A trigonal mineral, KFe₃(SO₄)₂(OH)₆; alunite group; amber yellow to dark brown; forms druses of minute crystals, crusts, and coatings; may be fibrous or fine-grained and massive; associated with volcanic rocks; thought to form under solfataric conditions at elevated temperatures and pressures; in some places with alunite; at many localities in the Western United States, Bolivia, Europe, and Russia.