Rietveld refinement and vibrational spectroscopic study of alunite from el Gnater, central Tunisia

The crystal structure of alunite (K0.72,Na0.28) Al3(SO4)2(OH)6 from El Gnater, central Tunisia, is refined by the Rietveld method. Raman and infrared data of this mineral are also given in order to provide some further information about the mineralogy and chemistry of this alunite. The crystal system is trigonal, space group R 3 m, with a = 6.9834(4) Å and c = 17.0899(11) Å. Final Rietveld refinement converges to R p = 0.16, R wp = 0.16, and R Bragg = 0.07. In the alkalic site, the occupancy of potassium and sodium is refined to 72 and 28%, respectively. The Raman and infrared spectra are investigated in order to improve previous assignments of the observed frequencies, especially for tetrahedral and octahedral vibration and OH group, which are discussed on the basis of unit-cell group analysis and by comparison with previously observed wavenumbers of natrojarosite and synthetic alunite.