Should Imilac (meteorite) be singular or plural?
Without knowing if there's an official statement on that, I can give my common sense opinion.

These are fragments that came from one single meteorite, or one single 
"formation". Let's say that we have a basalt flow, let's call it the Michael Basalt. It's fragmented, right? So there are a bunch of pieces of it. If I collect the pieces, those are still several pieces of the "Michael Basalt". If I would refer to basalts instead, that would imply that there were several flows, not only one. But there is one. So I have several samples of the singular Michael Basalt. Therefore, there are several samples of the singular Imilac meteorite.