| Sal ammoniac | |
|---|---|
| General | |
| Category | Halide mineral |
| Chemical formula | NH4Cl |
| Strunz classification | 3.AA.25 |
| Crystal symmetry | Isometric 4/m 3 2/m - Gyroidal |
| Unit cell | a = 3.859 Å; Z = 1 |
| Identification | |
| Molar mass | 53.49 g/mol |
| Color | Colorless, white, pale gray, may be pale yellow to brown if impure |
| Crystal habit | Crystals skeletal or dendritic; massive, encrustations |
| Crystal system | Isometric |
| Twinning | On {111} |
| Cleavage | Imperfect on {111} |
| Fracture | Conchoidal |
| Tenacity | Sectile |
| Mohs scale hardness | 1-2 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent |
| Specific gravity | 1.535 |
| Optical properties | Isotropic |
| Refractive index | n = 1.639 |
| Birefringence | Weak after deformation |
| Solubility | in water |
| References | [1][2][3] |
Sal ammoniac is a rare mineral composed of ammonium chloride, NH4Cl. It forms colorless to white to yellow-brown crystals in the isometric-hexoctahedral class. It has very poor cleavage and a brittle to conchoidal fracture. It is quite soft, with a Mohs hardness of 1.5 to 2, and has a low specific gravity of 1.5. It is water-soluble. Sal ammoniac is also the archaic name for the chemical compound ammonium chloride. The Romans called the ammonium chloride deposits they collected from near the Temple of Jupiter Amun (Greek Ἄμμων Ammon) in ancient Libya 'sal ammoniacus' (salt of Amun) because of proximity to the nearby temple.[4] Salts of ammonia have been known from very early times; thus the term Hammoniacus sal appears in the writings of Pliny,[5] although it is not known whether the term is identical with the more modern sal-ammoniac.[6] In any case, this salt ultimately gave ammonia and ammonium compounds their name.
It typically forms as encrustations formed by sublimation around volcanic vents. It is found around volcanic fumaroles, guano deposits and burning coal seams. Associated minerals include sodium alum, native sulfur and other fumarole minerals. Notable occurrences include Tajikistan; Mt. Vesuvius, Italy; and Parícutin, Michoacan, Mexico.
It is commonly used as a flux in the soldering of stained-glass windows. In both jewellery-making and the refining of precious metals, potassium carbonate is added to gold and silver in a borax-coated crucible to purify iron or steel filings that may have contaminated the scrap. It is then air-cooled and remelted with a one-to-one mixture of powdered charcoal and sal ammoniac to yield a sturdy ingot of the respective metal or alloy in the case of sterling silver (7.5% copper) or karated gold. Anything other than 24-karat gold has silver and copper added. Usually the addition of silica, zinc, and deoxidants in very small amounts relative to the pennyweight (dwt.) of gold are processed into gold from as low as 8-karat to as high as 23.5-karat gold. This is added to prevent porosity or cracking while milling the ingot further into wire, sheet, or tubing. Without those additives an otherwise poor-quality ingot will result in open crucible melting with a hand torch or blowpipe and flame, as was done before electric melting furnaces were invented for use in the precious metals industry.
Sal ammoniac has also been used in the past in bakery products to give cookies a very crisp texture, although that application is rapidly dying due to the general disuse of it as an ingredient. However, in some areas of Europe, particularly Scandinavia, it is still widely used in the production of salty licorice candy known as Salmiak or Salmiakki.