Schwertmannite

Images

Formula: Fe³⁺₁₆O₁₆(OH)_9.6(SO₄)_3.2.10H₂O
Valence: Fe³⁺₁₆O₁₆(OH)_9.6(S⁶⁺O₄)_3.2.10H₂O
Hydrated sulphate
Crystal system: Tetragonal
Specific gravity: 3.88 calculated
Hardness: 1 to 2
Streak: Yellow
Colour: Brownish yellow
Solubility: Rapid dissolution in cold, 5 M hydrochloric acid or in ammonium oxalate at pH 3 (strongly acid)
Environments

Hydrothermal environments

Schwertmannite is a secondary mineral precipitated by waters of pH 3 to 4.5 and high in iron sulphate content from the surface oxidation of metal sulphides. Associated minerals include goethite, jarosite, natrojarosite, ferrihydrite and sulphides (HOM).

Localities

Rotbachl, Rotbachlspitze, Pfitsch pass, Finkenberg, Schwaz District, Tyrol, Austria. After being frequently identified in precipitates from acid mine waters, a fully natural occurrence of schwertmannite has been found in a small steam draining a pyritic schist at the Pfitscher Joch. The characteristics of specimens from this location are in full agreement with those of synthetic samples and samples from acid mine waters described elsewhere. This natural occurrence also demonstrates the genetic relationship between schwertmannite, jarosite and goethite/ferrihydrite as products of pyrite oxidation, which is governed mainly the sulphate and iron concentrations of the stream (EJM 7.547-552).

At the type locality, Pyhäsalmi Mine, Pyhäjärvi, North Ostrobothnia, Finland, schwertmannite occurs as a poorly crystalline, yellowish brown, ochreous precipitate from acid, sulphate-rich waters. It typically consists of needles 2 to 4 nm thick and 60 to 90 nm long, which are in spherical to ellipsoidal aggregates 200 to 500 nm across.
Schwertmannite is known synthetically and from more than 40 localities in Europe, North America and Australia. All occurrences are related to the surface or near-surface oxidation of metal sulphides; optimum conditions for formation are pH 3 to 4.5 (acid) and S0₄ concentrations of 1000 to 3000 mg/L. Typical associates are jarosite, natrojarosite, goethite and ferrihydrite (AM 80.845-850).

Back to Minerals