Chrysotile

Not to be confused with the term Chrysolite.
Chrysotile

Chrysotile serpentine from the Salt River area, Arizona
General
Category Phyllosilicates
Kaolinite-serpentine group
Formula
(repeating unit)		 Mg3(Si2O5)(OH)4
(ideal)
Strunz classification 09.ED.15
Crystal system Monoclinic: clinochrysotile (most common), or orthorhombic: orthochrysotile and parachrysotile (both rare)
Identification
Formula mass 277.11 g/mol (ideal)
Colour Grey to green
Crystal habit Acicular
Crystal symmetry Clinochrysotile is monoclinic prismatic
H-M symbol: (2/m)
Space group: A2/m
Fracture Fibrous
Mohs scale hardness 2.5–3
Lustre Silky
Streak white
Diaphaneity translucent
Density 2.53 g/ml
Optical properties Biaxial
Refractive index nα = 1.569 nγ = 1.570
Birefringence 0.001 (max)
Dispersion relatively weak
Fusibility dehydrates at 550–750 °C (1,022–1,382 °F)
Solubility insoluble in water
fibres degrade in dilute acid
References [1][2]

Chrysotile or white asbestos is the most commonly encountered form of asbestos,[3] accounting for approximately 95% of the asbestos in the United States[4] and a similar proportion in other countries.[5] It is a soft, fibrous silicate mineral in the serpentine subgroup of phyllosilicates; as such, it is distinct from other asbestiform minerals in the amphibole group. Its idealized chemical formula is Mg3(Si2O5)(OH)4.[3] The material has physical properties which make it desirable for inclusion in building materials, but poses serious health risks when dispersed into air and inhaled.

Polytypes

Three polytypes of chrysotile are known.[6] These are very difficult to distinguish in hand specimens, and polarized light microscopy[4] 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.[7]

Name Crystal system Type locality mindat.org reference Unit cell parameters Crystal structure reference
Clinochrysotile monoclinic Złoty Stok*, Lower Silesia, Poland 1071 a = 5.3 Å; b = 9.19 Å; c = 14.63 Å; β = 93° [8]
Orthochrysotile orthorhombic Kadapa* district, Andhra Pradesh, India 3025 a = 5.34 Å; b = 9.24 Å; c = 14.2 Å [9]
Parachrysotile orthorhombic uncertain 3083 a = 5.3 Å; b = 9.24 Å; c = 14.71 Å [10]
Source: mindat.org.
*Złoty Stok and Kadapa have formerly been known as Reichenstein and Cuddapah respectively, and these names may appear in some publications.

Clinochrysotile is the most common 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.[11] 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 properties

SEM photo of Chrysotile.

Bulk chrysotile has a hardness similar to a human fingernail and is easily crumbled to fibrous strands composed of smaller bundles of fibrils. Naturally-occurring fibre bundles range in length from several millimetres to more than ten centimetres,[3] 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.[5] 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.[3][5]

Chemical properties

The idealized chemical formula of chrysotile is Mg3(Si2O5)(OH)4, although some of the magnesium ions may be replaced by iron or other cations. Substitution of the hydroxide ions for fluoride, oxide or chloride is also known, but rarer.[3] A related, but much rarer, mineral is pecoraite, in which all the magnesium cations of chrysotile are substituted by nickel cations.

Chrysotile is resistant to even strong bases (asbestos is thus stable in high pH pore water of Portland cement), 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 (1,022 °F), at which temperature it starts to dehydrate. Dehydration is complete at about 750 °C (1,380 °F), with the final products being forsterite (magnesium silicate), silica and water.[5]

The global mass balance reaction of the chrysotile dehydration can be written as follows:

Chrysotile (serpentine) → Forsterite + silica + water
\mathrm{2Mg_3Si_2O_5(OH)_4}   →   \mathrm{3Mg_2SiO_4 + SiO_2 + 4H_2O}

The chrysotile (serpentine) dehydration reaction corresponds to the reverse of the forsterite (Mg-olivine) hydrolysis in the presence of dissolved silica (silicic acid).

