Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Niter (US) or nitre (UK) is the mineral form of potassium nitrate, KNO3, also known as saltpeter (US) or saltpetre (UK). Historically, the term "nitre" cognate with "natrium", a Latin word for sodium has been very vaguely defined, and it has been applied to a variety of other minerals and chemical compounds, including sodium nitrate, sodium carbonate and potassium carbonate. This article is about the mineral form of potassium nitrate, which is the usual modern meaning. Because of its ready solubility in water, niter is most often found in arid environments. A major source is the Atacama desert in Chile. Potassium and other nitrates are of great importance for use in fertilisers, and, historically, gunpowder.