How is Persian blue rock salt made?

Question

How is Persian blue rock salt made and what does it contain that makes it look blue?

Answer 1

Persian blue salt is formed by evaporating sea water, as are most if not all salt deposits. In the geosciences, we call salt deposits like this evaporites. Persian blue salt is sodium chloride (the mineral name is halite).

According to this description:

Blue salt is one of the rarest salts known to mankind, and is extracted from a salt mine in the northern province of Semnan in Persia.

The intriguing blue color occurs during the forming of the salt's crystalline structure, as intense pressure is exerted on the salt deposits. The individual crystals fracture the light in an unusual way and the resulting blue (which is caused by an optical illusion), becomes visible.

Answer 2

I'm quite surprised no one stated the obvious: formation of F-centers in the crystal lattice of $\ce{NaCl}$. Despite the fact that in nature blue rock salt is rare, it's quite easy to synthesize some in the lab by heating metallic sodium and sodium chloride in an enclosed vessel.

$\ce{Na}$ vapor is capable of dissolving in the crystal lattice formed by $\ce{Na+}$ and $\ce{Cl-}$ ions occupying vacancies around defects and providing extra $\ce{Na+ + e-}$ pairs. Liberated electrons occupy positions of $\ce{Cl-}$ anions forming F-centers. Once the halite is cooled down, F-centers tend to associate in crystal space, leading to very intensive color.

In nature blue halite is usually formed by radiation accompanying radioactive transformations.

Interestingly enough, once dissolved, blue rock salt often sold for over 1 \$/g forms transparent colorless solution with the composition indistinguishable from the table salt costing 1 \$/kg.

There are also other factors, mostly impurities, that can lead to rock salt's crystals of virtually any color. Here is a part of the Table 1 from the review [1] regarding the shades of blue color:

Table 1. Common colors in rock salt.

$$ \begin{array}{ll} \hline \text{Color} & \text{Impurity} \\ \hline \ldots & \ldots \\ \text{blue} & \text{lattice defects/colour centers} \\ & \text{admixture of}~\ce{KBr + Au} \\ & \text{in sylvinite zone} \\ & \text{particles}~\pu{90-110 nm} \\ & \ce{Pb, Cu, Ag, Au}~\text{as pigments} \\ & \text{by adding metallic}~\ce{Na} \\ & \text{in}~\ce{Na}\text{-vapor} \\ & \ce{Cl-}~\text{removal by ionization} \\ & \text{rapid growth} \\ \text{violet} & \text{slow groth} \\ & \text{particles}~\pu{80-90 nm} \\ & +0.5\%~\ce{KCl}~\text{or}~\ce{RbCl} \\ & \text{exposure to radioactivity} \\ \text{purple} & \text{by cathode rays} \\ & \text{in sylvinite zone} \\ \text{black to dark blue} & \ce{Na}\text{-vapour + pressure} \\ & \text{large}~\ce{Fe2O3}~\text{platelets} \\ & \text{by cathode rays} \\ \ldots & \ldots \\ \hline \end{array} $$

References

1. Sonnenfeld, P. The Color of Rock Salt—A Review. Sedimentary Geology 1995, 94 (3), 267–276. https://doi.org/10.1016/0037-0738(94)00093-A.