A very interesting specimen of pyrite that has pseudomorphed pyrite.  I am fairly sure that the numerous small crystals are not epitaxial growths, as fractured surfaces on other pieces show internal voids between the replacement crystals.  Either way, these are very unusual specimens-- I obtained two on my first trip to Peru a few years ago, and this is only the second time I have seen them.  

There are a couple pseudomorphed cubes on this one, the outlines are somewhat visible in the two pictures to the right, though they are clearer in person.  The many small reflections make some of these rather difficult to photograph...