Comparison of the Raman Spectra of Natural and Synthetic K- and Na-jarosites at 298 and 77 K

Raman spectroscopy at 298 and 77 K has been used to characterise synthetic and natural jarosites of formula Mn(Fe 3+)6(SO4)4(OH)12 where M is K or Na. The natural jarosites are characterised by a well define hydroxyl stretching pattern in contrast to that of the synthetic jarosites. This difference is attributed to differences in hydrogen bonding in the mineral. Multiple Raman OH stretching bands are indicative of non-equivalent OH units in the structure. Jarosites are characterised by an intense sharp band between 1007 and 1010 cm-1attributed to the sulphate symmetric stretching mode. The band width for K-jarosite is 4.4 cm-1 at 298 K and 3.6 cm-1 at 77 K in comparison to the values for synthetic jarosite of 10.0 (298 K) and 7.3 cm-1 (77 K). Differences in the spectra between the natural and synthetic jarosites are found. Reasons for these differences in the spectra are provided.