Here's a beautiful specimen of golden PYRITE PSEUDOMORPHS after MARCASITE from the Nanasivik Iron Mine on Baffin Island, Nanavut Territory, Canada. This famous mine, which closed in 2002, was located above the Arctic Circle. This material has been analyzed by the Smithsonian and others and was proved to be pyrite, as a pseudomorphic replacement of marcasite. (This is not all that surprising, since pyrite and marcasite are polymorphs, and share the same chemical formula). Specimens collected in the 1990's have not deteriorated, so the material is quite stable. I picked out this specimen at the Bancroft Gemboree Gem, Rock & Mineral Show.