Barred olivine chondrule
A barred olivine chondrule is a chondrule made of parallel plates of olivine inside an olivine rim, like a grille inside a ring.
Chondrules come in several textures, and the barred olivine type is one of the most striking. Cut through the middle, it shows a set of thin, parallel olivine plates (the "bars"), separated by glass or fine crystals, all wrapped in a rim of olivine. Olivine is the green magnesium–iron silicate common in meteorites. Some barred chondrules have one set of bars; others have several sets at different angles.
The texture is a record of how the droplet cooled. Laboratory experiments show that bars form when a droplet melts almost completely, leaving few or no surviving crystal seeds, and then cools rapidly, so olivine grows quickly as plates. Porphyritic chondrules, with chunky crystals, are far more common and suggest less complete melting. Barred olivine chondrules therefore help researchers estimate the peak temperatures and cooling rates of chondrule-forming events.
You can find them in many chondrites, from carbonaceous meteorites like Allende to unequilibrated ordinary chondrites. On a polished slice they look like small discs with fine stripes. In a thin section under crossed polarisers, the bars often go dark together as the stage turns, showing they share the same crystal orientation.
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