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.
 

 

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.

 

 

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.

 

 

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.

 

 

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).

 

 

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.

 

 

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.

[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.

 

 

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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