Limonite

Limonite, a name applied generally to the hydrous sesqui-oxides of iron which generahy occur in an earthy or other massive non-crystalline form, or diffused as colouring matters through rocks of other composition, varying from yellow ochre to brown or even black. It is thrown down from water in marsh-lands where iron sulphide becomes sulphate and happens to co-exist with carbonate of lime in the water (bog iron-ore); and it is a common decomposition product of other minerals.