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I've been interested in bismuth crystallization since I found out they existed, but have a question on their formation.

According to Wikipedia, the stair-step structure of bismuth crystals is "the result of a higher growth rate around the outside edges than on the inside edges."

What would cause this higher growth rate around the edges? Cooling?

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Hopper crystals, that is, the "staircase" form, develop when a crystal forms so rapidly that material is used up before the interior finishes.

Table salt, $\ce{NaCl}$, also forms nice hopper crystals. If you want to try this, use "kosher" salt or CP $\ce{NaCl}$, which doesn't have anti-caking and iodide additives that would spoil the crystal structure.

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