Madocite is a mineral with a general formula of Pb17(Sb,As)16S41. Madocite was named for the locality of discovery, Madoc , Ontario. It is found in the marbles of the Precambrian It is orthorhombic (rectangular prism with a rectangular base) and in the point group mm2. Its crystals are elongated and striated along to a size of 1.5 mm.
Madocite is anisotropic and classified as having high relief. It also displays strong pleochroism.
Madocite is found in small clusters in marble pits (near Madoc, Ontario), and was originally categorized in the 1920s as an unidentified sulfosalt mineral in an assemblage of pyrite , sphalerite in marble. Later research was done by John L. Jambor in the 1960s who went to the site and collected samples of the assemblages.