IRON CRYSTALS: WIDMANSTÄTTEN PATTERN

IN THE CORES OF DIFFERENTIATED ASTEROIDS, IRON CRYSTALLIZED IN A PATTERN NOT FOUND ON EARTH.

IN THE METAL CORES OF PARTIALLY MOLTEN ASTEROIDS, iron-nickel alloys crystallized in a distinctive pattern known as the Widmanstätten structure. This pattern is named after one of the first people to observe it some 200 years ago, Count Alois de Widmanstätten. The pattern forms only deep inside planetary bodies that take millions of years to cool. Iron never crystallizes this way on Earth's surface, so any piece of metal showing this pattern on Earth is definitely from a meteorite.

The crystal patterns in iron meteorites range from very coarse to extremely fine. The thickness of the crystals in the pattern depends mainly on the amount of nickel they contain, and how slowly the metal cooled. With this information, one can estimate the size of the meteorite's parent body, because larger asteroids cool more slowly than small ones.

CRYSTALS IN CROSS SECTION

The crystal structure in iron meteorites is three-dimensional, so the pattern looks different depending on how you slice it. These meteorites have been cut with a saw and polished flat to reveal the crystals in cross section.

The actual 3-D structure is made of numerous flat plates of the iron-nickel alloy kamacite. When molten iron and nickel first cool and harden in a planet's core, they form a different alloy, taenite. But as the solid taenite cools, plates of crystallized kamacite grow through it. After millions of years, when the metal cools so much that crystals stop growing, the finished pattern freezes in place.

The crystals that form the Widmanstätten pattern grow diagonally at very specific angles. These crystals make precise eight-sided octahedrons, forming a dramatic octahedrite structure. All of the various patterns shown here can be found in the same meteorite, depending on the angle at which it is sliced.

CRYSTALS TOO BIG TO SEE

No crystal pattern is visible in Coahuila, because the entire meteorite is part of a single, immense crystal. Coahuila contains very little nickel, which enabled a single crystal of the low-nickel alloy kamacite to grow until it consumed all of the original taenite alloy. Meteorites like this are called hexahedrites.

COARSE CRYSTALS

In meteorites with the coarsest patterns, such as Linwood (above) and Mount Joy (right), the crystals are not flat plates. When flat crystals run into each other and cannot grow any wider, they become thicker instead to fill the available space. This produces an irregular pattern.

MEDIUM AND FINE CRYSTALS

Medium-grained meteorites like Owens Valley (below) and fine-grained ones like Carlton (left) have much thinner crystals than the coarse-grained samples above. Finer patterns generally indicate higher nickel content, because nickel limits the growth of the kamacite crystals that produce the Widmanstätten pattern.

Finer crystals are also produced by faster cooling. Gibeon (below, left), for example, has smaller crystals than Owens Valley (below), even though it has slightly less nickel. The most likely explanation is that Gibeon cooled more quickly than Owens Valley-indicating that it came from a smaller parent body.

LOOK CLOSELY

A dark band of iron sulfide marks the boundary between two sections of crystals that grew until they ran into each other.

CRYSTALS TOO SMALL TO SEE

Tinnie is an ataxite, meaning it has a very fine-grained crystal pattern. It cooled so quickly that the only crystals it contains are microscopic, as in ordinary steel. This mineral structure is more flexible and less breakable than iron with large crystals in it.

BROKEN CRYSTALS

Willamette once had a Widmanstätten structure similar to the one in Owens Valley (above). However, its crystals were reheated and recrystallized into new shapes. In this case, the heat came from collisions with asteroids in space. Other iron meteorites, however, were partially recrystallized on Earth when people used them as anvils.

HOW DO WE KNOW?

IS IT A METEORITE?

Iron meteorites are easier to spot than stony meteorites because of their distinctive appearance. But how do you know for sure if a piece of iron is a meteorite? All iron meteorites are strongly attracted to magnets-but so are many other metal objects.

A better test is to check for nickel content, because unlike most iron on Earth, iron meteorites always contain at least 4 percent nickel. The surest test is to cut a flat surface, polish it and bathe it in acid, as illustrated here. If a Widmanstätten pattern emerges, it is definitely a meteorite, because no such iron-nickel crystals ever form on Earth.

One tip of the giant iron meteorite in the center of this room, Ahnighito, has been cut and polished to reveal the Widmanstätten structure. But on the far side of the meteorite, a faint Widmanstätten pattern appears naturally on an uncut surface (left). This pattern was etched over thousands of years by natural processes.