Leadhillite is a lead hydroxide. It has the formula Pb4SO4(CO3)2(OH)2. Leadhillite crystallises in the monoclinic system, but develops pseudo-hexagonal forms due to crystal twinning. It forms transparent to translucent variably coloured crystals with an adamantine lustre. It is quite soft with a Mohs hardness and a relatively high specific gravity of 6.26 to 6.55.
It was discovered in 1832 in the Susannah Mine, Leadhills in the county of Lanarkshire. It is trimorphous and macphersonite (these three minerals have the same formula, but different structures). Leadhillite is monoclinic , susannite is trigonal and macphersonite is orthorhombic Leadhillite was named in 1832 after the locality.