Niter

Niter
General
Category	Nitrate minerals
Chemical formula	KNO3
Strunz classification	05.NA.10
Dana classification	18.1.2.1
Crystal symmetry	2/m 2/m 2/m
Identification
Color	white
Crystal habit	encrustations
Crystal system	orthorhombic
Cleavage	very good on {001}; good on {010}
Fracture	brittle
Mohs scale hardness	2
Luster	vitreous
Streak	white
Diaphaneity	transparent
Specific gravity	2.10 (calc.)
Solubility	soluble
References

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 (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.

Description

Niter is a colourless 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 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.

Occurrence

Because of its ready solubility in water, nitre is most often found in arid environments. A major source is the Atacama desert in Chile.

History

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 implied 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 .

Potassium and other nitrates are of great importance for use in fertilisers, and, historically, gunpowder. 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: as of 2007, Chilean potassium nitrate was reportedly trading at around USD 1000 per tonne.

In literature, Edgar Allan Poe invokes the use of nitre in the short story "The Cask of Amontillado" (1846), in which the main character, Montresor, describes the mineral deposits lining the walls of the underground catacombs, where he lures Fortunato to his death.

See also

• Aphronitre

References

• Etymology of "niter"
• Niter page on mindat.org
• Niter page on webmineral.com
• Mineral galleries
• Poe's The Cask of Amontillado