While visiting Harvard’s natural history museum on a trip to Boston, i’ve seen some minerals (or crystals. Not exactly sure of the difference between them) that seems to have naturally formed into near perfect cubes. How is that possible?
It seemed to be most frequent in pyrite.
Thanks in advance!
(Tagged as chemistry, but not sure if it fits)
In: Chemistry
Every crystal has a ‘shape’ (though some have more than 1, which can mix things up). This is the combination of the angles that the crystal’s molecules form with each other. Some common shapes are hexagons (like ice), or cubic (like pyrite). A decent list of them is on the [crystal system wikipage](https://en.wikipedia.org/wiki/Crystal_system), which is also a good place for further reading on the topic.
Not a mineral expert, but I thought in those cases the iconic bonds were naturally at 90°, similar to how bonds between atoms in a water molecule are 104.5°, leading to its tetrahedrol shape. Physicists?
When atoms clump together into a solid, there are two common ways that they settle. There is a crystal structure, and a glassy structure. Glassy structures are chaotic and inconsistent – the atoms are strewn about haphazardly.
As the name would imply, glass is a material like that.
Crystals, however, have a pattern. A repeating structure, like Sodium-Chlorine-Sodium-Chlorine-Sodium-Chlorine. Since the atoms can stick together in different ways, crystal structures can take on [countless patterns](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgEG88lYibXgF7c_owycV-EMpx-v9mH4HgKqcv6pBHYbA8FkXns639Ry5zsfQ5w74lghz701nF78eo_kZez9EwUzLK2prl8g7qvctKPy0xL20QGPm823q_oiptOn7-YWfzxX8X3hQANvBg/s1600/bravais.png).
When the crystal forms, atoms are sticking to it at random. Since the atoms have to snap into place to stay, there’s certain shapes where more or less atoms stick, depending on the crystal structure. For instance, if there’s a side that isn’t lined up flat with one of the ‘flat’ segments of the crystal structure, the atoms may stick more easily there until it becomes flat.
For this to work there must be a single large crystal, not several stuck together, so the atoms usually need a lot of time to slowly clump together and settle snugly into the pattern.
[Pyrite crystal structure](https://upload.wikimedia.org/wikipedia/commons/f/fd/Pyrite_crystal_structure.png). Notice anything?
Minerals have a crystal habit. At a fundamental level, atoms in crystalline arrange themselves in such a way as to minimise their internal energy. For some minerals, this is a cubic structure. Under optimal crystallisation conditions, crystals can get quite large and have few defects.
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