Characterization of the sulphate mineral amarantite – using infrared, Raman spectroscopy and thermogravimetry

Abstract

The mineral amarantite Fe2(3+)(SO4)O·7H2O has been studied using a combination of techniques including thermogravimetry, electron probe analyses and vibrational spectroscopy. Thermal analysis shows decomposition steps at 77.63, 192.2, 550 and 641.4°C. The Raman spectrum of amarantite is dominated by an intense band at 1017 cm(-1) assigned to the SO4(2-) ν1 symmetric stretching mode. Raman bands at 1039, 1054, 1098, 1131, 1195 and 1233 cm(-1) are attributed to the SO4(2-) ν3 antisymmetric stretching modes. Very intense Raman band is observed at 409 cm(-1) with shoulder bands at 399, 451 and 491 cm(-1) are assigned to the ν2 bending modes. A series of low intensity Raman bands are found at 543, 602, 622 and 650 cm(-1) are assigned to the ν4 bending modes. A very sharp Raman band at 3529 cm(-1) is assigned to the stretching vibration of OH units. A series of Raman bands observed at 3025, 3089, 3227, 3340, 3401 and 3480 cm(-1) are assigned to water bands. Vibrational spectroscopy enables aspects of the molecular structure of the mineral amarantite to be ascertained.

Structure refinement, hydrogen-bond system and vibrational spectroscopy of hohmannite, Fe3+2 [O(SO4)2]·8H2O

“…For example, as mentioned above hohmannite may occur associated with amarantite and butlerite. Both FTIR and Raman spectra obtained for hohmannite are significantly different from published spectra of amarantite (Frost et al, 2013) and butlerite/ parabutlerite (Cejka et al, 2011). The distinction between hohmannite and metahohmannite based on spectroscopic data is, however, much more subtle.…”

“…The n 2 (SO 4 2À ) modes are observed at wavenumbers >400 cm À1 (e.g. Ross, 1974;Knittle et al, 2001;Murphy et al, 2009;Frost et al, 2013) in the Raman spectrum as very intense absorptions (Fig. 6).…”

“…Vibrational analysis by the application of the correlation method yields six modes that are Raman active (6 A g + 6 A u ). The unequivocal assignment of these modes is difficult due to the concomitant occurrence in this spectral range of n 2 (SO 4 2À ) and FeÀO,OH modes (Frost et al, 2013;Murphy et al, 2009). Nevertheless, these low wavenumber modes are assigned to the Raman peaks located between 400 and 227 cm À1 .…”

“…Finally, Raman peaks that are found below 200 cm À1 can generally be classified as lattice modes (e.g. Frost et al, 2004Frost et al, , 2013.…”

A Raman and infrared spectroscopic study of the sulphate mineral aluminite Al2(SO4)(OH)4·7H2O

“…This band is assigned to the SO 4 2À m 1 symmetric stretching mode. The position of this band is very typical of a sulphate unit [20][21][22][23][24][25]. Several low intensity Raman bands are noted at 1069, 1094 and 1136 cm À1 .…”