Twinning
Twinning is when two or more parts of a crystal grow or deform in mirror-image orientations.
Twinning is a crystal defect in which separate regions of one crystal share some atoms but are oriented as mirror images or rotations of each other. Twins can form as the crystal grows, as it cools through a change of structure, or when it is deformed by force.
In meteorites, twinning appears in several places. Plagioclase feldspar commonly shows repeated thin twin lamellae, which look like barcode stripes under crossed polars. Some pyroxenes show twinning from their cooling history, and clinoenstatite in chondrules often has fine polysynthetic twins that may record rapid cooling or mild shock.
The best-known meteoritic example is in metal. Neumann lines, the fine straight lines seen in etched hexahedrites and in kamacite of other irons, are mechanical twins formed when the metal was shocked, probably by collisions in space. Their presence tells scientists the iron was struck hard enough to deform, but not reheated afterwards.
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