June 2012. I’m in Northwest Montana. The rocks here are very old – some are over a billion years old. There is a beautiful site where you can find limonite cubes. These are pyrite pseudomorphs. Originally, a cubic pyrite specimen formed. Over time, however, the pyrite cube was replaced with a secondary mineral, limonite, which took the form of the pyrite cube. I collected a nice specimen with a little help from mom and dad.