Sal ammoniac
| Sal ammoniac | |
|---|---|
Sal ammoniac from Tajikistan
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| General | |
| Category | Halide mineral |
|
Formula (repeating unit) |
NH4Cl |
| Strunz classification | 3.AA.25 |
| Crystal system | Isometric |
| Crystal class | Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m) |
| Space group | Pm3m |
| Unit cell | a = 3.859 Å; Z = 1 |
| Identification | |
| Formula 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 |
| 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 |
| Ultraviolet fluorescence | no |
| Absorption spectra | no |
| Solubility | in water |
| References | |
Sal ammoniac is a rare mineral composed of ammonium chloride, NH4Cl. It forms colorless, white, or 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 it 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 that 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. Salts of ammonia have been known from very early times; the term Hammoniacus sal appears in the writings of Pliny, but it is not known whether the term is identical with the more modern sal-ammoniac. In any case, that salt ultimately gave ammonia and ammonium compounds their name.
It typically forms as encrustations formed by sublimation around volcanic vents and 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; Mount Vesuvius, Italy; and Parícutin, Michoacan, Mexico.
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