ELi5: Why does Pyrite form in perfect cubes?
In: 12
Lots of elements can form crystals or cubes in the right environment.
Lets say you have solid pyrite that is then dissolved in a liquid. If the liquid say, begins to dry up then the solids will start to come back together. IF the liquid evaporates too fast or too slow or if there are a lot of other elements in the water that are also attracted to the pyrite or if the liquid is physically disturbed during the evaporation process it will prevent the crystallization process from continuing (other elements besides pyrite will attach to it and it won’t make the same cube) and a more random molecule will form. So, pyrite under certain conditions will crystalize while under others it won’t.
The short, unhelpful answer that a textbook would probably give you is that the atoms that make up pyrite lock together in a cube-shaped lattice, so therefore their crystals form cubes.
While that is correct, it’s not very helpful because atoms are *tiny*, and this cube is big enough to hold in your hand, how could such a nanoscopic effect possibly scale up like this?
Let’s say you had a bunch of toy cubes, about the size of common six-sided dice. Each cube has a magnet embedded in each face that allows it to snap-connect to any other cube if it gets close enough to any of the others. But the magnets aren’t very strong ones. Let’s also say you already had a huge collection of these small cubes already formed into a bigger cube-like shape, maybe, the size of a car. And finally, let’s say that there are now a countless amount of these toy cubes just floating around in the air, bumping into things in random directions. What’s gonna happen?
It’s inevitable that some small cubes are going to bump into your big cube and stick to it. And they’re going to snap perfectly into the grid when they do so. But, you gotta remember, these magnets are weak. It’s possible that as soon as a small cube snaps into the grid, another passing by cube will slam into it and break it back off again. New cubes can only snap onto your big cube with a single attachment point. There are no nooks and crannies they can slot into to anchor them down better. Getting a new cube to stick to the outside and last for any duration of time is rare.
One of the small cubes could get lucky, though. It bumps into the big cube and snaps into the grid. Now that it is there, it has created four places on either side of it where another cube can drift by and latch in on *two* sides, not just one. Before getting knocked off, it could cause another wayward cube to snap down next to it. That creates 3 new spots where new cubes would really like to park. With each new cube you add, you increase the “surface area” of where more cubes can come down and join, which further increases the likelihood that they do. If this can build up to a critical point where it can avoid being blasted off by random chance, suddenly you have a runaway chain reaction where many, many cubes start locking into the grid, because those L-shaped nooks are such attractive places for them to be.
The result of this is that, if a new layer on the face of the big cube can last long enough to get started, it very quickly fleshes out into a full layer. You usually don’t ever get partial layers formed, it’s either the *whole* layer, or no layer at all.
That’s the key to why macro-sized crystals form as big geometric shapes with such flat faces and pointy corners. They’re formed layer by layer. Almost never in partial layers, but almost always only in full, complete layers. And if you only build shapes in broad, flat layers, then no matter how small your original crystal was, even atomic sized, it will scale up to be a sharp geometric shape.
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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