Schwertmannite is an iron‑oxyhydroxysulfate mineral with the idealized chemical formula Fe₈O₈(OH)₆(SO₄)·nH₂O, which can also be expressed as Fe³⁺₁₆O₁₆(OH,SO₄)₁₂‑₁₃·10‑12H₂O. It crystallises in the tetragonal system (space group P4/m) and typically appears as opaque, brownish‑yellow encrustations or “pin‑cushion” aggregates. Physical properties include a Mohs hardness of 2.5–3.5 and a specific gravity ranging from 3.77 to 3.99.
Discovery and nomenclature
The mineral was first described in 1994 from an occurrence in Finland and was officially recognised as a new species in 1992. It is named after the German soil scientist Udo Schwertmann (born 1927) for his contributions to the study of iron‑sulfur geochemistry.
Occurrence and formation
Schwertmannite forms primarily in iron‑rich, acidic sulfate waters with pH values between about 2 and 4. Typical natural settings include:
- Acid mine drainage streams and lakes where oxidation of sulfide minerals releases Fe³⁺ and SO₄²⁻.
- Acidic sulfate springs such as the type locality at Pyhäsalmi, Finland.
- Coastal lowland soils and acid‑sulfate soils where seawater intrusion or flooding creates acidic, sulfate‑rich conditions.
The mineral often precipitates as an orange to brownish‑yellow solid that can later transform to more stable iron oxides (e.g., goethite) or sulfates such as jarosite when environmental conditions change (pH > 7.5 or < 2.5, respectively).
Structure and stability
Schwertmannite possesses a layered structure in which Fe³⁺ octahedra are linked by hydroxyl groups and sulfate ions occupy interlayer positions, with variable water content (the “n” in the formula). Its stability is limited to moderately acidic conditions; it dissolves or recrystallises when pH moves outside its narrow stability field.
Environmental relevance
Because of its high surface area and affinity for anions, schwertmannite plays a significant role in iron‑sulfur geochemistry. It can adsorb oxyanions such as arsenate, chromate, and phosphate, influencing the mobility of these contaminants in acid‑impacted waters. Consequently, it is studied both as a natural attenuator of pollution and as a material that can be synthetically produced for remediation applications.
References
- Bigham, J.M., Carlson, L., Murad, E. (1994). “Schwertmannite, a new iron oxyhydroxysulfate from Pyhasalmi, Finland, and other localities.” Mineralogical Magazine, 58, 641‑664.
- Bigham, J.M., Schwertmann, U., Carlson, L., Murad, E. (1996). “Schwertmannite and the chemical modelling of iron in acid sulfate waters.” Geochimica et Cosmochimica Acta, 54, 2743‑2758.
- Burton, E.D., Bush, R.T., Sullivan, L.A. (2007). “Reductive transformation of iron and sulfur in schwertmannite‑rich accumulations associated with acidified coastal lowlands.” Geochimica et Cosmochimica Acta, 71, 4456‑4473.
Further mineral data are available from Mindat.org and Webmineral.