Pyrite - Iron Cross Twin
Based on a specimen from Gachala, Cundinamarca Dept., Columbia.
One of the most recognizable crystal drawings in any mineralogy textbook is the diagram of the pyrite iron cross twin. It’s a penetration twin of two pyritohedra, and their interaction creates the distinctive cross-shaped pattern where the edges of the two pyritohedra intersect at 90 degrees. Below is a drawing from a crystallography textbook, with the model I made next to it.
I’ve actually made the wood model twice, in two different sizes. Alignment and assembly is very difficult due to all the concave areas, but I think the result is worth it. The model shown above is the small 5.5" version, and the one shown below next to a matching specimen is 7" across.
In my opinion, this is one of the most beautiful shapes in mineralogy. Even though the cross shape on the fourfold axis is what gives the twin its name, I think views down the other axes are equally lovely.
Down 2-fold axis
Down 3-fold axis
To see what a model of a single, untwinned pyritohedron looks like, have a look at my pyritohedron model.
Model details: Large model is 7" across. Oak and black walnut. Small model is 5.5" across. Yellowheart and black walnut.
Specimen details: Iron cross twin pyrite, Gachala, Cundinamarca Dept., Colombia. 4.2 cm