The nitrate group, NO3-, is bonded together strongly. Consequently, the group is not easily disrupted. Like the carbonate anionic group, the nitrate group is trigonal planar. The N-O bond in nitrates is stronger then the C-O bond in carbonates.
Nitrate compounds are nearly always highly soluble. Hence, minerals in this class are rare. The two most common examples are both found in very arid environments. Nitrates are powerful oxidizing agents. When a nitrate mineral is heated on a charcoal block, nitrates cause the charcoal to burn violently, because of the oxygen released.
Nitratite (formerly soda niter, also called nitratine) - NaNO3
The monovalent sodium ion is in six coordination to oxygens in the nitrate group. Nitratite is isostructural with calcite. H=1-2, G=2.29 The lower cation charge on sodium (compared to calcium in calcite) makes the mineral softer and lower melting than calcite. Crystals are rhombohedral, but it is usually found as an incrustation in soils. The major locality is in northern Chile (a caliche deposit in the Atacama desert), where it is mined as a source of nitrogen for fertilizer. It is colorless, but often is brown or yellow due to impurities. It is somewhat sectile, and is reported to have a "cooling" taste. A flame test will yield the bright yellow color of sodium.
Niter (aka saltpeter) - KNO3
The larger potassium ion makes niter isostructural with aragonite, rather than calcite. Niter is less common than nitratite. It is recovered from soils in several areas, including Chile, as a source of nitrogen. H=2, G=2.1 A flame test will yield a violet color. However, if sodium is present, the strong emission of sodium will mask the purple color of the potassium.
The source of the nitrate in Chile appears to be evaporite deposits from non-marine saline lakes. Other minerals present include sodium chloride, sodium sulfate, and minor amounts of other compounds. These include iodates, from which a large part of the world's supply of iodine is obtained. The source of these compounds is still unsolved.
Last revised: April 26, 2000