Crocidolite Asbestos

A unique feature of crocidolite asbestos is its dispersion of the birefringence. Crocidolite is naturally blue to blue-gray. Its birefringence is higher in the red wavelengths than in the blue wavelengths. As a result if the fiber is thin enough it will appear pink between crossed polarizing filters. The thicker fibers don't show this property because the red light is absorbed and the anomolous interference colors are hidden by the dense blue of the fibers. The blue fibers in this image are aligned with one of the polarizing filters show only show their natural blue color.

Transmitted Off Cross Polarized Light Illumination

Definition/Function:

Crocidolite asbestos, the blue asbestos, is a fibrous amphibole with the chemical composition Na₂Fe ₅[Si₈O₂₂](OH) ₂. It is one of the more hazardous asbestos minerals.

Significance in the Environment:

Crocidolite is the third most common commercial asbestos, behind Chrysotile and Amosite. It is used in the blue ceramic water pipe that was used widely in the past. It was commonly used in high temperature environments, like boiler. It is the only asbestos other than chrysotile that can be easily woven into cloth. It is rare as a natural mineral so when it is found in an environment it is usually the result of the degradation of an asbestos containing construction material.

Characteristic Features:

The blue color of Crocidolite is unique in the asbestos minerals though there are some other blue or blue-green mineral fibers. It has relatively high refractive indices, around 1.705. It has a dispersion of the birefringence that is visible in thin fibers. For these thin fibers they appear pink between crossed polarizers. For thicker fibers the blue absorbs most of the red end of the spectrum and the fibers appear blue. This is a very characteristic feature.

Associated Particles:

Crocidolite is often used with Chrysotile asbestos.

References:

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. 261-267, 1992
4. Ledoux, R. L. (ed), SHORT COURSE IN MINERALOGICAL TECHNIQUES OF ASBESTOS DETERMINATION, Mineralogical Association of Canada, 1979.
5. Levadie, Benjamin (ed), DEFINITIONS FOR ASBESTOS AND OTHER HEALTH-RELATED SILICATES, ASTM STP 834, 1984.
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.