Thenardite is a delicate sulfate mineral that comes from arid dry or saline lake deposits. It is similar to Mirabilite, a chemically similar mineral to Thenardite that, unlike Thenardite, contains water in its structure. Mirabilite is an
unstable mineral, and will dehydrate and lose its water when exposed to air, thereby changing to Thenardite. These altered specimens are in fact Thenardite
pseudomorphs after Mirabilite. Some of these crystals, especially those of outstanding crystal form, are artificially grown as by-products of
borax mining operations.
Thenardite is named after the French chemist Louis Jacques Thenard (1777-1826), a professor at the University of Paris.