Chrysotile

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Chrysotile

Chrysotile serpentine from the Salt River area, Arizona
General
Category Silicate mineral
(serpentine group)
Chemical formula Mg3(Si2O5)(OH)4
(ideal)
Identification
Molecular Weight 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
Refractive index 1.545–1.569; 1.553–1.571
Birefringence 0.008 (max)
Dispersion relatively weak
Streak white
Density 2.53 g/ml
Fusibility dehydrates at 550–750 °C
Solubility insoluble in water
fibres degrade in dilute acid
Diaphaneity translucent

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.

Contents

[edit] 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]

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° [6]
Orthochrysotile orthorhombic Kadapa* district, Andhra Pradesh, India 3025 a = 5.34 Å; b = 9.24 Å; c = 14.2 Å [7]
Parachrysotile orthorhombic uncertain 3083 a = 5.3 Å; b = 9.24 Å; c = 14.71 Å [8]
Source: mindat.org.
*Złoty Stok and Kadapa have formerly been known as Reichenstein and Cuddapah respecively, and these names may appear in some publications.

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.

[edit] 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]

[edit] Safety concerns

Chrysotile asbestos
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]

[edit] References

  1. ^ a b c d e U.S. Department of Health and Human Services (2005). "Asbestos." Report on Carcinogens, Eleventh Edition.
  2. ^ a b Occupational Safety and Health Administration, U.S. Department of Labor (2007). 29 C.F.R. 1910.1001. Appendix J.
  3. ^ a b c d Institut national de recherche sur la sécurité (1997). "Amiante." Fiches toxicologiques. n° 167. (in French)
  4. ^ Can. Mineral. 13 (1975): 227–43.
  5. ^ Burke, Ernst A. J. (2006). "A Mass Discreditation of GQN Minerals." Can. Mineral. 44: 1557–60.
  6. ^ Whittaker, E. J. W. (1956). "The structure of chrysotile II. Clinochrysotile." Acta Crystallogr. 9: 855–62.
  7. ^ Whittaker, E. J. W. (1956). "The structure of chrysotile III. Orthochrysotile." Acta Crystallogr. 9: 862–64.
  8. ^ Whittaker, E. J. W. (1956). "The structure of chrysotile IV. Parachrysotile." Acta Crystallogr. 9: 865–67.
  9. ^ 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.
  10. ^ International Agency for Research on Cancer (1998). "Asbestos." IARC Monographs on Evaluating the Carcinogenic Risks to Humans. Supplement 7.
  11. ^ Agency for Toxic Substances and Disease Registry (ATDSR), U.S. Department of Health and Human Services (2007). "Asbestos Toxicity." Case Studies in Environmental Medicine.

[edit] External links