Safety concerns

Chrysotile asbestos

Chrysotile has been included with other forms of asbestos in being considered to be a human carcinogen by the International Agency for Research on Cancer (IARC)[12] and by the U.S. Department of Health and Human Services.[3] These state that "Asbestos exposure is associated with parenchymal asbestosis, asbestos-related pleural abnormalities, peritoneal mesothelioma, and lung cancer, and it may be associated with cancer at some extra-thoracic sites".[13] In other scientific publications, epidemiologists have published peer reviewed scientific papers establishing that chrysotile is the main cause of pleural mesothelioma.[14][15][16]

Chrysotile has been recommended for inclusion in the Rotterdam Convention on Prior Informed Consent,[17] 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[18][19] for its opposition to including chrysotile in the Convention.[20]

According to EU Regulation 1907/2006 (REACH) the marketing and use of chrysotile, and of products containing chrysotile, are prohibited.[21]

Critics of safety regulations

1990s: Canada-European dispute GATT dispute

In May 1998, Canada requested consultations before the WTO and the European Commission concerning France's 1996 prohibition of the importation and sale of all forms of asbestos. Canada said that the French measures contravened provisions of the Agreements on Sanitary and Phytosanitary Measures and on Technical Barriers to Trade, and the GATT 1994. The EC claimed that safer substitute materials existed to take the place of asbestos. It stressed that the French measures were not discriminatory under the terms of international trade treaties, and were fully justified for public health reasons. The EC further claimed that in the July consultations, it had tried to convince Canada that the measures were justified, and that just as Canada broke off consultations, it (the EC) was in the process of submitting substantial scientific data in favour of the asbestos ban.[22]

2000s: Canadian exports face mounting global criticism

In the late 1990s and early 2000s, the Government of Canada continued to claim that chrysotile was much less dangerous than other types of asbestos.[23] Chrysotile continued to be used in new construction across Canada, in ways that are very similar to those for which chrysotile was exported.[24] Similarly, Natural Resources Canada once stated that chrysotile, one of the fibres that make up asbestos, was not as dangerous as once thought. According to a fact sheet from 2003, "current knowledge and modern technology can successfully control the potential for health and environmental harm posed by chrysotile".[25] The Chrysotile Institute, an association partially funded by the Canadian government, also prominently asserted that the use of chrysotile did not pose an environmental problem and the inherent risks in its use were limited to the workplace.[26]

However, under increasing criticism by environmental groups, in May, 2012, the Canadian government stopped funding the Chrysotile Institute.[27][28] As a result, the Chrysotile Institute has now closed.[29][30][31]

The Canadian government continues to draw both domestic and international criticism for its stance on chrysotile, most recently in international meetings about the Rotterdam Convention hearings regarding chrysotile. The CFMEU pointed out that most exports go to developing countries. Canada has pressured countries, including Chile, and other UN member states to avoid chrysotile bans.[32] In November, 2011, the Australian Upper House passed a motion to pressure the Canadian government to end policies supporting asbestos exports.[33]

In September 2012, governments in Quebec and Canada ended official support for Canada's last asbestos mine in Asbestos, Quebec.[34]

