Cloudy zone
The cloudy zone is a fine intergrowth of nickel-rich particles at the edges of taenite that records cooling rate and ancient magnetism.
The cloudy zone is a band, typically a few micrometres wide, near the edges of taenite grains in slowly cooled meteoritic metal. Seen at high magnification, it looks like a cloud of tiny islands of nickel-rich tetrataenite in a nickel-poorer matrix. It formed at low temperatures, around 300 to 400 °C, as the metal separated at the nanometre scale.
The size of the islands depends on how fast the metal cooled: slower cooling gives larger islands. Measuring them with a scanning electron microscope gives cooling rates that support those from nickel profiles.
The cloudy zone has another remarkable use. The tetrataenite islands can lock in the direction of a magnetic field present when they formed. Studies of pallasites and some irons have used this to argue that their parent asteroids had liquid metal cores generating magnetic fields, like a miniature version of Earth's dynamo, long after the asteroids formed.
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