Piece of coal eroding I'm my room

So I've had what appears to be a piece of bituminous coal with pyrite streaks in it sitting in my room for a few months now. I just returned from school for the weekend and noticed that it looks like it has been eroding while just sitting there on my dresser. It's easily crumbling and it has clear/white fibrous crystals on the surface now. I don't know if those crystals are from the coal or the pyrite.

Now, it gets really hot in my room when it's hot outside (especially during the day) and really cold when it's cold out and during the night. So, my initial thought was a sort of freeze-thaw cycle due to the slightly extreme temperature changes in my room every day and night. Does anyone have any ideas on why this rock may be eroding like this? (And if I'm even correct in that it is coal with pyrite?). Thank you for the help!!