See also

References

  1. ↑ Chrysotile on Mindat.org
  2. ↑ Chrysotile data on Webmineral
  3. 1 2 3 4 5 6 "Asbestos". Report on Carcinogens, Eleventh Edition [http://ntp.niehs.nih.gov/index.cfm?objectid=32BA9724-F1F6-975E-7FCE50709CB4C932#] (PDF). U.S. Department of Health and Human Services. 2005. Archived from the original (PDF) on June 8, 2011. External link in |title= (help)
  4. 1 2 Occupational Safety and Health Administration, U.S. Department of Labor (2007). 29 C.F.R. 1910.1001. Appendix J.
  5. 1 2 3 4 Institut national de recherche sur la sécurité (1997). "Amiante." Fiches toxicologiques. n° 167. (in French)
  6. ↑ Wicks, F. J.; Whittaker, E. J. W. (1975). "A reappraisal of the structures of the serpentine minerals". The Canadian Mineralogist 13 (3): 227–243.
  7. ↑ Burke, Ernst A. J. (2006). "A Mass Discreditation of GQN Minerals" (PDF). The Canadian Mineralogist 44 (6): 1557–60. doi:10.2113/gscanmin.44.6.1557.
  8. ↑ Whittaker, E. J. W. (1956). "The structure of chrysotile. II. Clino-chrysotile". Acta Crystallographica 9 (11): 855–62. doi:10.1107/S0365110X5600245X.
  9. ↑ Whittaker, E. J. W. (1956). "The structure of chrysotile. III. Ortho-chrysotile". Acta Crystallographica 9 (11): 862–64. doi:10.1107/S0365110X56002461.
  10. ↑ Whittaker, E. J. W. (1956). "The structure of chrysotile. IV. Para-chrysotile". Acta Crystallographica 9 (11): 865–67. doi:10.1107/S0365110X56002473.
  11. ↑ In principle, all polytypes of chrysotile should have three independent refractive indices: in practice, two of the three are so close as to be indistinguishable by experimental measurement.
  12. ↑ International Agency for Research on Cancer (1998). "Asbestos." However, the study states " In some of these case reports and in other studies, asbestos fibres have been identified in the lung. Amphibole fibres have usually predominated, but in a few cases mainly or only chrysotile fibres were found." IARC Monographs on Evaluating the Carcinogenic Risks to Humans. Supplement 7.
  13. ↑ Agency for Toxic Substances and Disease Registry (ATDSR), U.S. Department of Health and Human Services (2007). "Asbestos Toxicity." Case Studies in Environmental Medicine.
  14. ↑ See e.g., Smith, Allen "Chrysotile is the main cause of pleural mesothelioma", Amer.J.Indus.Med., Vol. 32, pp. 252 to 266 (1996)
  15. ↑ Tossavainen A, "Asbestos, asbestosis, and cancer: the Helsinki criteria for diagnosis and attribution" Scand J Work Environ Health 1997;23(4):311–316 (stating that all types of malignant mesothelioma can be induced by asbestos, with the amphiboles showing greater carcinogenic potency than chrysotile)
  16. ↑ Lemen RA 2010. Chrysotile Asbestos and Mesothelioma. Environ Health Perspect 118:a282-a282
  17. ↑ Rotterdam Convention: Chrysotile
  18. ↑ Attaran, Amir; Boyd, David R.; Stanbrook, Matthew B. (October 2008). "Asbestos mortality: a Canadian export". CMAJ 179 (9): 871–2. doi:10.1503/cmaj.081500. PMC 2565724. PMID 18936444.
  19. ↑ Collier, Roger (December 2008). "Health advocates assail Canada's asbestos stance". CMAJ 179 (12): 1257. doi:10.1503/cmaj.081806. PMC 2585122. PMID 19047602.
  20. ↑ MITTELSTAEDT, MARTIN (September 10, 2008). "Canada still blocking action on asbestos". The Globe and Mail (Toronto). Retrieved 2008-10-01.
  21. ↑ Amtsblatt der Europäischen Union, L 396 from 30-12-2006 (PDF 1,8 MB; S. 129)
  22. ↑ EC measures affecting asbestos products. World Trade Organization News. 29 October 1998
  23. ↑ Harper, Tim (3 July 2011). "Canada diminished by asbestos hypocrisy". The Chronicle Herald.
  24. ↑ Versailles, Guy (8 July 2015). "What Exactly Is Asbestos?". South Bay Mitigation.
  25. ↑ "Chrysolite Asbestos Fact Sheet" (PDF). Natural Resources Canada. 3 February 2009. Retrieved 2010-01-12.
  26. ↑ Vongdouangchanh, Bea (5 July 2011). "NDP MP Martin's one-man battle to ban asbestos mining in Canada gains traction". The Hill Times.
  27. ↑ Asbestos Advocacy Group Shuts Its Doors, Montreal Gazette
  28. ↑ Canadian Cancer Society Disappointed with Proposed Federal Government Funding for Chrysotile Institute, 3 March 2012
  29. ↑ R.I.P. Chrysotile Institute, The Mad Scientist Blog>
  30. ↑ Asbestos Advocacy Group Shuts Its Doors, Vancouver Sun, 29 April 2012
  31. ↑ "Minister Flaherty: Stop Funding The Chrysotile Institute", 2 Feb. 2011
  32. ↑ Stop Canada's Export Of Asbestos CFMEU Archived July 10, 2010, at the Wayback Machine.
  33. ↑ Australian Senate Slams Canada's Asbestos Industry, 17 November 2011
  34. ↑ Daniel Lak (17 Oct 2012). "Canada axes support for asbestos mining". Al Jazeera.

External links

This article is issued from Wikipedia - version of the Wednesday, May 04, 2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.