Why are carbonaceous chondrites so fragile?
They are porous and full of soft clay minerals and organic matter, so they crumble easily, break up in the atmosphere and weather quickly on Earth.
Carbonaceous chondrites are among the most scientifically valuable meteorites, but they are also the most fragile.
Soft minerals. Many, especially CI and CM chondrites, are made largely of clay minerals formed by water inside their parent asteroids. Clay minerals are soft and weak compared with the olivine and pyroxene of ordinary chondrites.
High porosity. Carbonaceous chondrites can have porosities of 20 to 35 per cent, with many tiny holes and cracks. Samples from Ryugu and Bennu are even more porous. This makes them light and easily broken.
Volatile content. They contain water, organic matter and salts. When exposed to Earth's air, they can absorb moisture, causing swelling and cracking. Some, such as Orgueil, grow crusts of salts such as sulfates over decades in collections.
Atmospheric survival. Their weakness means many carbonaceous meteoroids break into dust high in the atmosphere. Fireball observations suggest that a larger fraction of incoming material is carbonaceous than the proportion recovered among falls. The Tagish Lake meteorite of 2000, for example, was found in pieces on a frozen lake and had to be kept frozen to preserve its fragile material.
Weathering. On the ground, they disintegrate faster than other types, so fewer survive to be found, especially in wet climates.
Because of this, curators handle carbonaceous chondrites carefully, storing them in dry nitrogen and minimising handling. Their fragility is one reason why sample-return missions, which deliver pristine material, are so important.