Hisingerite
Hisingerite is an iron(III) phyllosilicate mineral with formula Fe2Si2O5(OH)4·2H2O. A black or dark brown, lustrous secondary mineral, it is formed by the weathering or hydrothermal alteration of other iron silicate and sulfide minerals.
It was first described in 1828 for an occurrence in Riddarhyttan, Vastmanland, Sweden. It was named after Wilhelm Hisinger (1766–1852), a Swedish chemist.