Raman anisotropy in serpentine minerals, with a caveat on identification

Abstract

The serpentine minerals lizardite, polyhedral serpentine, chrysotile, antigorite and 15-sector and 30-sector polygonal serpentine have been studied by micro-Raman spectroscopy, using selected samples, that had been previously characterized. The appropriate crystal orientations were determined by optical microscopy of petrographic sections. Oriented spectra, obtained using Nd-YAG green laser radiation (532 nm), were deconvolved, extracting wavenumber and intensity values for the peaks, possibly overlapping and forming complex bands. Relevant Raman anisotropy is common and relevant in serpentine. Depending upon the orientation of the impinging laser, significant wavenumber shifts occur (up to 10 cm À1 , mostly in polyhedral serpentine and lizardite). Furthermore, also, important intensity variations (up to 1 order of magnitude) occur in polyhedral serpentine, lizardite, chrysotile and antigorite as well. On the one hand, the possibly characteristic peaks have been identified and discussed. On the other hand, caution is suggested as far as the micro-Raman characterization of polyphasic, variably oriented serpentine minerals is concerned.

The intracrystalline microstructure of Monte Fico lizardite, by optics, <i>μ</i>-Raman spectroscopy and TEM

“…As far as phase identification is concerned, we make reference again to Compagnoni et al (2021). In the case of lighter domains, the two sharp peaks at 3686 and 3705 cm −1 well match the values of 3688 and 3706 cm −1 previously reported for the ω-ω section of lizardite.…”

“…1e, right), being the one with better phase resolution. Furthermore, as shown by Compagnoni et al (2021), this orientation does not present anisotropic Raman effects. Therefore, a simpler interpretation may be expected.…”

“…During a µ-Raman study of serpentine minerals (Compagnoni et al, 2021), the careful microscopic examination of prismatic lizardite-1T crystals from the Monte Fico quarry, the island of Elba, showed "spongy" intracrystalline microstructure, especially evident in sections perpendicular to the c axis. This observation aroused renewed interest over a mineral occurrence largely studied after its first report (Mellini and Viti, 1994).…”

Crystallographic orientation mapping of lizardite serpentinite by Raman spectroscopy

“…Mellini and Viti (1994) and Viti and Mellini (1997) initially characterised samples of the Monte Fico lizardite vein by optical microscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Since then, the sample has been studied using a diverse range of approaches, including neutron diffraction (Gregorkiewitz et al, 1996), thermal analysis (Viti, 2010), Mössbauer and infrared spectroscopy (Fuchs et al, 1998;Trittschack et al, 2012), Raman spectroscopy (Rinaudo et al, 2003;Auzende et al, 2004;Trittschack et al, 2012;Capitani et al, 2021;Compagnoni et al, 2021), X-ray diffraction (Hilairet et al, 2006), and calorimetry (Gailhanou et al, 2018). Additionally, Monte Fico lizardite has been used as a starting material for several rock deformation experiments (Hirose et al, 2006;Viti andHirose, 2010, 2009;Amiguet et al, 2014;Tesei et al, 2018).…”

“…Recently, Compagnoni et al (2021) and Capitani et al (2021) reported a significant Raman anisotropy effect in the serpentine minerals lizardite and polyhedral serpentine, which results in wavenumber shifts depending on the orientation of the crystals relative to the impinging excitation laser. In particular, they noted a shift of more than 10 cm −1 in the position of the main band in the OH-stretching region for lizardite.…”