounts of iron and low amounts of iron
metal. There are more chondrules in ordinary chondrites than in
carbonaceous chondrites, and less fine-grained matrix. Most ordinary
chondrites were heated on their parent asteroids and underwent
metamorphism which changed their structures.
Eros is an S-type asteroid visited by the NEAR spacecraft in
2000. S-type asteroids are the most common kind of asteroid and
may be where the ordinary chondrites come from. Meteorites are
launched from asteroids by impacts of smaller bodies onto the
asteroid. These impacts leave craters that you can see all over
the asteroid.
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[pictured top] The many
chondrules in an ordinary chondrite are all different. This
is an SEM image of the Bishunpur (LL3.1) ordinary chondrite.
White grains are iron-nickel metal and iron sulfide. Grey
grains are silicate minerals, mostly olivine and pyroxene.
The different shades of grey show different chemical compositions.
Mineral grains in each chondrule have different sizes, shapes
and compositions. This is a back-scattered electron image
taken in the SEM. The white line at the top of the image
is a 1 mm scale bar.
[pictured
above] Ordinary chondrite Olivenza (LL5). Ordinary
chondrites do not have much matrix but they have a lot of
iron-nickel metal and iron sulfide. The metal and sulfide
look shiny in the photograph. |
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|  ENSTATITE
CHONDRITES
Enstatite chondrites come from the inner asteroid belt, close
to Mars.
When enstatite chondrites formed there was not much oxygen around.
This meant that some unusual minerals formed. Strange sulfide
minerals found in enstatite chondrites are never found on Earth
where there is plenty of oxygen. Enstatite chondrites also contain
iron metal: EH chondrites have high metal contents and EL chondrites
have lower metal contents. Like ordinary chondrites, enstatite
chondrites were metamorphosed on their parent asteroids.
| [pictured
above] Enstatite chondrites like Abee (EH) contain
a lot of metal and all sorts of sulfide minerals that are
very rare on Earth. The metal and sulfides look shiny in the
photograph. |
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