Schwertmannite is an iron-oxyhydroxysulfate mineral with an ideal chemical formula of Fe8O8(OH)6(SO4)·nH2O.
Schwertmannite (with a distinct "pin cushion" morphology) commonly forms in iron-rich, acid sulfate waters in the pH-range of 2 - 4.. The mineral was first recognised officially as a new mineral from a natural acid-sulfate spring occurence at Pyhasalmi, Finland (Bigham et al., 1994). However, it is more commonly reported as an orange precipitate in streams and lakes affected by acid-mine drainage (Bigham et al., 1996). Schwertmannite is also known to be central to iron-sulfur geochemistry in acid-sulfate soils associated with coastal lowlands (Burton et al., 2007).
Bigham, JM, Carlson, L, Murad, E (1994) Schwertmannite, a new iron oxyhydroxysulfate from Pyhasalmi, Finland, and other localities. Mineral Mag 58, 641-664.
Bigham, JM, Schwertmann, U, Carlson, L, Murad, E (1996) Schwertmannite and the chemical modeling of iron in acid sulfate waters. Geochimica et Cosmochimica Acta 54, 2743-2758.
Burton, ED, Bush, RT, Sullivan, LA (2007) Reductive transformation of iron and sulfur in schwertmannite-rich accumulations associated with acidified coastal lowlands. Geochimica et Cosmochimica Acta 71, 4456-4473