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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.

Pyrite - Pseudoicosahedron

Based on a specimen from Bingham, UT

Some crystal habits are so distinctive they end up with their own names. This one is one of them, known as the pseudoicosahedron. It’s found in pyrite crystals when octahedral and pyritohedral faces occur in a particular proportion relative to each other. When the sizes of the two kinds of faces are exactly balanced, the 8 octahedron faces and 12 pyritohedron faces are all triangular, and are quite difficult to tell apart. The whole shape appears at first glance to be composed of 20 equilateral triangles, and resembles (but is not equivalent to) an icosohedron, the 20-sided Platonic solid with 5-fold symmetry.

Pyrite - Pyritohedron

Based on a specimen from Huanzala, Peru

One of the first things you learn in a crystallography class is that one, two, three, four, and six-fold symmetries are all possible, but a true 5-fold axis of rotation is impossible in crystals. (Objections relating to quasicrystals are noted, but the structure of quasicrystals is not periodic as required by the strict definition of a crystal. I will write a post on quasicrystals if/when I get my hands on a specimen!) Some crystals, however, appear at first glance to have this forbidden symmetry, although a careful check of their interfacial angles will prove that they do not.