Sal ammoniac is a rare mineral composed of
ammonium chloride, NH
4Cl. 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. Salts of ammonia have been known from very early times; thus the term
Hammoniacus sal appears in the writings of
Pliny, although it is not known whether the term is identical with the more modern
sal-ammoniac. In any case, this salt ultimately gave
ammonia and
ammonium compounds their name.