Raman spectroscopy of halotrichite from Jaroso, Spain

Raman spectroscopy complimented with infrared ATR spectroscopy has been used to characterise a halotrichite FeSO4·Al2(SO4)3·22H2O from The Jaroso Ravine, Aquilas, Spain. Halotrichites form a continuous solid solution series with pickingerite and chemical analysis shows that the jarosite contains 6% Mg2+. Halotrichite is characterised by four infrared bands at 3569.5, 3485.7, 3371.4 and 3239.0 cm−1. Using Libowitsky type relationships, hydrogen bond distances of 3.08, 2.876, 2.780 and 2.718 Å were determined. Two intense Raman bands are observed at 987.7 and 984.4 cm−1 and are assigned to the ν1 symmetric stretching vibrations of the sulphate bonded to the Fe2+ and the water units in the structure. Three sulphate bands are observed at 77 K at 1000.0, 991.3 and 985.0 cm−1 suggesting further differentiation of the sulphate units. Raman spectrum of the ν2 and ν4 region of halotrichite at 298 K shows two bands at 445.1 and 466.9 cm−1, and 624.2 and 605.5 cm−1, respectively, confirming the reduction of symmetry of the sulphate in halotrichite.