chrysotile

Chrysotile
Chrysotile serpentine from the Salt River area, Arizona
General
Category	Silicate mineral (serpentine group)
Chemical formula	Mg3(Si2O5)(OH)4 (ideal)
Identification
Molar mass	277.11 g/mol (ideal)
Color	grey to green
Crystal habit	acicular
Crystal system	monoclinic or orthorhombic
Fracture	fibrous
Mohs scale hardness	2½–3
Luster	silky
Streak	white
Diaphaneity	translucent
Density	2.53 g/ml
Refractive index	1.545–1.569; 1.553–1.571
Birefringence	0.008 (max)
Dispersion	relatively weak
Fusibility	dehydrates at 550–750 °C
Solubility	insoluble in water fibres degrade in dilute acid

Chrysotile or white asbestos is the most commonly encountered form of asbestos,^[1] accounting for approximately 95% of the asbestos in place in the United States^[2] and a similar proportion in other countries.^[3] It is a soft, fibrous silicate mineral in the serpentine group of phyllosilicates: as such, it is distinct from other asbestiform minerals in the amphibole group. Its idealized chemical formula is Mg₃(Si₂O₅)(OH)₄, in which some of the magnesium ions may be substituted by iron or other cations. Substitution of the hydroxide ions for fluoride, oxide or chloride is also known, but rarer.^[1] A related, but much rarer, mineral is pecoraite, in which all the magnesium cations of chrysotile are substituted by nickel cations.

Polytypes

Three polytypes of chrysotile are known.^[4] These are very difficult to distinguish in hand specimens, and polarized light microscopy^[2] must normally be used. Some older publications refer to chrysotile as a group of minerals—the three polytypes listed below, and sometimes pecoraite as well—but the 2006 recommendations of the International Mineralogical Association prefer to treat it as a single mineral with a certain variation in its naturally-occurring forms.^[5]

Clinochrysotile is the commonest of the three forms, found notably at Asbestos, Quebec, Canada. Its two measurable refractive indices tend to be lower than those of the other two forms.^[9] The orthorhombic paratypes may be distinguished by the fact that, for orthochrysotile, the higher of the two observable refractive indices is measured parallel to the long axis of the fibres (as for clinochrysotile); whereas for parachrysotile the higher refractive index is measured perpendicular to the long axis of the fibres.

Physical and chemical properties

Bulk chrysotile, whose hardness is about the same as that of a human fingernail, is easily crumbled to fibres that are, in fact, bundles of fibrils. Naturally-occurring fibre bundles range in length from several millimetres to more than ten centimetres,^[1] although industrially-processed chrysotile usually has shorter fibre bundles. The diameter of the fibre bundles is 0.1–1 µm, and the individual fibrils are even finer, 0.02–0.03 µm, each fibre bundle containing tens or hundreds of fibrils.^[3]

Chrysotile fibres have considerable tensile strength, and may be spun into thread and woven into cloth. They are also resistant to heat and are excellent thermal, electrical and acoustic insulators.^[1][3]

Chrysotile is resistant to even strong bases, but the fibres are attacked by acids: the magnesium ions are selectively dissolved, leaving a silica skeleton. It is thermally stable up to around 550 °C, at which temperature it starts to dehydrate. Dehydration is complete at about 750 °C, with the final products being silica and forsterite (magnesium silicate).^[3]

Safety concerns

Chrysotile asbestos

Chrysotile, as well as other forms of asbestos, is considered to be a human carcinogen by the International Agency for Research on Cancer (IARC)^[10] and by the U.S. Department of Health and Human Services.^[1] Asbestos exposure is associated with parenchymal asbestosis, asbestos-related pleural abnormalities, mesothelioma, and lung cancer, and it may be associated with cancer at some extra-thoracic sites.^[11]

Chrysotile has been recommended for inclusion in the Rotterdam Convention on Prior Informed Consent,^[12] an international treaty that restricts the global trade in hazardous materials. If listed, exports of chrysotile would only be permitted to countries that explicitly consent to imports. Canada, a major producer of the mineral, has been harshly criticized by the Canadian Medical Association^[13][14] for its opposition to including chrysotile in the Convention.^[15]

References

External links

• International Chemical Safety Card 0014
• Webmineral
• Mindat clinochrysotile
• Mindat orthochrysotile
• Mindat parachrysotile
• "Asbestos-containing Floor Tile and Mastic Abatement: Is there Enough Exposure to Cause Asbestos-related Disease?"
• Deer William Alexander, Howie Robert Andrew, Zussman Jack, An Introduction to the rock-forming minerals, ISBN 0-582-30094-0, OCLC 183009096 pp. 344-352, 1992
• Ledoux, RL (ed), Short course in mineralogical techniques of asbestos determination, Mineralogical Association of Canada, pp. 35-73, 185, 1979.
• http://www.microlabgallery.com/ChrysotileFile.aspx Photomicrographs of parachrysotile and clinochrysotile
• Nolan, RP, Langer AM, Ross M, Wicks FJ, Martin RF (eds), "The health effects of chrysotile asbestos", The Canadian Mineralogist, Special Publication 5, 2001.