Petrologic type
The petrologic type is a number from 1 to 7 added to a chondrite's group to show how much it was altered by water or heat on its parent body.
In 1967 the scientists W. R. Van Schmus and J. A. Wood proposed a scale that is still the backbone of chondrite classification. The number tells you what happened to the rock after it formed. Type 3 chondrites are the least changed: chondrules are sharp, and minerals vary in composition. Types 2 and 1 record increasing aqueous alteration, meaning water reacting with the rock to make clays and other minerals. Types 4, 5 and 6 record increasing heating, called thermal metamorphism, which blurs chondrules and evens out mineral compositions. Type 7 is used for chondrites heated almost to melting.
The number follows the chemical group: Orgueil is CI1, Murchison is CM2, Allende is CV3 and Chelyabinsk is LL5. Type 3 chondrites are further divided from 3.00 to 3.9, because small amounts of heating matter greatly for studying early Solar System processes.
On a cut slice you can often estimate the type. Crisp, well-defined chondrules suggest type 3, while chondrules that blend into the background suggest types 5 or 6. Official assignment requires microscope work and mineral analyses, such as how uniform olivine compositions are.
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