In the last post, we took a walk in a cave that water has carved out in the easily solvable mineral anhydrite, a calcium sulphate. Anhydrite is among the minerals we call salts, that form when atoms with a postive charge – a.k.a. cations – combine with negatively charged atoms, a.k.a. anions. Most cations are metals, in this case calcium Ca2+, that combine with a sulphate ion SO42- to the stable salt CaSO4. If you add two water molecules per unit, you get ordinary gypsum.
Salts like anhydrite and gypsum typically form when a sea or bay in a hot, dry climate gets isolated from the rest of the ocean, the water evaporates and is not replaced by supply from rivers or rain. The most famous of these events are the Messinian salinity crisis from 5,96 to 5,5 million years ago, when the Gibraltar strait was closed and the Mediterranean ocean repeatedly nearly dried up and left behind thick layers of salt.
Ordinary salt is, of course, also a salt, NaCl, the combination of the positive cation Na+ and negative anion chlorine, Cl–. Salts of various types have different limist for how concentrated they can become before they preciptitate, and therefore lies in fairly nicely stratified layers.
So there is no surprise that there also are salt mines in the Perm area. The town of Solikamsk has long been a centre for salt mining. We went to a smaller place, Khokhlovka, to see an old salt factory built in timber. Here, the miners drilled wells Down to reach the salt layer, pumped hot water through that solved the salt, and then boiled it to refine salt at the surface. These salt works are long time closed down, but the buildings have been taken care of. So, let’s go and look at old buildings!