Charlesite is fluorescent and so is Clinohedrite, although not all clinohedrite is fluorescent. Matrix looks like axinite? I can't see the hexagonal symmetry but that is probably do to my eyes and the photo so I lean towards clinohedrite and quite possibly datolite. Put a UV lamp on it.
Well - I did try UV. That's how I knew it wasn't willemite (of which there are a few rather strangely shaped "xls" in this material). But I didn't see any response at all - neither orange clinohedrite nor blue charlesite nor cream from a gypsum coating. But I concluded that the thing was just too small to tell (except for no willemite of course).
Anyway - thanks for the input. It's nothing important - just trying to empty out my UK boxes.
Well - Fig 140 is a pretty complex crystal so you'd have to simplify it to match this guy.
But the main problem I have in thinking datolite is that (in my experience) the datolite xls in this environment are very glassy and nearly transparent. The one charlesite that I do have (a much larger xl that was kindly glued next to a clinohedrite by one of the previous owners) is opaque. Of course that's probably due to the gypsum coating.
Anyway - don't knock yourself out. It's not that important. But maybe I'll take a crack at exposing the xl better. What have I got to lose?