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.
REFERENCES