Serpentine is a polymorphous, magnesium-rich mineral typically found in metamorphic rocks. Its non-fibrous modifications are known as lizardite and antigorite, and the fibrous type is called chrysotile. Chrysotile fibers are formed by sheetlike crystals that roll into tight tubes and may exhibit several forms. The fibers may be compared to fine threads or hairs. Chrysotile usually occurs in bundles.
Curved, wavy and kinked shapes are common; some straight fibers and bundles may also occur. Splayed ends are common. Bundles of long, thin fibers can bend at angles more acute than 90 degrees without breaking, whether the material has been treated during a milling process or is derived directly from a serpentinite. Shorter fibers can be moderately brittle, and are generally thicker. Thick, straight sticks of chrysotile, produced during a milling process, may be found occasionally. These short fibers form bundles that can be considerably different in appearance from those of typical chrysotile. The crystal system of chrysotile is monoclinic and its optic sign is negative. Under plane light chrysotile appears colorless to pale green or brown and is non-pleochroic. The birefringence is low. Under polarized light chrysotile typically exhibits interference colors ranging from very dark grays to light grays, which increase with thickness. Chrysotile fibers are often obscured by other materials in the sample, such as clay, and only the ends of fibers may be exposed. The ends of chrysotile bundles and fibers are distinctive. Chrysotile bundles usually separate into individual fibers which have split ends.
Low percentages of chrysotile can be very difficult to detect. If a PLM analyst finds small fibers that cannot be identified, and suspects that asbestos may be present in a sample, the analyst should continue to search through the sample for larger and more representative fibers, so that a positive identification can be made. If only small or thin portions of fibers are detected, the analyst should reprepare the sample until confident that the presence or absence of asbestos has been ascertained. The analyst should be aware of other fibrous materials that resemble chrysotile: among these is polyethylene, morphologically and optically similar to chrysotile, except polyethylene has a more ribbonlike morphology and slightly higher birefringence. If an analyst suspects a sample of containing polyethylene, the sample should be heated with a flame or hot plate. Polyethylene will incinerate at about 135°C, chrysotile at much higher temperatures.
Another fiber that can resemble chrysotile in minute portions is cellulose. Analysts should look for a larger fiber that will show the twisted, ribbonlike morphology and the higher birefringence that indiocate cellulose. Chrysotile is the most common type of asbestos in building materials. The most useful characteristics of chrysotile fibers are high tensile strength and resistance to extreme temperatures. The mineral has been widely used in fireproofing and as a binder. Its strong, flexible fibers have been woven into fireproof cloth. Chrysotile is also chemically resistant and has been used to make laboratory bench tops that prevent chemical stains.
