epsomite and mirabilite are small amounts of crystallized salts that form hairlike tendrils from ceilings, walls, and floors. They can also have the look of wisps of cotton. These salts form more during the winter months when humidity is low and disappear as the humidity rises.
Epsomite is a hydrous magnesium sulfate mineral with formula MgSO4·7H2O or simply MgSO4. Epsomite forms as encrustations or efflorescences on limestone cavern walls and mine timbers and walls, as a volcanic fumaroles, and as rare beds in evaporite layers. It was first systematically described in 1806 for an occurrence near Epsom, Surrey, England, for which it was named.