# Portable Raman Spectrometer for In Situ Analysis of Asbestos and Fibrous Minerals
## 3 Results and Discussion
### Evaluation of the Performance of pRS  
Raman spectra recorded with pRS were compared with spectra obtained with bench-top high-resolution micro-Raman spectrometer (HRS). A comparison of different working conditions is reported in Table 2. HRS was used on petrographic polished thin cross-sections, optimizing the working conditions, such as spot analysis, objective, filter, time, and number of acquisitions, to obtain the best spectra virtually achievable with the given instrument. It is worth noting that pRS used for this work does not allow for fine-tuning the instrumental parameters, but the time of acquisition only. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Mining Site** & \multicolumn{3}{c}{**Spectral Quality \({}^{*}\)**} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\ \hline  & & **Tr** & **Ctl** & \multicolumn{2}{c}{**Atg**} & \multicolumn{2}{c}{**Lz**} & **Blg** & \multicolumn{2}{c}{**Di**} & \multicolumn{2}{c}{**Chi**} & **Sap** & **OI** & **Grt** & \multicolumn{2}{c}{**Mixed Phases**} \\ \hline Valmalenco & 90\% & & 6 & 2 & & & & & & 1 & \\ Balangero & 84\% & & 6 & 8 & 3 & 3 & 5 & 3 & 1 & & 3 \\ Tontouta & 70\% & 3 & 5 & 11 & & & & & & 2 & & 1 \\ \hline \multicolumn{10}{l}{\({}^{*}\) Spectral quality = number of analyses with sufficient quality to distinguish mineral phases/number of total analyses performed \(\times\) 100.} \\ \multicolumn{10}{l}{Abbreviations: \(\text{Tr}\), tremolite; Ctl, chrysotile; Atg, antigorite; Lz, lizardite; Blg, balangeroite; Di, diopside; Chl, chlorite; Sap, saponite; Ol, olivine; Grt, garnet.} \\ \end{tabular} \\ \end{table} 
Table 1: Overview of the Raman spectra performed at the exploitation sites of Valmalenco (central Alps, Italy), Balangero (western Alps, Italy), and Tontouta (Massif du Sud, New Caledonia). Portable Raman spectrometer (pRS) allowed the identification of asbestos and asbestos-like minerals in the low- to highly-altered peridotite matrix. Raman spectra of non-asbestos or asbestos-like minerals are reported in Supplementary Material S3. Figure 3: Estimation of performance of portable Raman on differently cohesive, from massive to friable-powdered, asbestos-bearing veins from Tontouta Ni- open pit mine. (**a**) antigorite; (**b**) tremolite. Raman spectra were acquired in the extended spectral region (100-4000 cm\({}^{-1}\)). In Figure 4, spectra of chrysotile, fibrous tremolite, fibrous antigorite, and balangeroite collected with pRS and HRS are compared. Despite obvious differences in signal-to-noise ratio and spectral resolution (8 cm\({}^{-1}\) and 4 cm\({}^{-1}\), respectively), Raman spectra showed very similar profiles at both low- (100-1800 cm\({}^{-1}\)) and high-wavenumber (3500-3800 cm\({}^{-1}\)) regions. The presence of the diagnostic features in the OH-stretching region (3500-3800 cm\({}^{-1}\)), with the main peaks located at about 3698 cm\({}^{-1}\) for chrysotile, 3668 and 3695 cm\({}^{-1}\) for antigorite [33], 3672 cm\({}^{-1}\) for tremolite [54], and 3670 cm\({}^{-1}\) for balangeroite [55], allowed the discrimination among fibrous mineral phases easily. The OH stretching peaks in high wavenumbers were often partially covered by fluorescence interference. The annoying effect of fluorescence was particularly evident in spectra obtained with pRS for tremolite samples (Figures 2b and 4c). Nevertheless, the identification of the asbestos-tremolite variety as the predominant phase was not compromised due to the well-resolved low-frequency pattern. Due to the intimately mixed nature of asbestos-type occurrences, even at the sub-micron scale, some level of contamination due to the simultaneous interaction of the laser beam with crystals of several mineral phases is likely to occur. Obtaining informative spectra on mixed-mineral varieties is rather challenging.