Petrologic subtype
Petrologic subtypes divide type 3 chondrites from 3.00 to 3.9 according to how much heating they experienced.
Type 3 chondrites are the least heated of the ordinary and carbonaceous chondrites that were not altered by water. Because they range from almost pristine to fairly heated, they are divided into petrologic subtypes from 3.00 to 3.9.
Subtypes were first defined for ordinary chondrites in 1980 using thermoluminescence, the glow given off by feldspar crystals that grew as chondrule glass crystallised during heating. Today, researchers also use the chromium content of olivine, the structure of organic carbon, the composition of chondrule glass and other measures. Low subtypes such as 3.0 to 3.2 are the most primitive.
Semarkona, an LL3.00 chondrite that fell in India in 1940, is one of the least altered ordinary chondrites known and is a benchmark for research. Bishunpur, LL3.15, is another. Primitive type 3 chondrites keep presolar grains, original chondrule glass and short-lived isotope records that heating destroys in higher types.
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