<p>Rock salt (halitite) from the Permian of Poland (4.8 cm across at its widest).
</p><p>This interesting rock is halitite, also called rock salt.  As its name suggests, it is entirely composed of halite (NaCl - sodium chloride).  Halitite is the most common type of evaporite, a crystalline-textured, chemical sedimentary rock formed by the evaporation of water (usually seawater), and the precipitation of dissolved minerals.  Most halitites are clear to whitish to grayish.  This sample is from Poland's Klodawa Salt Mine and has intensely blue-colored halite in places.  The coloration is not due to impurities, but rather to structural defects at the atomic level.  Blue-colored halite and rock salt tend to occur in proximity to sylvite (KCl) and other potassium salt minerals.  Potassium-40 irradiation from these minerals causes atomic-scale defects, resulting in blue coloration.  Blue halite at this mine also occurs in tectonically deformed rock salt, where tectonite formation has occurred (extreme stretching).
</p><p>Geologic unit &amp; age: Klodawa Salt Dome, Zechstein Formation, Upper Permian
</p><p>Locality: Klodawa Salt Mine, ~1 to 2 km south of the town of Klodawa, central Poland
</p>
<hr><p>Most info. from:
</p>
Zelek et al. (2014) - Lattice deformation of blue halite from Zechstein evaporite basin: Klodawa Salt Mine, central Poland.  Mineralogy and Petrology 108: 619-631.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
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<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/15359708243/">Rock salt (halitite) (Klodawa Salt Dome, Zechstein Formation, Upper Permian; Klodawa Salt Mine, central Poland) 2</a>