Raman spectroscopy of anhydrous and hydrated aluminum sulfates: Experience from burning coal heaps

Abstract

Hydrated aluminum‐rich sulfates belong to less widespread secondary minerals on Mars, which are probably connected with hydrothermal alterations. On Earth, such sulfates result from acidic weathering of aluminosilicates, and their formation is controlled by factors such as pH, temperature, or water activity. Physical–chemical conditions can be reconstructed if specific sulfate phases are detected, and therefore, the investigation of sulfate assemblages in high‐temperature settings is important for planetology and exobiology research. Raman spectroscopy is a powerful analytical tool for the discrimination of sulfates, and the degree of hydration, which is a sensitive marker of temperature conditions, can be tracked using this method. However, spectral similarities of sulfates and metastability and rapid transformations may hinder their correct identification. We take advantage of in situ and laboratory Raman spectroscopy to characterize uncommon anhydrous and hydrated Al‐NH4 sulfate mineralogy which forms under elevated temperature at the Anna burning coal waste dump, Alsdorf, Germany, and can be considered as analogous to fumarolic environments. We detected a suite of hydrated Al sulfates (e.g. alunogen, khademite, and tschermigite) in the medium‐temperature zones (~50°C). To minimize possible rehydration processes, we deployed two miniaturized Raman spectrometers (532 and 1064 nm) for field investigations in the high‐temperature zone (~130°C to 150°C). Mixtures of anhydrous (godovikovite and millosevichite) and hydrated phases along with intermediate phases were detected in the field as well as in the laboratory. Such observations are consistent with the premise that the degree hydration of sulfates increase with decreasing temperature. These results confirm that Raman spectroscopy is, despite several analytical challenges, capable of distinguishing Al sulfates, and their hydration states, within complex aggregates and crusts, and future applications in planetary research are promising.

Atmospheric aerosol particles before, during, and after a convective dust event on the Jornada experimental range as measured using Raman hyperspectral imaging

“…However, gypsum and sodium sulfate present in the same RS would be consistent with particle transport from the Lake Lucero/White Sands area, a source of sodium sulfate and gypsum 13,76 . A tertiary explanation would be that the peak is an aluminum sulfate, which can have a SO 4 − symmetric stretch in the region described 77 . The tape used as a substrate is aluminum coated Mylar, and so a sufficiently acidic sulfate drop 78 could potentially react with the aluminum to form an aluminum sulfate or similar.…”

Evaluation of carbonification of coals using a portable Raman spectrometer

“…Miniaturized (portable and handheld) Raman spectrometers presently have a good record of applications in the fields of geology: studies involving minerals from different groups, [ 38–42 ] sulfate mineralogy, [ 43–49 ] arsenate mineralogy [ 50 ] and organic minerals. [ 51,52 ] These applications cover a wide range of samples or environments and illustrate that portable Raman instrumentation is a unique method for field studies where the stability of samples can be a factor, or their in situ distribution is to be documented.…”