Bixbyite - cube and trapezohedron
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.
Spinel Pair
Based on specimens from Russia and Canada
At the 2023 East Coast show in Springfield MA, I was delighted to find two complex spinel crystal specimens from two different dealers. One, from Siberia, was large, sharp, and highly symmetrical like something out of a textbook. The other, from Baffin Island, Canada, was a pleasing shade of dark blue but distorted in a way that made its jumble of faces difficult to sort out. I bought them both, but upon further investigation in my hotel room that evening I was disappointed to find that both crystals were bounded by the exact same forms! Although I was initially grumpy about the coincidence, I eventually realized that models of these crystals would make a very nice pair. The completed set makes the similarities between these two seemingly very different crystals readily apparent.
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.