Sylvite's potassium chloride (KCl) in natural mineral form. It forms crystals in the isometric system very similar to normal rock salt, halite (NaCl). (The two are, in fact, isomorphous. Sylvite's colorless to white with shades of yellow and red due to inclusions. It's a Mohs hardness of 2.5 and a specific gravity of 1.99. It's a refractive index of n=1.490) . Sylvite has a salty taste with a distinct bitterness. Sylvite's one of the last evaporite minerals to precipitate out of solution. As s… (More on Sylvite)