Characterization of the sulphate mineral amarantite - Fe23+(SO4)O•7H2O using infrared, Raman spectroscopy and thermogravimetry

The mineral amarantite Fe-2(3+) (SO4)O center dot 7H(2)O 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 degrees C. The Raman spectrum of amarantite is dominated by an intense band at 1017 cm(-1) assigned to the SO42- v(1) symmetric stretching mode. Raman bands at 1039, 1054, 1098, 1131, 1195 and 1233 cm(-1) are attributed to the SO42- v(3) 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 v(2) bending modes. A series of low intensity Raman bands are found at 543, 602, 622 and 650 cm(-1) are assigned to the v(4) 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. (C) 2013 Elsevier B.V. All rights reserved.