This twin exhibits the interpenetrating docecahedra very clearly, as the striations on the crystals are very fine. The twinned crystals are evenly matched and offset. In excellent condition – a textbook pyrite iron-cross twin!
NB about the photographs: All of the specimens from this find are black-to-dark brown in colour. The lustre varies somewhat. In order to capture the crystal forms in the photos, reflectors have been used (as always, on this website) and this may give the impression some twins are a brighter metallic colour – however, this is simply the effect of the reflectors.
About These Iron-Cross Twins
“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 is exhibited in specimens from several localities, but rarely does one find good-sized complete crystals. They are usually a few mm, sometimes closer to around 1 cm or so. Iron-cross twins have always been sought after by collectors. These remarkable specimens from workings near Gachalá, Colombia were first seen on the market in 2017 and are superb iron-cross twins. The workings have continued to produce and the fine specimens from 2022 are among the finest quality to date. In this size range the quality is superb and they are like crystal models.
These specimens from Gachalá are often labeled “‘limonite’ pseudomorphs after pyrite”, but this is not accurate for most specimens. Most are pyrite crystals with a thin surface veneer of goethite, and a few are partially or significantly pseudomorphed. Accordingly, they are labeled and described here as pyrite with a goethite coating.