Equilibrated chondrite
An equilibrated chondrite is one heated enough inside its parent asteroid that its main minerals have evened out to nearly the same composition.
Inside a warm asteroid, iron and magnesium atoms slowly migrate between neighbouring grains of olivine and pyroxene. Given enough heat and time, every grain of the same mineral ends up with nearly the same composition. A chondrite in this state is described as equilibrated. The term covers petrologic types 4, 5 and 6 and, by extension, type 7.
Equilibration is useful for classification. Because olivine compositions are uniform, a handful of microprobe analyses gives a reliable average fayalite value, and that average falls in different ranges for H, L and LL chondrites, allowing a confident group assignment. The degree of recrystallisation then sets the type: chondrules remain fairly distinct in type 4, are blurred in type 5, and are hard to pick out in type 6, where plagioclase feldspar has grown into visible grains.
Most ordinary chondrites in collections are equilibrated. Chelyabinsk, the LL5 chondrite that fell in Russia in 2013, is a well-known example, and Kernouvé, an H6 chondrite that fell in France in 1869, shows strong recrystallisation. On a cut face, look for a fairly uniform grey interior with metal flecks and faint, ghostly chondrules.
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