Abstract
Alunogen (Al(2)(SO4)(3).17H2O) is a common secondary mineral in the terrestrial environment (acid mine drainage, volcanic or coal-fire fumaroles), and is also formed through the acidic weathering of aluminosilicates. Moreover, alunogen has been suggested as a part of the Al-bearing deposits on Mars.
The identification of alunogen in secondary sulfate mixtures by Raman spectroscopy strictly depends on good knowledge of alunogen spectral features and band positions. However, comprehensive Raman data of alunogen of natural origin are lacking.
This study reports on Raman spectra obtained from two natural specimens originating from a burning coal dump at the Schoeller mine, Kladno, Czech Republic, along with the additional characterizations by infrared spectroscopy, X-ray diffraction, and electron microprobe. For comparison purposes, a Raman spectrum of a synthetic analogue was also obtained.
The studied specimens have (Al(1,99)Fe(0.01)(3+))(2) (SO4)(3).17H2O as their calculated empirical formula, and the structural parameters correspond to the previously reported data for alunogen. Both natural specimens and the synthetic analogue showed uniform Raman spectra with no extensive band splitting in the sulfate vibrational regions.
The most intensive Raman band associated with the symmetric stretching vibration of the SO4 tetrahedra (ν(1)) is located at 992 cm(-1). A multicomponent band was observable in the characteristic region for OH-related vibrations.
A small variation in the spectral intensity of the hydroxyl bands suggests that the studied specimens could possibly be slightly dehydrated.