Distinguishing the Raman spectrum of polygonal serpentine

Positively identifying serpentine mineral types is important for a wide range of disciplines in the Earth sciences and health sciences. Although Raman spectroscopy has been widely applied as a tool to distinguish four of the main serpentine minerals (i.e., antigorite, lizardite, chrysotile, and polygonal serpentine), some uncertainty remains as to whether all four varieties have unique Raman spectra. In this paper, submicron Raman spectroscopy mapping was performed directly on electron-transparent regions of serpentine samples that were unambiguously identified by transmission electron microscopy (TEM). The increased spatial resolution of the Raman mapping technique (~370 nm), combined with the detailed characterization provided by TEM, indicates that polygonal serpentine has the same Raman spectrum as lizardite and therefore cannot be spectrally distinguished from lizardite. Furthermore, the Raman spectral profile that has previously been reported as unique to polygonal serpentine is likely to represent a mixture of chrysotile and polygonal serpentine/lizardite. To positively discriminate between lizardite and polygonal serpentine requires TEM investigation.