There was a huge woodshop where my dads shop is now, it burned down and due to the extreme temperature from the solvents, paints, and varnishes burning everything but the cast iron melted. I still find huge chunks of aluminum/alloys that look just like the one you have here. They often have inclusions of other metals and chunks of glass. I've also found similar chunks around a house that burned down which had aluminum siding. Pretty much ruins those sights for future detecting.