Raman anisotropy in serpentine minerals, with a caveat on identification

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−1, 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. Field identification of serpentine minerals often fails. These millimetric greenish spheres (actually, polyhedral serpentine from Gew Graze, Cornwall) may be confused with prehnite. More serious problems occur with fibrous minerals looking like chrysotile asbestos, but being the less harmful palygorskite. In these cases, even a portable Raman is helpful. In the lab, micro‐Raman equipment recognizes the phase and reveals subtle features, such as anisotropy, that modifies both peaks wavenumber and intensities.