Analysis of serpentine polymorphs in investigations of natural occurrences of asbestos

Our plain-English summary of Analysis of serpentine polymorphs in investigations of natural occurrences of asbestos, by J. Wagner (Environmental Science. Processes & Impacts, 2015).

The study

• Author: J. Wagner
• Journal: Environmental Science. Processes & Impacts
• Published: 2015;17(5):985-96
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

This work investigates potential analytical variability in environmental investigations of natural occurrences of asbestos (NOA) due to intergrown serpentine minerals. Franciscan complex and serpentinite rock samples were obtained from likely NOA sites in coastal Northern California with geographic information system (GIS) maps, then analyzed using polarized light microscopy (PLM), transmission electron microscopy with energy-dispersive X-ray analysis and selected area electron diffraction (TEM/SAED/EDS), and environmental scanning electron microscopy with EDS (ESEM/EDS). Non-asbestos serpentine fibers were superficially similar to chrysotile but were differentiated quickly using TEM morphology criteria and reference SAED overlays. 94 NOA fibers were classified as asbestiform chrysotile (62%), polygonal serpentine (34%), lizardite scrolls (2%), and lizardite laths (2%). Chrysotile fibril widths (mean = 42 nm) were significantly different from those of polygonal serpentine and lizardite laths (167 and 505 nm, respectively), but not lizardite scrolls (37 nm). Due to differing preparations and microscope resolutions, TEM analyses investigated a distinct, smaller population of particles (0.01-10 μm) than did PLM analyses (10-100 μm). A higher proportion of asbestiform phases in the finer fraction could potentially bias TEM bulk percent asbestos determinations. ESEM/EDS of intermediate particle size ranges revealed 20-200 μm, elongated particles with intermixed asbestiform and non-asbestiform structures on their surfaces. These particles were too thick and complex to be resolved by PLM, and too massive to be detected by TEM. These large particles are likely to exist in samples prepared by mechanical crushing or grinding, but are not likely to be generated by "releasable asbestos" methods.

In plain English

This was a laboratory study of rock, not of people. It examined how microscope methods cope with naturally occurring asbestos (NOA) when chrysotile is mixed with similar-looking serpentine minerals. Three small serpentinite and Franciscan complex rocks from two sites in the Northern California coastal range were examined by polarized light microscopy (PLM), transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM). Of 94 fibers classified by TEM, 62% were chrysotile, 34% polygonal serpentine, 2% lizardite scrolls and 2% lizardite laths. Chrysotile fibrils averaged 42 nm wide, significantly narrower than polygonal serpentine (167 nm). Lizardite scrolls (37 nm) were not significantly different from chrysotile. TEM preparations held mostly much smaller particles than PLM preparations. ESEM showed large, elongated particles with mixed asbestiform and non-asbestiform structures on their surfaces that neither PLM nor TEM could resolve. The author concluded that non-asbestos serpentine fibers could be told apart from chrysotile quickly with TEM. The author also suggested that, because TEM preparations under-represented larger, non-asbestiform particles, TEM could overestimate asbestos mass percentages in bulk aggregate analyses. Sampling was deliberately non-representative.

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 mineralogical study comparing analytical methods (PLM, TEM with electron diffraction and X-ray analysis, and ESEM) on rock subsamples deliberately taken from fibrous areas
• Who was studied: Three small rocks (2 to 4 cm across), Rocks A and B from Location 1 and Rock C from Location 2. The rocks were serpentinite and Franciscan complex (mélange) from the Northern California coastal range. In total, 94 fibers were examined by TEM. No sampling dates are given.
• Exposure: No human exposure was studied. The material was natural occurrences of asbestos in serpentinite and Franciscan complex rock, specifically serpentine minerals: chrysotile, polygonal serpentine, lizardite and antigorite (antigorite was looked for but not found).
• Main results:
  • Of 94 NOA fibers examined by TEM, 62% were classified as asbestiform chrysotile, 34% as polygonal serpentine, 2% as lizardite scrolls and 2% as lizardite laths.
  • No antigorite fibers were identified.
  • Mean fibril width was 42 nm for chrysotile and 167 nm for polygonal serpentine, a statistically significant difference. The two width distributions overlapped at about 90 nm.
  • Lizardite scrolls (mean width 37 nm) were not significantly different in width from chrysotile (42 nm; p = 0.6), which the author said confirmed that the two can be confused.
  • Lizardite laths were non-asbestiform and significantly wider than chrysotile (range 110 to 900 nm).
  • Reference electron diffraction overlays gave good matches to the diffraction patterns measured from the NOA fibers.
  • Chrysotile and polygonal serpentine fibers had significantly different iron-to-silicon ratios (0.08 and 0.22, respectively, p less than 0.05).
  • In TEM preparations, most structures measured 10 to 1000 nm wide, with only a few large particles 10 to 20 micrometres wide. The most prominent particles in PLM preparations were 100 to 300 micrometres.
  • ESEM showed complex, elongated particles, typically 10 to 25 micrometres wide, carrying both asbestiform and non-asbestiform structures. These particles resembled fibers that could not be resolved by PLM or TEM.
  • Measured diffraction row spacings deviated by up to 40% from the expected value. Adjusting the microscope stage tilt brought them back to the expected value.
  • The author suggested that the scarcity of larger, non-asbestiform particles on TEM grids could lead to overestimates of asbestos mass percentages in aggregate analyses by TEM.
• The authors' conclusion: The author concluded that non-asbestos serpentine fibers (polygonal serpentine, lizardite laths and scrolls), though superficially similar, can be differentiated quickly from chrysotile using TEM morphology, fiber widths and reference diffraction overlays. The author also concluded that TEM and PLM looked at different particle populations, and that a higher asbestos percentage in the fine fraction could bias TEM bulk percent asbestos results.
• Limitations the authors state:
  • Sampling was non-representative (fibrous areas were targeted on purpose), so the true prevalence of polygonal serpentine is uncertain.
  • Only two lizardite laths were measured, giving low statistical power.
  • SEM/EDS could not identify mineral phases.
  • Manual grinding was used. Other pulverization procedures might break down large particles more efficiently.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Serpentine Group, Polymorphs, Serpentinite, Polarized Light Microscopy (PLM), Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED), Scanning Electron Microscopy (SEM), Chrysotile.

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