Crystal faces and
grain boundaries
Iron meteorites
possess a three-dimensional crystal structure that originates in their molten
phase within differentiating asteroids. Initially, these meteorites form as
large—or in some cases, such as the Coahuila hexahedrite,
massive—austenite crystals composed of iron and nickel. As the molten
alloy cools, two distinct phases emerge: kamacite, which crystallizes at a
lower temperature, and taenite, which solidifies at a higher temperature.
During further cooling, kamacite plates grow within the taenite matrix, giving
rise to the characteristic Widmanstätten pattern.
This transformation
unfolds over tens of millions of years, with the temperature dropping at a rate
of just a few degrees per million years—from approximately 1700°C
down to 500°C. Below 450°C, solid-state crystal growth (nucleation)
slows significantly and eventually ceases. The resulting crystal size, plate
dimensions, and thickness depend primarily on the initial nickel content and
the cooling rate. However, determining the precise size of the original
crystals from the asteroid’s core requires more data than is typically
available.
Meteorite samples
offer partial insights into crystal size through observable features like
crystal boundaries and faces—relatively flat surfaces that define a
crystal’s growth limits and may indicate twinning. These features become
evident in cut or sectioned specimens, though the evidence is often incomplete
or obscured, necessitating careful analysis and interpretation. Even then,
individual grain and twin boundaries remain hidden until a meteorite is sliced
open, a labor-intensive process. Larger samples are particularly valuable, as
they provide a clearer view of the internal structure, but after over 4 billion
years since formation, crystal size estimates remain
approximate.
The seven images in
this series (Figures 1–7) showcase a sectioned slice of the Gibeon IVA
iron meteorite. Weighing 61.4 lbs. (27.9 kg) and measuring 60.96 cm x 45.72 cm,
this full slice includes a 1 cm³ nickel cube in the lower right corner of
Figure 1 for scale. These images highlight the intricate crystal patterns
preserved within, offering a glimpse into the meteorite’s ancient
history.
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