Niter, or nitre (chiefly British), is the mineral form of potassium nitrate , KNO3, also known as saltpeter or saltpetre. Historically, the term niter was not well differentiated from natron, both of which have been very vaguely defined but generally refer to compounds of sodium joined with carbonate ions.
Related minerals are soda niter (sodium nitrate), ammonia niter or gwihabaite (ammonium nitrate), nitrostrontianite (strontium nitrate), nitrocalcite (calcium nitrate ), nitrobarite (barium nitrate) and two copper nitrates, gerhardtite and buttgenbachite; in fact all of the natural elements in the first three columns of the periodic table and numerous other cations form nitrates which are uncommonly found for the reasons given, but have been described. Niter was used to refer specifically to nitrated salts known as various types of saltpeter (only nitrated salts were good for making gunpowder) by the time niter and its derivative nitric acid were first used to name the element nitrogen , in 1790.
Because of its ready solubility in water, niter is most often found in arid environments and often in conjunction with other soluble minerals like halides, and rarer carbonates and sulphates. A major source of sodium nitrate mineral ("Chile saltpeter" or nitratine in Chile. Potassium and other nitrates are of great importance for use in fertilizers. Much of the world's demand is now met by synthetically produced nitrates, though the natural mineral is still mined and is still of significant commercial value.