Clast
A clast is a single fragment of rock or mineral that is embedded in a larger rock, especially a breccia.
When a rock forms from broken pieces, each piece is called a clast, from the Greek word for broken. Clasts can be lithic, meaning fragments of an older rock, or mineral clasts, meaning single broken crystals. The finer material that holds them together is often called the matrix or groundmass.
Clasts are like pages torn from different books and bound into one. Each can come from a different place or time, so studying clasts reveals which rocks existed on a parent body and how impacts mixed them. Occasionally a clast is something completely foreign, carried in by an impacting object. Scientists call such foreign pieces xenoliths or exotic clasts, and they can be the only known samples of some materials.
The Kaidun meteorite, which fell in Yemen in 1980, is famous for an extraordinary variety of clasts from many different meteorite types packed into one stone. In howardites, pale clasts of eucrite and greenish diogenite clasts sit side by side. To recognise clasts, look on a cut face for patches whose colour, grain size or texture differs from their surroundings, often with sharp, angular edges.
Related terms
Level: Explorer Ages 10 and up