Characterisation of mackinawite by Raman spectroscopy: Effects of crystallisation, drying and oxidation

Iron(II) sulphides were precipitated by mixing FeCl2 · 4H2O (or FeSO4 · 7H2O) and Na2S aqueous solutions with various [Fe(II)]/[S(-II)] concentration ratios at [Fe(II)] = 0.1 mol L−1. They were analysed by micro-Raman spectroscopy and X-ray diffraction immediately after precipitation and after various times of ageing in suspension at room temperature. In any case, the initial precipitate was nanocrystalline mackinawite. Its Raman spectrum is made of two sharp peaks at 208 ± 1 and 282 ± 1 cm−1. For [Fe(II)]/[S(-II)] ⩾ 1, ageing of the precipitate led to crystalline mackinawite, as testified by X-ray diffraction. The Raman spectrum of crystalline Fe(II) mackinawite shows three main peaks at 208, 256 and 298 cm−1. Drying of nanocrystalline mackinawite under an argon flow favoured crystallisation. The removal of interparticle and surface adsorbed water molecules led to coalescence of the nanoparticles and increase of the size of the domains of coherent scattering. For [Fe(II)]/[S(-II)] = 1/2, the precipitate still consisted of nanocrystalline mackinawite after 70 days of ageing. Finally, the early oxidation stages of mackinawite led to the formation of Fe(III) cations inside the tetrahedral sites of the crystal structure. The most oxidised form of Fe(III)-containing mackinawite is characterised by a Raman spectrum with sharp peaks at 125, 175, 256, 312 and 322 cm−1, and a large vibration band at 350–355 cm−1 that may be attributed to stretching modes of Fe(III)-S tetrahedrons. Analyses of the rust layers of a roman ingot that remained 20 centuries in the Mediterranean Sea revealed the presence of iron sulphides, more likely generated by sulphate-reducing bacteria. Micro-Raman analyses demonstrated the presence of nanocrystalline mackinawite and Fe(III)-containing mackinawite.