(from Latin mirabilis, “wonderful”; named by the German chemist J. R. Glauber), also called Glauber’s salt, a mineral of the sulfate class with a chemical composition of Na₂ [SO₄]-10H₂O; it contains 19.24 percent Na₂O, 24.85 percent SO₃, and 55.91 percent H₂O. Mirabilite crystallizes in the monoclinic system, forming crystals ranging from short prismatic to acicular. It also forms grainy or powdery aggregates and microcrystalline incrustations. Mirabilite dissolves readily in water to form a bitter saline solution. In dry form it rapidly loses water and undergoes subsequent conversion into a white, powdery anhydrous mineral known as thenardite (Na₂SO₄). Mirabilite has a hardness of 1.5–2 on Mohs’ scale and a density of 1,480 kg/m³.

Mirabilite is a typical chemogenic product formed in salt lakes, shallow bays, and hot springs; it also occurs as an efflorescence. Because of its poor solubility at low temperatures, especially in the presence of NaCl, mirabilite is often deposited from saline waters during the winter season; this process is peculiar to the gulf of Kara-Bogaz-Gol in the Caspian Sea (USSR), Great Salt Lake in Utah (USA), and elsewhere. Mirabilite occurs in association with thenardite, astrakhanite, gypsum, and halite. It is primarily used in the chemical industry for the production of soda and sodium hydroxide; it is also used in the glassmaking, dyeing, and other industries, as well as in medicine.