Composition of Amphiboles in the Tremolite–Ferro–Actinolite Series by Raman Spectroscopy

Amphiboles are an important family of rock forming minerals, whose identification is crucial in provenance studies as well as in many other fields of geology, archaeology and environmental sciences. This study is aimed to find a quick way to characterize Ca-amphiboles in the tremolite (Ca2Mg5Si8O22(OH)2)–ferro–actinolite (Ca2Fe5Si8O22(OH)2) series. Raman spectroscopy is established as technique to perform non-destructive and quick analysis, with micrometric resolution, able to give the composition in terms of Mg/(Mg + Fe2+) ratio. To exploit the method, a preliminary characterization is performed by Scanning Electron Microscopy coupled with Energy-dispersed X-ray Spectroscopy (SEM-EDS). Two independent methods to evaluate the composition from the Raman data (aiming to an accuracy of about 5%), using the low-wavenumbers part of the spectrum and the OH stretching bands, are developed. The application of the proposed method to micro-Raman mappings and the possible use of handheld Raman spectroscopy to have compositional information on Ca-amphiboles are discussed.