Unequilibrated chondrite
An unequilibrated chondrite is one heated so little on its parent body that its minerals still vary in composition from grain to grain.
Chondrites are assembled from components that formed in different places and conditions, so at first each olivine or pyroxene grain has its own composition. Heating inside an asteroid lets atoms diffuse between grains until compositions even out, which geologists call equilibration. A chondrite that escaped significant heating keeps its original variety and is called unequilibrated. In practice the term usually means petrologic type 3, and unequilibrated ordinary chondrites are often abbreviated UOCs.
These rocks are prized because they preserve the most information about the solar nebula: sharp chondrules with glassy material between crystals, fine-grained matrix, and sometimes presolar grains. Subtypes from 3.00 to 3.9 grade them by the small amount of heating they did experience.
The benchmark is Semarkona, an LL3.00 chondrite that fell in India in 1940 and is one of the least metamorphosed ordinary chondrites known. To recognise an unequilibrated chondrite on a cut face, look for abundant, crisp, often colourful chondrules packed closely together. Confirmation comes from electron microprobe analyses showing a wide spread in olivine iron content, together with other measures such as thermoluminescence or the structure of carbon.
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