Formula: FeSO4.7H2O Monoclinic
Description: Melanterite forms fibrous to stalactitic masses, concretions and crusts due to the oxidation of sulfides such as marcasite and pyrite. It is commonly intermixed with other sulfates including jarosite, copiapite gypsum, alunogen, epsomite and pickeringite. The small grain size of such material in addition to their pulverant nature makes their distinction difficult. Despite the fact that melanterite is reported from only a few localities in the state, it may be expected to form wherever abundant marcasite or pyrite is exposed at the earth’s surface. This condition is met frequently at the old mines of the Upper Mississippi Valley zinc-lead district in Lafayette, Iowa and Grant Counties. Here old dump piles and mine workings rich in sulfides become coated with crusts of oxides and sulfates within months of their exposure to surface conditions. Indeed marcasite and pyrite specimens should be coated to prevent their deterioration as they reside in display or storage areas. Although the long white to greenish melanterite crystals that form on these display samples may be attractive, the specimens themselves soon crumble and are destroyed by the weathering. One may assume that all of the listed occurrences of pyrite, marcasite, sphalerite and galena in the zinc-lead district are also sites where melanterite and its associates can be found.
LAFAYETTE COUNTY: Pale bluish green melanterite is found as translucent stalactites associated with gypsum on mine walls in the lower stopes of the Galena level in the east 1/2 SW sec. 4 T.1N. R.2E. near Shullsburg (Heyl et al., 1959).
— Efflorescences of colorless to green needle-like crystals of melanterite, having a bitter taste, are reported from the marcasite-rich portions of the Etna Mine SE SE sec. 2 and NW NW sec. 11 T.1N. R.1E. near Benton (Bain, 1907).