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Pyrite Iron-Cross Twins – May 2018

I’ve posted a small number of excellent pyrite iron cross twins from Colombia in this update (click here).
“Iron-cross twins” are twinned crystals exhibiting the forms of two 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.
The iron cross twin law is well known and exhibited in specimens from various localities, but rarely does one find good-sized complete crystals. These remarkable specimens are from a find about a year ago near Gachalá, Colombia. They are superb iron-cross twins.
These were being sold as “limonite” pseudomorphs after pyrite, but they are not. They are pyrite crystals with a thin surface veneer of goethite.