Coupling SEM-EDS and confocal Raman-in-SEM imaging: A new method for identification and 3D morphology of asbestos-like fibers in a mineral matrix

Abstract

Asbestos consists in natural minerals crystallized in a specific habit and possessing in particular properties. In the case of Naturally Occurring Asbestos, usual methods applied to the identification of mineral fibers and the determination of their possible asbestiform nature seems not efficient, especially in the case of mineral fibers included in mineral matrix. We present a new in-situ method based on the use of confocal Raman-in-SEM imaging implemented in a Scanning Electron Microscope as an efficient method for in-situ mineralogy. The limitation of conventional methods is discussed. We applied 2D-Raman imaging to the identification of sub-micrometric fibers included in different mineral matrix. We were able to identify actinolite fibers down to 400 nm in diameter, included in feldspar, quartz and/or calcite matrix. Moreover, Confocal Raman allows the collection of 3D data that would provide access to critical information on the morphology of the amphibole fibers in the volume, such as aspect ratio, fibers distribution and amphibole volume fraction. We performed this method on various examples of rocks containing actinolite fibers of mean structural formula is: Na_0,04-0,12 Mg_2,79-3,73 Al_0,29-0,58 K_0,01 Ca_1,79-1,98 Mn_0,01-0,09 Fe2+_0,99-1,91 Fe3+Si_7,64-7,73 O₂₂(OH)₂. We demonstrated that coupling confocal Raman imaging and SEM is a new and efficient in-situ method for identification and morphological characterization of amphibole fibers.

Keywords: 3D analysis; Fibrous amphiboles; In-situ asbestos diagnosis; Naturally occurring asbestos; Raman-in-SEM imaging.