ELi5: Why does Pyrite form in perfect cubes?
In: 12
You can think of minerals as a repeating set of different molecules. Naturally they occur in different patterns and have different symmetrical properties (such as rotation about an axis). A set of symmetrical operations about one point is called a point group. And a set of point groups is called a space group which ultimately defines the structure/symmetry of the minerals. There are 32 point groups, giving rise to 230 space groups.
These space groups are categorized and given names, pyrite is in the isometric or cubic crystal class. But this really just means that all axes of the mineral are equal length from each other, so this could be a cube or octahedron depending on how they crystalize.
The cubic shape you see is all about “cleavage”, which is determined by the planes of weaknesses in the mineral. Muscovite is easiest to picture because it is weakest along the K-O bond rather than the Al-S [bond](https://www.thisoldearth.net/Mineral_cleavage.cfm)
Because unit cell is cubic, the breaking off of a weaker plane results in a cube. And if it crystalizes as an octohedron the pyrite might take the shape of an octohedron. Basically any shape that can be made with the symmetrical conditions of the isometric system is possible.
This website has a list of the crystalized possibilities for pyrite [link](https://m.minerals.net/mineral/pyrite.aspx)
Tldr: It just happens to be a possibility out of the many ways you can arrange molecules into crystals. And because it is a cubic crystal it tends to break off into cubic looking minerals
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