Chrysotile

Three polytypes of chrysotile are known.[8] These are very difficult to distinguish in hand specimens, and polarized light microscopy[6] 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.[9]

Clinochrysotile is the most common of the three forms, found notably at Val-des-Sources, Quebec, Canada. Its two measurable refractive indices tend to be lower than those of the other two forms.[13] 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.

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,[5] 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.[7]

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.[5][7]

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.[5] 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 when 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.[7]

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

${\displaystyle {\ce {{\overset {Chrysotile\ (serpentine)}{2Mg3Si2O5(OH)4}}->[{750^{\circ }C}][{dehydration}]{\overset {Forsterite}{3Mg2SiO4}}+{\overset {silica}{SiO2}}+{\overset {water}{4H2O}}}}}$

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

Previously, in the 1990s it was used in asbestos-cement products (like pipes and sheets).[14]

Magnesium sulfate (MgSO₄) may be produced by treating chrysotile with sulfuric acid (H₂SO₄).[15]

Chrysotile asbestos

Chrysotile has been included with other forms of asbestos in being classified as a human carcinogen by the International Agency for Research on Cancer (IARC)[16] and by the U.S. Department of Health and Human Services.[5] 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".[17] In other scientific publications, epidemiologists have published peer reviewed scientific papers establishing that chrysotile is the main cause of pleural mesothelioma.[18][19][20]

Chrysotile has been recommended for inclusion in the Rotterdam Convention on Prior Informed Consent,[21] 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[22][23] for its opposition to including chrysotile in the Convention.[24]

According to EU Regulation 1907/2006 (REACH) the marketing and use of chrysotile, and of products containing chrysotile, are prohibited.[25]

• Erionite
• Serpentinite reactions – Rock formed by hydration and metamorphic transformation of olivine
• Antigorite

• The Chrysotile Institute
• International Chemical Safety Card 0014
• "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.