Sylvite is similar to halite in chemistry & physical properties, but is scarcer. Sylvite is KCl - potassium chloride. It has a nonmetallic luster and varies from clear to white to other colors due to impurities - often red from hematite. Sylvite is soft (H=2), water soluble, and has cubic cleavage. Sylvite rock (sylvitite) flows under intense pressure from burial, as does rock salt (halitite). Sylvite flows much more readily than does halite. Some shafts drilled through sylvite are known to have been quickly closed shut by sylvite flow. The quickest way to identify sylvite is by taste. It has a strong bitter salty taste. Halite lacks the sharp bitter taste of sylvite.
Sylvite is an evaporite mineral - it forms by the evaporation of seawater (along with halite, anhydrite, gypsum, and other minerals). Sylvite has economic significance as the principal source of potassium. The K is most frequently used as an ingredient in agricultural fertilizers.
Sylvite (red) mixed with halite (clearish gray) (field of view 2.6 cm across).