Use of Raman spectroscopy to characterize and distinguish minerals of the alunite supergroup

The alunite supergroup consists of more than 50 representative minerals with the general formula A B 3 (XO 4) 2 (OH) 6 , where A may be occupied by Na, is the trivalent cation , Al 3+ or Fe 3+ , but also Cu 2+ or Zn 2+ and X is the tetrahedral anion, S, P or As [1, 2]. The minerals of this supergroup form numerous complex solid solutions which crystallize in a rhombohedral system, space group R 3 m. The general structure can be described as a combination of BO 2 (OH) 4 – octahedra and XO 4 – tetrahedra. The ions in A-site are located between the BO 2 (OH) 4 – octahedral sheets, in an icosahedral site. The minerals of the alunite supergroup occur in a wide range of geological environments, including metamorphic, sedimentary, magmatic systems and soils. Their large domain of stability, the complex solid solutions between the numerous end-members and their capacity to incorporate heavy metals such as As, Pb, Ba and radionucleides such as Sr, Th, U in their lattice, makes them minerals of interest and potential tools in mineral exploration and resources, but also as traps for pollutants. Although Raman spectra of synthetic and natural minerals of the alunite supergroup have already been published (e.g.[3, 4]), the attributions of some bands are still debated. It is the same for the interpretations of the wavenumber variations of some vibrational bands, with the elemental composition, which are sometimes superficials. In this work, acquisition of new Raman spectra was performed on natural minerals differing from one to another by their composition in A-, Band X-sites. The minerals used are from advanced argillic zones hosted by volcanic rocks in Bulgaria [5] and are defined as K-alunite (KAl 3 (SO 4) 2 (OH) 6), Na-alunite (NaAl 3 (SO 4) 2 (OH) 6), K-jarosite (KFe 3 (SO 4) 2 (OH) 6) and woodhouseite (CaAl 3 (SO 4)(PO 4)(OH) 6). Their Raman spectra are presented in figure 1. Comparison of these spectra one to one (K-alunite versus Na-rich alunite for Na-K substitution, K-alunite versus K-jarosite for Al-Fe substitution, and K-Na alunite versus woodhouseite for SO 4-PO 4 substitution) combined with crystalline parameters determined by Sato et al. [6] has brought new insight to the interpretation of the vibrational spectra and band assignments. Thus, the region below 650 cm-1 is complex because of bands due to various deformation modes of …