Archives

Pyrite Iron-Cross Twins – July 2023

I’ve posted an update with superb new pyrite iron-cross twins from Colombia (click here). The remarkable specimens from workings near Gachalá were first seen on the market in 2017 and are great 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 fantastic and they are like crystal models. The specimens in this update were selected out of stacks of flats, and are top pieces from the locality’s 2022 production.

Pyrite Iron-Cross Twin, Gachalá, Cundinamarca, Colombia – 3.3 cm

“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.

Pyrite Iron-Cross Twin, Gachalá, Cundinamarca, Colombia – 2.9 cm

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.

Pyrite Iron-Cross Twin, Gachalá, Cundinamarca, Colombia – 4.3 cm

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.

Pyrite Iron-Cross Twin, Gachalá, Cundinamarca, Colombia – 3.9 cm