Marcasite, FeS2, is very similar to pyrite and has the same orthorhombic crystal system but with a tabular habit. It will oxidize to ferrous sulphate but may also become pyrite.
Marcasite is a low temperature mineral and common in sedimentary rock and can be found in Czechoslovakia, Germany, and the U.S.
Buerger, M. J. 1931. The crystal structure of marcasite. American Mineralogist. 16:361-395.