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.

Based on a specimen from Thomas Range, UT

This model was inspired by a beautifully symmetric but tiny crystal of bixbyite. There are little concavities along the edges of the main crystal, as well as the expected trapezohedral corner modifications. A little investigating convinced me that the faces that made up the concave regions were also trapezohedron faces, but what were they doing in the centers of cube edges, and why were they so nicely symmetrical? And what, if anything, did they have to do with the striations that crossed in the center of every cube face?

Based on a specimen from Topaz Mountain, UT

This beautiful specimen of bixbyite on topaz was just crying out for a model! Nothing too complicated here, just a cube and trapezohedron in nicely balanced proportions. Cubes of bixbyite often have trapezohedral modifications on their corners, but often the modifying faces are tiny. This crystal has comparatively large trapezohedron faces, which makes an attractive combination of forms. The photos below are taken with the same orientation, looking down a 4-fold axis. The other photo was taken looking down a 3-fold axis, and really showcases the unique wood that makes up the trapezohedron faces.

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.

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.