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Amosite and Glass Fiber from Acoustic Ceiling Tile Under the Microscope with Dispersion Staining

Amosite Condenser Central Stop Dispersion Staining

The D-line refractive index along the length of this sample of amosite asbestos is about 1.679. It matches the refractive index of the Cargille High Dispersion 1.680 liquid at a wavelength of about 600 nanometers. At this wavelength the Cargille liquid has a refractive index of about 1.679, as does the amosite. This Type of Dispersion Staining produces two colors, one for the body and one for the halo outside the particle. The body color here is a sky blue. The halo color is an orange/red. The two colors provide a comparison for assigning the wavelength at which the liquid and the particle match. Phase contrast dispersion staining has a very shallow depth of field and retains the high resolution of the particle image. This type of dispersion staining works best with small particles or fibers. It is ideal for asbestos fiber identification.

Transmitted Phase Contrast with one linear Polarizing Filter Oriented Perpendicular to the Fiber Length


Glass fiber is a fiber composed of a material in a "glassy" state. A "glass" is any of a large class of materials that solidify from a molten state without crystallization and with random molecular orientation. They are regarded physically as supercooled liquids rather than true solids (based on the definition in the AMERICAN HERITAGE DICTIONARY). Chemically they generally contain silicon along with a number of other elements though carbon-based glasses are also common in some environments. It is estimated that there are in excess of 50,000 different chemical compositions for glass (MATERIALS HANDBOOK, by George S. Brady and Henry R. Clauser). Glass is easily formed at elevated temperature and can be made into fibers intentionally, as an artifact of thermal forming, or as a natural process as in the case of volcanic glass fiber (Pele's Hair).

"Glass fiber", as identified by light microscopy, is defined by its morphology and a few optical properties. Its cylindrical shape (a result of surface tension at elevated temperature) is best seen by the relief gradient at the edge of the fiber when viewed with transmitted oblique illumination. The random molecular structure is demonstrated by the fact that the fiber is isotropic (disapears in all orientations when viewed between linear polarizing filters at 90 degrees to one another). The brittle nature of the fiber is shown by the lack of plastic deformation at the terminations of the fiber.

Glass fiber from acoustic ceiling tile is identified by the binders and fillers stuck to the outside of the fiber and the lack of impacted debris adhering to the fibers. Ceiling tile is a molded material and the molding process imposes certain compositional demands and limitations. A binder must be present in sufficient quantity to impart some rigidity to the tile. The binder is relatively expensive so it is often extended by adding mineral fillers. Flexible fibers, such as paper fiber or plastic fiber, may be added to impart greater durability to the tile. The presence of these materials physically attached to the glass fiber identifies the fiber as being from acoustic ceiling tile. The type of acoustic ceiling tile is identified by the composition of the attached materials.

Amosite asbestos is the fibrous form of amphibole in the cummingtonite-grunerite family. Its chemical composition is (Mg, Fe, Mn)7[Si8O22](OH) 2. It is the next most common commercial asbestos after Chrysotile.

Significance in the Environment:

Glass fiber on surfaces has been associated with a number of health complaints and with the "Sick Building Syndrome" in general. This is not true of glass fiber collected in air samples. The reason for this seems to be that in most cases in offices, schools, and homes the transport to the upper respiratory system is the result of mechanical disturbance and transport from contaminated surfaces and not from elevated airborne loading (see references in the microlabnw paper cited below).

Amosite is a hazardous material and exposure should be very carefully limited. It is one of the more hazardous of the asbestos minerals. Exposure to asbestos fiber will not result in health symptoms associated with the work place early in the exposure. There are no early symptoms of the onset of asbestos related disease.

Amosite asbestos was used primarily for boiler and steam pipe insulation and for sound proofing products like acoustic ceiling tile. It is not a common form of amphibole and so its presence in the environment generally indicates the disturbance of an asbestos containing construction material.

Characteristic Features:

Glass fiber may be distinguished from bird feather barbules by the low birefringence of the barbules and by accessory structures (nodes) on the barbules, if present.

Glass fiber may be distinguished from silaceous phytoliths by accessory structures on the phytoliths, the low refractive index of the phytoliths (less than 1.500), by surface texture on the phytoliths, or by the non-cylindrical shape of the phytolith. These properties exclude most silaceous phytoliths from confusion with glass fiber unless a lower refractive index glass fiber is present, such as fused quartz (refractive index of 1.48). Amosite fibers tend to be straight and are rather stiff. The fibers tend to be bundles and the terminations are often Well broomed. They show good dispersion colors when dispersion staining is used and they are mounted in a high dispersion 1.680 refractive index oil.

Associated Particles:

The source of glass fiber in an environment is identified by the types of materials associated with the glass fiber. Chemical composition is often unreliable because of variations in the chemical composition of the glass itself and because of surface films or particle adhering to the fiber that alter its chemical signature. Binders and the fillers, extenders, and pigments that may be associated with the binders attached to the fiber are a good indication of the fibers source. In addition to the binder there are often other fiber types attached to the glass fiber by the binder that further clarify the source of the glass fiber.

The two most common non-fibrous materials that amosite is associated with are magnesia and gypsum. A very common block form of insulation containing amosite was simply designated as Magnesia 80. It is 80% magnesia (magnesium oxide) and 20% asbestos. The asbestos content ranges from 20% amosite to various blends of amosite and chrysotile. The gypsum formulation was similar though often calcite (limestone) would be blended into the gypsum.

When used in acoustic ceiling tile it is generally mixed with glass fiber and an epoxy binder.


1. Asbestos Textile Institute, HANDBOOK OF ASBESTOS TEXTILES, 3RD EDITION, 1967.
2. Campbell, W.J., R.L. Blake, L.L. Brown, E.E. Cather, and J.J. Sjoberg, IC 8751; SELECTED SILICATE MINERALS AND THEIR ASBESTIFORM VARIETIES, US Dept. of the Interior, Bureau of Mines Information Circular, 1977
3. Deer, W. A., R. A. Howie, and J. Zussman, AN INTRODCUTION TO THE ROCK-FORMING MINERALS, ISBN 0-582-30094-0, pp. 22-6, 1992
4. Ledoux, R. L. (ed), SHORT COURSE IN MINERALOGICAL TECHNIQUES OF ASBESTOS DETERMINATION, Mineralogical Association of Canada, 1979.
6. Riordon, P. H. (ed), GEOLOGY OF ASBESTOS DEPOSITS, Society of Mining Engineers, 1981.
7. World Health Organization, ASBESTOS AND OTHER NATURAL MINERAL FIBRES, Environmental Health Criteria 53, 1986.
8. Brady, George S. and Henry R. Clauser, MATERIALS HANDBOOK, 11th Edition, ISBN 0-07-007069-5, pp. 341-350
10. Parker, Sybil P. (ed), McGRAW-HILL ENCYCOPEDIA OF PHYSICS, ISBN 0-07-045253-9, p.27.