Mirabilite, also known as "Glauber's salt", is a hydrous sodium sulfate mineral: Na2SO4·10H2O. It is a vitreous, colorless to white monoclinic mineral which forms as an evaporite from sodium sulfate bearing brines. It is found around saline springs and along saline playa lakes. Associated minerals include gypsum, halite, thenardite, trona, glauberite, and epsomite.
Mirabilite is unstable and quickly dehydrates in dry air, the prismatic crystals turning into a white powder (Na2SO4)). Mirabilite is soft with a hardness of 1.5–2, and has a low specific gravity of 1.49.
Thenardite (Na2SO4)) is an orthorhombic anhydrous sodium sulfate mineral found in volcanic caves on Mt. Etna, Italy. Thenardite absorbs water and converts to mirabilite.
Mirabilite is used as a purgative in the traditional Chinese materia medica; in Mandarin it is called máng xiāo.