Saltpeter

Niter
A niter crystal under a polarizing microscope
General
Category	Nitrates
Formula (repeating unit)	KNO3
Strunz classification	5.NA.10
Dana classification	18.1.2.1
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Cmc21
Identification
Color	white
Crystal habit	druse or acicular
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.)
Refractive index	nα = 1.332 nβ = 1.504 nγ = 1.504
Solubility	soluble
References

Niter, or nitre ("chiefly British variant"), 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 or potassium joined with carbonate or nitrate ions. Three related minerals are soda niter (sodium nitrate), ammonia niter (ammonium nitrate), and strontium nitrate. Niter was used to refer specifically to nitrated salts known as various types of (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. A major source of sodium nitrate mineral ("Chile saltpeter", that is, nitratine) is the Atacama desert in Chile. Potassium and other nitrates are of great importance for use in fertilizers 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.

Niter is a colorless to white mineral crystallizing in the orthorhombic crystal system. It usually is found as massive encrustations and efflorescent 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.

Niter as a term has been known since ancient times, although there is much historical confusion with natron (an impure sodium carbonate/bicarbonate), and not all of the ancient salts known by this name or similar names in the ancient world contained nitrate. The name is from the Greek νιτρων nitron from Ancient Egyptian netjeri, related to the Hebrew néter, for salt-derived ashes (their interrelationship is not clear).