Schwertmannite is an iron-oxyhydroxysulfate mineral with an ideal chemical formula of Fe₈O₈(OH)₆(SO₄)·nH₂O or Fe³⁺₁₆O₁₆(OH,SO₄)_12-13·10-12H₂O. 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.