Saltpeter is a white, translucent, lustrous mineral composed of potassium nitrate, KNO3. It crystallizes in the orthorhombic system in prismatic crystals, which have a hardness of 2 and a specific gravity of 2.1, and these crystals exhibit perfect domal cleavage. The mineral, which is also called niter, forms delicate crusts on the surfaces of rocks and stonewalls, and occurs as a component of surface soil in Spain, Iran, Egypt, and India. In the U.S. it occurs in loose soil of the limestone caves of Kentucky, Tennessee, and the Mississippi Valley. Saltpeter is of commercial importance as a fertilizer, in the manufacture of glass, as a food preservative, and in some medicines as a diuretic. Saltpeter was once used in gunpowder and is now found in explosives, fireworks, and matches. In addition, this substance is employed in fluxes used in metallurgy. It is important as a source of nitrogen in the manufacture of nitrogen-containing compounds, particularly nitric acid, and as an oxidizing agent in many industrial chemical processes.

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