Schwertmannite

Schwertmannite is an iron-oxyhydroxysulfate mineral with an ideal chemical formula of or. It is an opaque tetragonal mineral typically occurring as brownish yellow encrustations. It has a Mohs hardness of 2.5 – 3.5 and a specific gravity of 3.77 – 3.99.
It was first described for an occurrence in Finland in 1994 and named for Udo Schwertmann, a soil scientist at the Technical University of Munich, Munich, Germany.
Schwertmannite commonly forms in iron-rich, acidic 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 occurrence at Pyhäsalmi, Finland. However, it is more commonly reported as an orange precipitate in streams and lakes affected by acid mine drainage. Schwertmannite is also known to be central to iron-sulfur geochemistry in acid sulfate soils associated with coastal lowlands.