Sylvite, also called sylvinite when impure, is potassium chloride (KCl) in natural mineral form. It is colorless to white with shades of yellow and red due to inclusions, has a hardness of around 2.5 on Mohs’ Scale and has a distinctively bitter salty taste. Sylvite is a chemical sedimentary rock, laid down through the evaporation of sea-water such deposits are collectively termed evaporites.
Locally, there are many hundreds of metres of Permian evaporite deposits, both sylvite and rock salt (halite), beneath Teesside and North Yorkshire which were deposited around 260 million years ago when the ancient Zechstein Sea became land-locked and evaporated. Middlesbrough formerly fostered a thriving salt industry, and sylvinite (for use as fertiliser) and halite (essential for keeping roads ice-free in winter) are still mined locally at Cleveland Potash’s mine near Boulby which descends over a kilometre beneath the surface to reach the Permian strata.
Sylvite crystals