## 1 Introduction  
Identification and characterization of the minerals constituting rocks, soils or dispersed in different matrixes has been at all times object of study for researchers working in the earth sciences. Mineralogists in particular applied optical microscopy (OM), scanning electron microscopy coupled with electron dispersive spectroscopy (SEM/EDS), X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), etc., to obtain a complete characterization of the mineral phases. The normally used techniques often require preparation of the sample before analysis. Considering that the samples are often constituted by different crystalline phases, in variable amounts, which can be sometimes mixed with amorphous phases, it is obvious that the preparation of the samples for analysis may induce changes, loss of important information, and difficulties in the identification of phases in very low amounts. Micro-Raman spectroscopy, where an OM is coupled with the spectroscope, allows overcoming these inconveniences, mainly when OM is equipped with crossed nicols. In fact, the technique can be applied directly on the samples, in a non-invasive way, and directing the laser beam in areas selected by the OM and characterized by optical peculiarities, heterogeneity in the sample can be easily highlighted. Moreover, Raman spectroscopy, supplying information on the vibrational modes of the chemical bonds, allows studying also materials in amorphous, gaseous, or liquid states. Our research group operates in Alessandria, where the minerals regulated as \"asbestos\" have been, and are up till now, a serious problem consequent to the presence of an Eternit factory in Casale Monferrato, Alessandria (Italy), producing large quantities of asbestos-cement since the beginning of the last century until 1985 [1; 2]. The asbestos are fibrous minerals with a high impact on the human health. In fact, to the inhalation of the asbestos fibers has been ascribed the development of severe respiratory diseases, in particular mesothelioma, asbestos, and pulmonary carcinoma [3; 4]. Other malignancies affecting extra-respiratory systems have been recognized as related to inhaled/ingested asbestos fibers. In particular, IARC recognized that the larynx and ovary tumors can be related to asbestos exposure [5] and various researchers are developing studies to verify the relationships between gastrointestinal cancers (GI) and asbestos [6; 7; 8; 9; 10]. We have been therefore interested in the identification of a technique able to identify surely fibers of asbestos, even when they are present in very low amounts. On the mineralogical point of view, the minerals regulated as \"asbestos\" [5; 11; 12] are:  
* actinolite, theoretical chemical formula Ca\({}_{2}\)(Mg, Fe\({}^{2+}\))\({}_{5}\)[Si\({}_{8}\)O\({}_{22}\)](OH)\({}_{2}\);
* amosite, theoretical chemical formula (Fe\({}^{2+}\), Mg)\({}_{7}\)[Si\({}_{8}\)O\({}_{22}\)](OH)\({}_{2}\);
* anthophyllite, theoretical chemical formula (Mg, Fe\({}^{2+}\))\({}_{7}\)[Si\({}_{8}\)O\({}_{22}\)](OH)\({}_{2}\);
* crocidolite, theoretical chemical formula Na\({}_{2}\)Fe\({}^{2+}\)\({}_{7}\)Fe\({}^{3+}\)\({}_{2}\)[Si\({}_{8}\)O\({}_{22}\)](OH)\({}_{2}\);
* tremolite, theoretical chemical formula Ca\({}_{2}\)Mg\({}_{5}\)[Si\({}_{8}\)O\({}_{22}\)](OH)\({}_{2}\),  
when they exhibit fibrous habit. All these phases belong to amphibole group of silicates, where double chains of [SiO\({}_{4}\)] tetrahedra are each other linked by Na, Ca, Mg, Fe\({}^{2+}\), or Fe\({}^{3+}\) cations, Figure 1a [12; 13]. Being phases growing in a natural system, then with complex chemical composition of the mother solutions, very often homo- and etero-valent diadocly in the different crystallographic sites may occur. The real chemical formula therefore can differ from the theoretical one. As an example, in tremolite iron is not foreseen but substitutions of Mg by Fe\({}^{2+}\) in the octahedral sites can be frequently observed.