Raman spectroscopic study of six synthetic anhydrous sulfates relevant to the mineralogy of fumaroles
Abstract
Fumaroles, vents that emit hot gases and vapor, are an accompanying phenomenon of volcanic activity. Such phenomena are also observed within the framework of self-ignited burning coal seams and coal heap fires, if less commonly.
The high temperatures and chemical reactions between the gas and solid phases are responsible for extensive alteration of surrounding rocks, resulting in mineral encrustations of unusual compositions. Rare anhydrous sulfates (millosevichite, mikasaite, efremovite, godovikovite, sabieite, and steklite) are signature minerals of fumarole encrustations.
Comprehensive Raman data for these anhydrous phases are required for the successful identification of natural samples by Raman spectroscopy. Six synthetic equivalents of the natural anhydrous sulfates were prepared by heating of the hydrated analogues and were investigated using both a bench-top Raman microspectrometer and a portable Raman spectrometer.
This comparative approach can help further steps for the successful deployments of miniature Raman tools in situ under field conditions. The studied anhydrous sulfates displayed distinctive Raman spectra of their crystalline phases.
Compared with their fully hydrated counterparts, a shifting of bands of the (1) symmetric stretching mode was observed in the Raman spectra of all samples. Isostructural millosevichite and mikasaite have very distinctive Raman spectra; however, structurally related godovikovite, sabieite, and steklite show very similar spectral shapes.
For ammonium-bearing phases (efremovite, godovikovite, sabieite), the Raman signatures of the NH4 group were observable at >2,800cm(-1) and in the 1,400-1,800cm(-1) region. Our measurements show that the performance of a light-weight portable Raman spectrometer with near infrared excitation was sufficient for the unambiguous discrimination of the investigated sulfates.