Meteorite Monday: Basic chondrite classification

One of my first tasks at the Cascadia Meteorite Laboratory is to photograph thin sections (PTS) for the samples that haven’t had any work done on them yet. We have over 600 meteorites in the collection, a large number of which haven’t had PTS work done. I’ve photographed around one hundred of them so far and there’s plenty more work to be done. Doing this has been a great way to learn how to classify meteorites. Just like with any terrestrial rock, meteorite classification starts with nothing more than a hand sample, a thin section and a petrographic microscope.

The first thing you look for are the chondrules. Are they super crisp and well defined? Then you possibly have a type three ordinary chondrite. The presence of glass is also a good sign of a type three. If those chondrules are set in a rather dark matrix, chances are you have a carbonaceous chondrite. Type three’s and carbonaceous chondrites have seen the least amount of metamorphism (or thermal alteration).

A carbonaceous chondrite thin section- those bright spherical inclusions are chondrules. Notice how apparent they are? (Thin section from the Cascadia Meteorite Laboratory. Image taken by me).

Once you get into the type four, five and six you begin to see an increase in metamorphism. Namely, your chondrules become less apparent as they basically get destroyed by the thermal process. At the type six level, the chondrites display relict chondrules. These are faint outlines of where a chondrule used to be- a ghost of its former self.

These four thin section views show the state of chondrules as they progress up the metamorphic ladder. (Image from Gary Huss as used by the Hawaii Institute of Geophysics and Planetology)

Beyond the type six, there’s the type seven. I alluded to this in last week’s post, but didn’t really expand on it. The meteoritics community has been debating this one since at least the 1970’s with the publication of Pyroxenes in the Shaw L-7 Chondrite. I’m not going to delve too much into the type seven classification here. It’s a tricky one with the definition changing from researcher to researcher. As such, I’m going to address that in its own separate post.


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