RAMAN AND IR SPECTROSCOPIC STUDIES OF ALUNITE-SUPERGROUP COMPOUNDS CONTAINING Al, Cr3+, Fe3+ AND V3+ AT THE B SITE

Abstract

Raman and IR spectroscopies, X-ray diffraction and chemical analysis have been used to characterize synthetic compounds of the alunite supergroup of formula AB 3 (SO 4 ) 2 (OH) 6 , with A = Na + , K + , H 3 O + and B = Al, Cr 3+ , Fe 3+ , V 3+ . For each A-site cation, the proportion of B-site vacancies decreases in the same order as the effective ionic radius of the B-site cations (Al > Cr > Fe > V). Raman and IR spectra of the compounds with B = Al and Fe 3+ (alunite-and jarosite-group phases) show very close agreement with previously published spectra. The spectra for Cr-and V-based analogues show similarities to the alunite-and jarosite-group phases, and particularly the latter. The Raman and IR spectra are similar in the high-wavenumber ranges (around 3400-3500 cm -1 ), but are different in the intermediate and, particularly, the lower ranges, <700 cm -1 , probably owing to the different responses of Raman and IR to SO 4 and metal-O (M-O) group symmetries. The a parameter correlates well with n 1 SO 4 , n 2 SO 4 and n 4 SO 4 Raman and n 4 SO 4 IR wavenumbers. There are differences in the OH-stretching regions, and shifts in the main nSO 4 and M-O peaks of the alunite-group phases, and Cr-, Fe-and V-based analogues with substitution of K, Na or H 3 O at the A site in the Raman and IR spectra. Raman and IR spectroscopies are therefore useful in distinguishing these compounds. SOmmaiReNous nous sommes servis de la spectroscopie Raman, la spectroscopie infrarouge, la diffraction X et l'analyse chimique pour caractériser les composés synthétiques du supergroupe de l'alunite, dont la formule générale est AB 3 (SO 4 ) 2 (OH) 6 , avec A = Na + , K + , H 3 O + et B = Al, Cr 3+ , Fe 3+ , V 3+ . Pour chaque cation au site A, la proportion de lacunes au site B diminue dans la même séquence que le rayon ionique du cation logeant au site B (Al > Cr > Fe > V). Les spectres de Raman et IR de ces composés ayant B = Al et Fe 3+ , c'est-à-dire les composés des groupes alunite et jarosite, concordent très étroitement avec les spectres déjà §

Crystallization of Jarosite with Variable Al3+ Content: The Transition to Alunite

“…The peaks compare well with literature [9,15,27,28], where the sulphate bands are found between 940 and 1300 cm −1 . The main differences between the alunite to jarosite spectrum are that the sulphate band is shifted to higher wavenumbers, as are the bands in the 400-700 cm −1 region.…”

“…The calculated compositions are shown in Table 2. The values for Fe, K and Al were all within expected ranges [9,14]. These results showed that Al substitution above 20% did not occur until the initial solution Al/(Al + Fe) content had increased above 0.9.…”

“…Acid-mine drainage is a serious environmental concern with jarosite a major mineral constituent of these systems [6,7]. The formation of jarosite in acid-mine drainage systems can be beneficial in that heavy metals can be incorporated into the structure, along with consumption of some of the acid [7,8], and thereby limit environmental harm [9,10]. Jarosite is also produced in some industrial hydrometallurgical operations (such as zinc processing) to remove unwanted iron and improve metal concentrates [11,12].…”

“…These investigations mainly focus on the thermodynamics [14] of formation and in particular the difference between Na and K containing jarosites [13]. There is a lack of information in these manuscripts on how the infrared spectroscopy or thermal behaviour alters for the entire solid solution from 0 to 1, but there is previously published infrared spectroscopy information on jarosite and alunite [9,15,27,28] looking at the formation of the Fe and Al end members of the jarosite family to determine their spectroscopic characteristics [9]. Similarly, the work of Rudolph et al [29] looked at the formation of synthetic alunites and their characterisation by diffraction and thermal techniques.…”

A first-principles determination of the orientation of H3O+ in hydronium alunite

“…The alunite structure often contains A-and Bsite vacancies (Dutrizac and Jambor 2000;Greedan 2001), and "excess", non-OH water, which is assumed to be in the form of hydronium (or "oxonium," H 3 O + ) that substitutes for K + on the A site (Ripmeester et al 1986;Alpers et al 1989). As Lager et al (2001) point out, the existence of hydronium is supported by K + deficiencies in the structure and the fact that hydronium alunite end-members without any alkalis can be synthesized (Murphy et al 2009). …”