This old specimen, of which supplier didn't had no more info about its original owner, was accompanied by a label where we can read: "Goethite pseud. Magnesite". Consulting the works "Minerales de España" by Salvador Calderón (1910) and the modern version of Miguel Calvo (2009), they mention the presence of pseudomorphic substitutions of goethite (limonite) after siderite in this Aragonese locality: Ojos Negros mines. They preserve the rhombohedral form or, less commonly, as "(...) lenticular pseudocrystals or in the form of short prisms with a hexagonal section and rounded faces that can exceed one centimeter in length (...)" (Calvo, 2009).
Therefore we can deduce that what we are showing is one of these curious pseudomorphs of goethite after siderite or magnesite. A good sized specimen, full of crystallographic history, chemistry and a classic Spanish locality.