Methods of forming potassium sulfate include calcining polyhalite ore particles to convert the polyhalite ore particles to a water-soluble composition. At least a portion of the water-soluble composition is dissolved in an aqueous medium to form an aqueous solution comprising K+, Mg2+, and SO42− ions and a calcium-containing solid. The calcium-containing solid is separated from the aqueous solution to form a filtrate comprising K+, Mg2+, and SO42− ions. A potassium-containing salt is dissolved in the filtrate to increase the concentration of K+ and SO42− ions to from a concentrated liquor, and K2SO4 is crystallized from the concentrated liquor. A system for processing polyhalite ore includes a countercurrent leaching apparatus, a first mix tank, an evaporator, and at least one crystallizer.