Asbestos samples from insulation delagging operations are analysed,
by light microscopy techniques, in their thousands in the U.K.
each year. The Health and Safety Executive has produced a new
publication to assist analysts (HSE, 1994). This draws together
commonly used mineralogical techniques into one systematic method
complete (for the first time in this series) with colour photographs.
Any laboratory performing analyses using validated methods should have available reference material for comparison and calibration purposes. Certified reference standards (CRSs), preferably traceable to primary standards, are essential for analytical determinations. It is important that these CRSs have been produced and characterized in a technically valid manner, and that details of homogeneity trials, for example, used in certifications should be provided to confirm their acceptability.
Producing reference material for asbestos is not straightforward, however. Asbestos is a term used for the fibrous forms of several naturally occurring silicate minerals. For regulatory purposes in the U.K., the Control of Asbestos at Work Regulations (CAWR) 1987, as amended by the CAWR (Amendment) regulations, 1992, define asbestos as any of the minerals chrysotile, crocidolite, amosite, fibrous anthophyllite, fibrous actinolite or fibrous tremolite. Natural occurrences of these minerals are found throughout the world, and there may be compositional variations in some of the types that lead to slightly different optical properties. The first three have had widespread commercial use, are much more commonly
encountered than the others and there is general consensus that recently mined products are representative of the type. Chrysotile asbestos is defined as a fibrous mineral in the serpentine mineral group, while crocidolite asbestos is the fibrous variety of riebeckite. Amosite is really the trade name of fibrous grunerite. Distinguishing between the asbestos varieties of anthophyllite and tremolite can be difficult as they have similar optical properties. Also all asbestos amphiboles occur frequently as prismatic non-asbestos varieties. These can be fractured along cleavage planes producing fragments which resemble asbestos fibres. Actinolite asbestos does have more distinct optical properties, but is seldom seen in large concentrations in commercial materials.
The samples for the HSE reference material sets were obtained from various sources around the world and the samples homogenized, characterized and validated by the Institute of Occupational Medicine (IOM, 1994). The sets of asbestos reference standards (together with documentation) are currently available free from the address below. There is a small charge to cover the cost of packaging and despatch. An additional charge is made for laboratories outside of the U.K.
The use of these reference sets should reduce the time which a laboratory needs to characterize their own reference material, possibly using more expensive techniques such as X-ray diffraction. They are also excellent for training purposes and were used to illustrate identification in HSE's recent publication (HSE, 1994). It is hoped these samples will become an important part of the of the laboratory's quality system.
Observations and properties pertaining to the minerals which may aid the analyst are given below, and Table 1 lists the main optical properties of the asbestos types.
The HSE Asbestos Reference Minerals arc available from The Institute of Occupational Medicine, 8 Roxburgh Place, Edinburgh, U.K., Tel. 0131-667 5131.
DESCRIPTIONS OF THE PROPERTIES OF THE REFERENCE MATERIALS
Sample 1: chrysotile (source-Cassiar McDame Mountain, British Columbia, Canada)
This sample is typical of the chrysotile fibre used in a large variety of products, from textiles to lagging materials. The fibres are long and appear white, curly and with silky lustre. The sample is composed of 99% chrysotile, the remainder being a small amount of irregular shaped quartz (low birefringence, refractive index close to chrysotile) and opaque iron oxide particles.
Sample 2: chrysotile (source-Shabani, Zimbabwe)
The fibres in this sample are white and appear curly and silky. This material is slightly different from Sample 1 as it contains small amounts of pleochroic chrysotile, which changes from pale brown to dark brown, but otherwise exhibits all of the optical properties of chrysotile. It is 98.5% pure, with small amounts of quartz and iron oxide particles.
Sample 3: amosite (source-Penge Mine, South Africa)
This sample is typical of amosite seen in lagging material. In appearance it is long, tough and flexible, with needle-like fibres having a vitreous lustre. It is fairly homogeneous in composition and weakly pleochroic (which is rarely visible) with some fibres changing from pale brown to colourless. There are also a few quartz and graphite particles present.
Sample 4: crocidolite (source-Coretsi Mine, Kuruman in Griqualand, South Africa)
The crocidolite in this sample is very homogeneous, and typical of that seen in lagging material with the fibres appearing long and fine, with a silky bluish-grey lustre. On examination they exhibit strong pleochroism, from blue-grey to a deep blue colour. The fibres are occasionally intergrown with fine grains of opaque iron oxides and yellow-brown grains of iron hydroxides. Small irregular quartz particles can also be seen.
Sample 5: tremolite asbestos (source-Saltworks Mine, Death Valley, San Bernadino County, California, U.S.A.)
The fibres in this sample are fine and white, with a silky lustre. On examination by light microscopy they are colourless and non-pleochroic and exhibit parallel extinction. Tremolite crystal cleavage fragments are present which exhibit a range of extinction angles, depending on their orientation between crossed polars. There are also cleavage rhombs of calcite showing high birefringence and ribbon shaped talc fibres with the higher refractive indices close to 1.59.
Sample 6: actinolite asbestos (source-Devon, U.K.)
The actinolite fibres appear fine, long and have a greenish colour and a silky lustre. Under light microscopy the fibres in this sample are mainly pale green to bluish-green and weakly pleochroic. They exhibit parallel extinction between crossed polars. Some of the actinolite fibres are altered to a brown colour. There are also prismatic cleavage fragments of actinolite displaying oblique extinction angles up to 20°. A few organic fibres have been detected.
Sample 7: anthophyllite asbestos (source-Karelia, Finland)
These fibres appear of moderate length and are grey-white with a silky lustre. On examination by light microscopy they are colourless, non-pleochroic and display parallel extinction. The sample also contains talc fibres, ribbon-like and kinked, with a refractive index close to 1.59. Platey talc and chlorite crystal (refractive index 1.56-1.60) can also be seen.