IIRC, the reason they can do this is based on a loophole in a statute. I think that's the right statute anyway.The statute is about commemorative coins. The statute lays out the specification for various coins (including things like silver dollars), and says how much those coins are worth. It also says: So a plain reading of the statute is that the Secretary can mint and issue a platinum coin of any denomination, including one worth a trillion dollars. It can't be paper currency, because that wouldn't fit inside the terms of the statute.