Characterization of the sulphate mineral amarantite – [formula omitted] using infrared, Raman spectroscopy and thermogravimetry

Abstract

The mineral amarantite Fe23+(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 1017cm−1 assigned to the SO42-ν1 symmetric stretching mode. Raman bands at 1039, 1054, 1098, 1131, 1195 and 1233cm−1 are attributed to the SO42-ν3 antisymmetric stretching modes. Very intense Raman band is observed at 409cm−1 with shoulder bands at 399, 451 and 491cm−1 are assigned to the ν2 bending modes. A series of low intensity Raman bands are found at 543, 602, 622 and 650cm−1 are assigned to the ν4 bending modes. A very sharp Raman band at 3529cm−1 is assigned to the stretching vibration of OH units. A series of Raman bands observed at 3025, 3089, 3227, 3340, 3401 and 3480cm−1 are assigned to water bands. Vibrational spectroscopy enables aspects of the molecular structure of the mineral amarantite to be ascertained.