Crystals of coexisting alunite and jarosite, Goldfield, Nevada

Alunite (KAl₃ (SO₄ )₂(OH)₆) and jarosite (KFe₃(SO₄)₂(OH)₆) coexist in optically continuous crystals in a sample from the Goldfield mining district, Esmeralda County, Nev. Beam scans with an electron microprobe indicate that no intermediate minerals of the solid-solution series alunite-jarosite occur in these crystals. An Eh-pH diagram demonstrating the equilibrium relations between alunite, jarosite, and geothite suggests the crystals most likely developed by a rise in Eh, which oxidized ferrous to ferric iron, thus allowing jarosite to precipitate.