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

Our plain-English summary of 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, by G. Wille, D. Lahondere, U. Schmidt, et al (Journal of Hazardous Materials, 2019).

The study

• Authors: G. Wille, D. Lahondere, U. Schmidt, J. Duron, X. Bourrat
• Journal: Journal of Hazardous Materials
• Published: 2019;374:447-458
• Cited by: 6 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

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: Na0,04-0,12 Mg2,79-3,73 Al0,29-0,58 K0,01 Ca1,79-1,98 Mn0,01-0,09 Fe2+0,99-1,91 Fe3+Si7,64-7,73 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.

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In plain English

This laboratory methods study, by researchers in France, tested a way to identify very thin mineral fibers while still inside rock. The authors examined three rock samples from France, prepared as polished thin sections without grinding. They combined standard microscopy and chemical analysis with Raman imaging inside a scanning electron microscope, in flat (2D) and depth (3D) views. Actinolite, an amphibole, was present in all three samples. Conventional microprobe analysis of a thin fiber gave abnormal readings. Raman imaging identified actinolite fibers as thin as about 400 nm in diameter. In 3D, fibers in sample 2 appeared to be bundles, and the thinnest appeared asbestiform. Sample 3 showed fibers below the surface. In one 3D-imaged area of sample 1, most fibers looked wide. The authors thought these more likely elongated crystals than compact fiber bundles; if so, another technique would be needed. The authors concluded that the combined system allows an accurate in-situ diagnosis of rocks containing amphiboles. They noted that 3D imaging is limited by how transparent the surrounding mineral is to the laser. No people or health effects were studied.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory methods study (mineralogical analysis of rock samples) testing coupled scanning electron microscopy with chemical analysis (SEM-EDS) and confocal Raman imaging inside the SEM (RISE), in 2D and 3D, alongside polarized light microscopy and electron microprobe (EPMA)
• Who was studied: Three rock samples collected in France: two from the southern part of the Armorican massif (an amphibolite and a hydrothermally altered basalt) and one from the southwestern Massif Central (a dolerite), each cut by mineral veins. They were prepared as polished geological thin sections. No collection dates were given.
• Exposure: Fibrous amphiboles embedded in a mineral matrix, mainly actinolite (found in all three samples), with magnesio-hornblende and magnesio-ferri-hornblende mainly in sample 1. No human exposure was studied.
• Main results:
  • By EPMA, the analyzed amphibole fibers larger than 3 μm were classified as calcic amphiboles.
  • Actinolite fibers in sample 1 were iron-rich and magnesium-poor (iron 15.9–17.8 wt % and magnesium 12.5–13.1 wt %, respectively).
  • Actinolite fibers in samples 2 and 3 had lower iron (9.8–12.6 wt %) and higher magnesium (15.9–18.3 wt %).
  • In sample 1, EPMA identified a thick fiber (Xa) as actinolite. A thin fiber (Xf) gave abnormal results, so actinolite could not be identified in it, or only with great difficulty.
  • 2D Raman imaging of an area of sample 1 showed actinolite fibers in an albite matrix, ranging from several μm in diameter down to about 400 nm.
  • Sample 2 had fine fibers in a matrix identified as orthoclase, with calcite present. Sample 3 had thin fibers in a quartz matrix with calcite particles.
  • In 3D, the actinolite fibers in sample 2 appeared elongate with a high aspect ratio. The authors took this as confirming fiber bundles, the thinnest appearing asbestiform.
  • In 3D, sample 3 showed numerous thin fibers with no preferred orientation, including fibers and fiber bundles below the surface that were not visible at the surface.
  • In the 3D view of the area of sample 1 that had been imaged in 2D, most fibers looked wide. The authors considered this more likely to reflect micrometric elongated crystals than a compact bundle of fine actinolite fibers.
• The authors' conclusion: The authors concluded that combining SEM-EDS with confocal Raman imaging in the SEM allows an accurate in-situ diagnosis of rocks containing amphiboles, including their mineral nature and whether their shape is asbestiform. They stated it can identify fibers thinner than 400 nm and can be done before any operation likely to put fibers into the air.
• Limitations the authors state:
  • 3D Raman imaging is limited mainly by how transparent the matrix is to the laser beam.
  • For the imaged area of sample 1, if the wide-looking features are elongated crystals rather than compact bundles of fine fibers, a different technique (such as SEM-FIB or TEM) is required to make a determination.

Topics

Medical terms in this study, explained in our Lexicon: Habit, naturally occurring asbestos (NOA), Mineral Fiber, Asbestiform, Mineralogy, Actinolite, Amphibole Group, Aspect Ratio (Length-to-diameter ratio).

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