The crystal structure and vibrational spectroscopy of jarosite and alunite minerals
These data depict the crystal structure and vibrational spectroscopy of jarosite and alunite minerals. The alunite supergroup of minerals is a large hydroxy-sulfate mineral group, which has seen renewed interest following their discovery on Mars. Numerous reviews exist concerning nomenclature, formation, and natural occurrence of this mineral group. Sulfate minerals in general are widely studied and their vibrational spectra are well characterized. However, no specific review concerning alunite and jarosite spectroscopy and crystal structure has been forthcoming. This review focuses on the controversial aspects of the crystal structure and vibrational spectroscopy of jarosite and alunite minerals. Inconsistencies regarding band assignments especially in the 1000–400 cm−1 region plague these two mineral groups and result in different band assignments among the various spectroscopic studies. There are significant crystallographic and magnetic structure ambiguities with regards to ammonium and hydronium end-members, namely, the geometry these two ions assume in the structure and the fact that hydronium jarosite is a spin glass. It was also found that the synthetic causes for the super cell in plumbojarosite, minamiite, huangite, and walthierite are not known.
Figure 1 shows the crystal structure of jarosite projected down the a axis showing sulfate tetrahedra and FeO6 octahedra. Figure 2 depicts the magnetic structure of jarosites showing (a) the impossibility of all moments coupling antiferromagnetically in a triangular array, (b) one of the many degenerate ground states that is obtained by rotating the moments 120°, and (c) the q = 0 array of magnetic moments that is at T < 70 K. Adapted from, Wills, A.S., and Harrison, A. (1996) Structure and magnetism of hydronium jarosite, a model Kagomé antiferromagnet. Journal of the Chemical Society, Faraday Transactions, 92, 2161–2166. Figure 3 is the crystal structure of super cell plumbojarosite projected down the a axis showing SO4 and FeO6 as polyhedra. Figure 4 shows the typical Raman spectrum of jarosite [KFe3(SO4)2(OH)6]. Figure 5 is the typical infrared spectrum of jarosite [KFe3(SO4)2 (OH)6]. Figure 6 depicts the Raman and infrared spectra of super cell plumbojarosite from 900–1300 cm−1.