The Widmanstätten Pattern



Formation of the Widmanstätten Pattern

The Widmanstätten pattern develops through a process known as cooling and crystallization within the meteorite’s parent body. Here’s how it happens:

  • Origin in Asteroids: Iron meteorites derive from the metallic cores of differentiated asteroids or larger planetary bodies. As these bodies form, iron and nickel separate to create a metallic core, while lighter materials develop into the mantle and crust.

  • Slow Cooling: A key factor in the formation of the Widmanstätten pattern is the gradual cooling of the meteoritic metal within the parent body. Over millions of years, as the core cools slowly, two main phases emerge: kamacite, which is a nickel-poor iron alloy, and taenite, a nickel-rich iron alloy.

  • Diffusion and Growth: The slow cooling process allows nickel and iron atoms to diffuse through the metallic matrix. As temperatures decrease, kamacite and taenite crystals begin to crystallize. The differing concentrations of nickel and iron in these phases lead to variations in their crystal structures.

  • Cutting and Etching: To expose the Widmanstätten pattern, a meteorite sample needs to be cut, polished, and subsequently treated with acid (commonly nitric acid). This technique selectively erodes the softer taenite, leaving the harder kamacite visible, which reveals the interconnected pattern.

Characteristics of the Widmanstätten Pattern

The outcome of this process is a striking arrangement of bands and lines that vary in size and complexity, making each specimen unique. The Widmanstätten pattern typically appears as an assortment of interlocking whorls or mosaics, providing valuable information about the meteorite's cooling history and formation conditions.

A well-defined Widmanstätten pattern suggests that the meteorite has experienced a prolonged cooling period, giving researchers insights into the temperatures and timescales associated with its formation. Each pattern acts as a distinctive fingerprint, assisting scientists in classifying the meteorite and tracing its origins back to specific parent bodies in the asteroid belt.