Crystal faces and grain boundaries in an iron meteorite

Primary and Secondary Structures - Meteorites
New England Meteoritical Services

Crystal faces and grain boundaries in an iron meteorite.

Meteorite - Gibeon, IVA Iron.

Figure 1. Scale bar 4 cm.

Gibeon, IVA Iron.

Crystal faces and grain boundaries

The crystal structure of an iron meteorite is three-dimensional.

In the molten phase, iron meteorites begin as large or, as in the case of some hexahedrites, Coahuila for exmple, giant austenite crystals.

The alloy kamacite forms when molten iron and nickel cool in a differentiating asteroid. The cooling process also forms the alloy taenite, which becomes a solid at a higher temperature than kamacite. As the taenite cools further, plates of crystallized kamacite grow through it (this is the developing Widmanstatten pattern).

This cooling profile takes many millions of years. The temperature drops at a few degrees per million years, from 1700 ºC to around 500 ºC, where solid-state crystal growth (nucleation) slows down, finally stopping around 450 ºC. Tens of millions of years or more are needed for this.

Ultimately, crystal size, plate size, and thickness will be determined mainly by the initial amount of Ni available and the cooling rate. We would need more data to learn the actual crystal size of one or several crystals from the original asteroid core.

Meteorites provide some of this data, which, to a limited degree, gives us a coarse idea of size. This evidence includes actual crystal boundaries and faces, which can be observed and analyzed to determine the crystal structure. However, this evidence is often incomplete or obscured, requiring further analysis and interpretation to understand the crystal size and structure fully.

We're still not there, though. Individual grain and twin boundaries are not revealed until cutting or sectioning, which is considerable work. We also need large meteorites to section so that we can "look" inside.

Faces can also be crystal boundaries. They are relatively flat or smooth surfaces by which a crystal is bounded during crystal growth and where twinning might occur. Portions of faces and boundaries can be seen in prepared slices, and the larger the specimen, the better. However, as we are approximately over 4 billion years from the formative event, crystal size determinations can only be approximate.

The seven images in this series, Figures 1 through 7, are from a sectioned Gibeon iron, IVA meteorite. It's a full slice, 61.4 lbs. (27.9 kg), measuring 60.96 cm x 45.72 cm. For size reference, note the 1 cm- sized Ni cube in the lower right section of the meteorite image in Figure 1.