Niter or nitre is the mineral form of potassium nitrate, KNO₃, also known as saltpeter. It 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. It is a member of the nitronatrite group of minerals which includes other nitrates of sodium (nitratine), ammonium, barium, magnesium and others. R-phrases   S-phrases   Supplementary data page Structure and properties n, εr, etc. ... In crystallography, the orthorhombic crystal system is one of the 7 lattice point groups. ... A crystal system is a category of space groups, which characterize symmetry of structures in three dimensions with translational symmetry in three directions, having a discrete class of point groups. ... For other uses, see Cave (disambiguation). ... It has been suggested that twin boundary be merged into this article or section. ... It has been suggested that Selenite be merged into this article or section. ... Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer. ... Relative density (also known as specific gravity) is a measure of the density of a material. ... The refractive index (or index of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. ... Nitratine, also known as cubic nitre or soda niter, is a mineral, the naturally occurring form of sodium nitrate, NaNO3. ...

Niter has been known since ancient times. The name is from Hebrew neuter, for salt derived ashes.

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 before burying him alive as a form of vulgar revenge. Edgar Allan Poe (January 19, 1809 – October 7, 1849) was an American poet, short story writer, playwright, editor, literary critic, essayist and one of the leaders of the American Romantic Movement. ... The Cask of Amontillado (sometimes spelled The Casque of Amontillado) is a short story, written by Edgar Allan Poe and first published in the November 1846 issue of Godeys Ladys Book. ...

References

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