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Re: PETROLOGIC GRADE 5
The term petrologic is really a misnomer, as it implies a chemistry
related identification. It really should be considered a petrographic
classification, as it is in terms of descriptive observation.
Petrography is describing features that are noticeable in hand sample,
and in thin section, and is purely descriptive in purpose. In chondrites
these features would be chondrules, igneous glass, and to what degree
certain minerals formed in the chondrite. These are textural features
only.
However, The use a water, carbon, and to what degree the opx, and
olivine have similar compositions is chemistry based. Again, these are
descriptive, and not genetic. This can be seen in the fact that there
are H,L, and LL chondrites, and the petrographic type gives a
description of the particular chondrite in question.
As for a type 5, it can be that the chondrules are not that delineated,
but it may have all the other features that make it a type 5 chondrite.
This should be considered as well when one thinks of petrographic types.
Frank
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