The spectroscopic characterization of the sulphate mineral ettringite from Kuruman manganese deposits, South Africa

Abstract

a b s t r a c tThe mineral ettringite has been studied using a number of techniques, including XRD, SEM with EDX, thermogravimetry and vibrational spectroscopy. The mineral proved to be composed of 53% of ettringite and 47% of thaumasite in a solid solution. Thermogravimetry shows a mass loss of 46.2% up to 1000 • C. Raman spectroscopy identifies multiple sulphate symmetric stretching modes in line with the three sulphate crystallographically different sites. Raman spectroscopy also identifies a band at 1072 cm −1 attributed to a carbonate symmetric stretching mode, confirming the presence of thaumasite. The observation of multiple bands in the 4 spectral region between 700 and 550 cm −1 offers evidence for the reduction in symmetry of the sulphate anion from T d to C 2v or even lower symmetry. The Raman band at 3629 cm −1 is assigned to the OH unit stretching vibration and the broad feature at around 3487 cm −1 to water stretching bands. Vibrational spectroscopy enables an assessment of the molecular structure of natural ettringite to be made.

Structural and spectroscopic characterization of ettringite mineral –combined DFT and experimental study

“…4) and bands with a very low intensity for symmetric OeH w vibrations at 2922 and 2832 cm À1 , respectively. The both calculated and experimental bands agree well also with the measured IR and Raman spectra by Myneni et al [12] and Frost et al [16] (Table 6). For example, the FTIR spectrum by Myneni exhibits the broad bands around 3500 and 3300 cm À1 .…”

“…These vibrations are easily distinguished in the calculated spectrum À the band at 3713 cm À1 is assigned to the asymmetric mode whereas the band at 3672 cm À1 corresponds to symmetric mode. Previous experiments [12,16] interpreted only one wavenumber value for both vibrations ( Table 6). The calculated spectrum also allowed us to distinguish the mixed asymmetric OeH stretching vibrations of water molecules (OH w ) and OeH stretching vibrations coming from the [Al(OH) 6 ] 3À unit (OH Al ) appearing at 3617 cm À1 (shoulder).…”

“…Experimental data published previously [12,16] were only partially described due to a complex character of the ettringite spectra (they are also collected in Table 6). The red lines in Fig.…”

“…In that work the OeH w asymmetric vibrations were assigned at 3560 cm À1 and symmetric ones at 3356 cm À1 . Frost et al [16] assigned the band at 3629 cm À1 to the OeH Al stretching vibrations and the broad band, centred at 3398 cm À1 , to OeH w vibrations. The latter band was deconvoluted into component bands at 3398, 3241, and 3116 cm À1 in the FTIR spectrum, and 3487, 3386, and 3260 cm À1 in the Raman spectrum, respectively.…”

“…These studies reported an assignment of several well-distinguished bands to vibrational modes of the basic functional groups (e.g. stretching and bending of eOH, CO 2À 3 , SO 2À 4 , [Si(OH) 6 ] 2À , and [Al(OH) 6 ] 3À groups) later complemented by the interpretation of Raman spectra [12,16]. However, a complete analysis of both IR and Raman spectra is not possible because both spectra are complicated with many overlapping unresolved bands; thus, their identification can be problematic.…”