This KT boundary clay layer has large amounts of iridium .
Iridium is a very rare chemical.
On Earth, most of the iridium is near the earth's center, not near the surface. Some meteors have lots of iridium in them. A meteor could have brought the iridium when it hit the earth. The iridium from the meteor would have mixed with the other dust from the impact. A meteor 10 kilometers in diameter would contain enough iridium for the clay layer.