Distinction of jarosite-group compounds by Raman spectroscopy

Raman spectra (200-1300 cm^-1) were measured for synthesized jarosite-group compounds [MFe₃(SO₄)₂(OH)₆, M⁺ = K⁺, NH₄⁺, Na⁺, Ag⁺, and 1/2Pb²⁺). The Raman spectra of jarosite-group compounds are characterized by a tendency for the wavenumbers assigned to two vibrational modes of SO₄^2-, ν₁(SO₄^2-) and ν₃(SO₄^2-), and three vibrational modes of Fe-O bonds, to decrease with increase in the c unit-cell parameter. The wavenumbers assigned to the ν₂(SO₄^2-) and ν₄(SO₄^2-) vibrational modes are independent of the value of c. For plumbojarosite, the peaks corresponding to the ν₁(SO₄^2-) and ν₃(SO₄^2-) vibrational modes are broad owing to two overlapping peaks assigned to two types of sulfate groups, SO₄^2- ions adjacent and not adjacent to Pb²⁺ ions. Raman spectra can serve to identify the specific type of jarosite-group compound in poorly crystalline or low-concentration geochemical samples.