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", an old 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 (also "soda nitre" or "cubic nitre"), sodium carbonate and potassium carbonate. This article is about the mineral form of potassium nitrate, which is the usual modern meaning.

Niter is a colorless to white mineral crystallizing in the orthorhombic crystal system. It usually is found as massive encrustations and effervescent growths on cavern walls and ceilings where solutions containing alkali potassium and nitrate seep into the openings. It occasionally occurs as prismatic acicular crystal groups, and individual crystals commonly show twinning. It is most common in arid environments. It is a soft mineral equal to gypsum on the Mohs scale and has a low specific gravity of 2.1. It has refractive indices of nα=1.332, nβ=1.504, and nγ=1.504. It readily dissolves in water.

Niter has been known since ancient times. The name is from Hebrew néter, for salt derived ashes. It may have been used as, or in conjunction with soap, as inferred by Jeremiah 2:22, "For though thou wash me with nitre, and take thee much sope . . ." However, it is not certain which substance (or substances) the Biblical "neter" refers to, with some suggesting sodium carbonate. A term (ἀφρόνιτρον) which translates as "foam of nitre" was a regular purchase in a fourth-century AD series of financial accounts, and since it was expressed as being "for the baths" was probably used as soap .

In literature, Edgar Allan Poe invokes the supposed Saint of Nitre repeatedly in the short story "The Cask of Amontillado" (1846), in which the main character uses the nitre to his "advantages", as it slowly murders his enemy. Fortunato's health worsens, until Montresor takes his revenge in the form of immurement.


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