Seeing the trailers for Star Wars: The Last Jedi, I was mesmerized by the planet Crait, with its bright red rock deposits kicked up by passing ships. On the day of the film’s release, I was again mesmerized by the planet, however this time I got a much better look at the geology of the place.
The closer look at the red rocks involves a trench-line view in the same flats that you see in the trailer. It was this view that immediately let me know what those rocks were. To solidify my inkling, a soldier in the movie even tastes a bit of it and says, “Salt.”
However, salt is not commonly the bright and deep reds seen in the move. Usually salt, especially that found in salt flats, is white. Your typical table salt — NaCl (sodium chloride) — is a mineral named halite. This is also the type of salt you would see at the Bonneville Salt Flats in Utah, which the plains of Crait are reminiscent of.
There is a kind of salt that can be red, though, and it’s called sylvite. Sylvite is KCl, potassium chloride. Instead of being “salty,” like halite, sylvite has a more bitter-salty taste. Sylvite also forms into crystals, much like those seen when the ships and soldiers were plowing through the deposits on Crait.
Sylvite, like halite, is an evaporite, meaning that it forms from the evaporation of water that has a lot of the specific elements in it (in this case K and Cl). This is commonly seen in dry areas that have large, flat plains, like the Bonneville Salt Flats. The resulting rock is often brittle, and easily broken, like the rocks seen in The Last Jedi.
So even though we don’t have sylvite flats here on Earth (it is found mixed with halite but not as the dominant mineral), it is entirely possible that this type of deposit could develop with the right type of water chemistry in a galaxy far, far away.
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