Applying portable Raman spectrometers for field discrimination of sulfates: Training for successful extraterrestrial detection

Abstract

The presence of sulfates containing Mg, Fe, and Ca (in several hydration states) has been confirmed on Mars from data acquired by both orbital and in‐situ measurements. It has been suggested that the sulfates on Mars originated deep the past from the evaporation of water at accumulation points on the Martian surface. Consequently, they can be seen as highly relevant to any search for the presence of extraterrestrial life. Miniaturized Raman spectrometers (which permit analysis of the mineralogical and molecular composition of a specimen) will be included in rovers on future planetary exploration missions. Portable miniature Raman spectrometers operating in the near‐infrared (785 nm) and in the green (532 nm, currently selected for the ExoMars mission) were used on‐site to estimate the mineralogical compositions of sulfate weathering crusts at Valachov (Bohemian Massif, Czech Republic). Spectra of sufficient quality were quickly obtained, and the characteristic shifts of the sulfate ν1 band of the minerals allowed for the discrimination of several sulfates. Superficial crusts contained gypsum, jarosite, rozenite, fibroferrite, magnesiocopiapite, and melanterite as major components. The current testing has confirmed the favorable performance of handheld instruments for the discrimination of structurally similar sulfates of relevance for studies on Mars. Copyright © 2017 John Wiley & Sons, Ltd.

Raman spectroscopic study of six synthetic anhydrous sulfates relevant to the mineralogy of fumaroles

“…The Raman spectrum of synthetic sabieite is very similar to the Raman spectrum of sabieite that was collected at burning dump Heřmanice, Silesia, Czech Republic, although some differences can be found (compare in Table S2). The Raman spectrum of steklite obtained is in excellent agreement with the previously reported Raman data for this phase …”

“…Portable Raman instruments are expected to be very useful in the study of metastable fumarolic encrustations . Commercially available portable spectrometers are of sufficiently good performance that it is possible to discriminate the fundamental vibration modes of the sulfate tetrahedron and thus to identify common sulfates . However, the use of these spectrometers faces several issues connected with the reading and interpretation of the Raman spectra obtained; some of these are addressed below.…”

“…Despite a moderate fluorescence background of the 785 nm instrument, all major Raman bands associated with the sulfate group or M–O vibrations are well developed and can be easily distinguished when baseline correction is applied (see Figures and ). In particular, the Raman spectra of anhydrous alums are nearly equivalent, although it should be emphasized that these data were obtained while measuring artificial phases under ideal conditions and results may differ when analyzing natural samples …”

“…It has been documented from Czerwionka‐Debiensko, Poland; and from the Anna mine dump, Alsdorf, Germany; however, it is often found as an admixture in godovikovite crusts. Powdery coatings of nearly pure sabieite were also discovered at the Heřmanice dump, Czech Republic . At the site of its original discovery, it is formed by the dehydration of lonecreekite; whereas sabieite, associated with a high‐temperature burning process, probably crystallizes from interactions between the solid and gaseous phases.…”

“…Moreover, mixtures of hydrous and/or anhydrous minerals often occur as a result of fumarolic processes. Our previous in situ study of encrustations at the burning Heřmanice dump, Ostrava, Czech Republic, showed that the interpretation of mineral compositions of sulfate mixtures by Raman spectroscopy can be challenging . Therefore, knowledge of the Raman spectra of the expected phases is essential for discrimination and recognition of the individual minerals in fumarole encrustations.…”