Asbestos Analysis in the TEM

Asbestos is a generic term used for two specific groups of mineral fibres that possess the common properties of having the ability to be spun into cloth that is both fire and acid resistant. It has also commonly been used as insulation materials in buildings and fillers in tiles and roofing materials. It was also widely used in the automotive industry for brake pads.

Asbestiform minerals can be divided into two main classes on the basis of their crystal structure. These are serpentine and amphiboles. The sole member of the serpentine group is chrysotile asbestos which is by far the most common asbestos mineral. There are five recognized asbestiform varieties of amphiboles. These are crocidolite, amosite, anthophyllite, tremolite and actinolite. Of these amphibole minerals only crocidolite and amosite can be classed as commonly used commercially although the others may be present as contaminant minerals in some cases.

Although most routine measurements are performed using polarized light or phase contrast optical microscopy, more in depth studies often involve the use of electron microscopy. The SEM has higher resolution than the optical microscope and also has the benefit of chemical analysis using EDS. However, in the majority of these studies TEM is the preferred route due to superior image and spectral resolution and the ability to obtain diffraction patterns from fibres which adds another dimension to identification of these minerals.

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Figure 1. Chrysotile, Mg₃Si₂O₅(OH)₄: This is a hydrated silicate high in Mg compared to most other forms of asbestos. Fibres tend to be very fine and often appear to be quite flexible.

Figure 2. Amosite, (Mg,Fe)₇Si₈O₂₂(OH)₂: An Mg and Fe rich silicate that usually contains minor concentrations of Mn. Fibres are straight and relatively inflexible.

Figure 3. Crocidolite, Na₂Fe₃²⁺Fe₂³⁺Si₈O₂₂(OH)₂: This silicate mineral is Fe rich and contains Na which is generally not found in other fibrous minerals. The fibres appear to be needle like in character and finer than amosite.

Although selected area electron diffraction can easily distinguish between chrysotile and amphibole minerals, the lattice parameters of the amphibole group are so similar that it is exceedingly difficult to tell them apart by diffraction alone. As can be seen, the spectra for different asbestos minerals are quite characteristic for each variety and the other fibrous type materials can usually be differentiated by chemistry.

These minerals are commonly identified from their structure as seen in the TEM and their chemistry. The chemical footprint of these and other associated minerals may be stored in a database for comparison with samples being analysed. Consequently INCAEnergy TEM with Spectrum Compare and Spectrum Matching is the perfect tool for the analysis of these minerals. Spectrum Compare can be used for a quick comparison of different fibre types by identifying elements present in each sample and comparing them on a best fit basis with those stored in the database. Inca Spectrum Matching is ideal for setting up a database of asbestos and associated minerals for quick and accurate analysis of samples collected for mineralogical analysis by matching them on a best fit basis with those stored in the stored database.

Figure 4. Spectrum Compare showing differences between various types of asbestos.

Figure 5. Spectrum Matching showing match indices for spectra in sample compared with those in stored database.

Although SEM analysis of asbestos fibres is less common, samples that are well dispersed may also be analysed using INCA Energy SEM software. There are SEM methods described by the Health and Safety Laboratory (UK Government) and VDI (The Association of German Engineers). SEM is also commonly used for the analysis of bulk materials for the detection of asbestos.

Anthophyllite, (Mg,Fe)₇Si₈O₂₂(OH)₂ (image courtesy USGS).

Tremolite-Actinolite asbestos, Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂

Conclusions

Asbestos identification is highly reliant on chemistry as other techniques such as selected area electron diffraction and morphology are insufficient for complete characterisation of these materials. This can be performed in both SEM and TEM using an INCAEnergy EDS system for mineral identification, mineral comparison and best fit matching using a custom database. The system may be tailored to the individual facility using SEM, TEM, STEM or a combination of the tools. Consequently, fibres may be characterised and counted under the electron beam and reliable results delivered in reasonable time. Quant results can also be obtained along with database matching to confirm that phases that may be present.