Fine-grained rim
A fine-grained rim is a coating of dusty material wrapped around a chondrule or inclusion in a primitive chondrite.
A fine-grained rim is a shell of very fine dust, often only tens of micrometres thick, that surrounds a chondrule, CAI or other object in some primitive chondrites. It is especially clear in CM chondrites, where rims of dark, fine material coat almost every chondrule.
There are two main ideas for how rims formed. One is that chondrules swept up dust while drifting in the solar nebula, gathering a coating before they were packed into an asteroid. The other is that rims formed later on the parent body, as chondrules tumbled in fine material or as water altered their outer layers.
Rims are studied closely because they can preserve presolar grains and organic matter and record the conditions of the disc just before assembly. Their texture may also show that rocks were compacted by impacts, as rims around chondrules are sometimes flattened in one direction, giving the rock a weak alignment called a petrofabric.
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