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.
The wood model appears below next to a specimen with the same form. Notice how the colors in the wood model make it easier to distinguish the two different types of faces.
In the wood model, the octahedron faces are yellow, while the pyritohedron faces are striped. At each vertex, two octahedron faces and three pyritohedron faces come together, suggesting 5-fold symmetry. However, when examined closely, that illusion is dispelled. The figure below shows the two shapes side by side, viewed from a slightly different angle.
You can see the pseudoicosahedron looks slightly skewed in comparison with the icosahedron. That’s because the octahedron face triangles are a little bit different from the pyritohedron face triangles. The fact that there are two different types of triangles, and the fact that they correspond to isometric crystal forms that aren’t actually related to an icosahedron means that the resemblance between the two shapes, uncanny as it is, is only an illusion. Pyrite crystals that form with this habit retain the characteristic symmetry of their crystal class, and do not have 5-fold axes of symmetry.
For more about the way pyrite crystals can sometimes mimic 5-fold symmetry without actually posessing it, see my post about pyritohedra.
Model details: 4.5" across. Pyritohedron faces are zebra wood. Octahedron faces are osage orange.
Specimen details: Pyrite crystal with pyritohedron and octahedron faces. Bingham Pit, Bingham, Utah. 2.5 cm across.