Hisingerite
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Hisingerite is a mineral, an iron silicate, formula Fe2Si2O5(OH)4.2H2O. A black or dark brown lustrous secondary mineral, it is formed by the weathering of other iron silicates. It is named after Wilhelm Hisinger, a Swedish chemist.
There is also Aluminian Hisingerite which is when one of the iron atoms is replaced by aluminium.