Iron Cross Twin Pyrite otherwise known as Goethite PS. After Pyrite, is a pseudomorph pyrite, meaning it has been replaced with goethite on the surface. "Iron-cross twins" are twinned crystals exhibiting the forms of two interlocking pentagonal dodecahedra (also known as pyritohedra). These twins display edges crossing at 90 degrees, and in an idealized/model twin, these edges form a cross. In nature, the edges that form the cross are sometimes not continuous or are not equal to one another, so a pyrite crystal that is twinned according to the iron-cross law may or may not exhibit an actual cross shape, but in any event the relationship of the crystals exhibits the "crossed" symmetry.