Chrysotile

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	9.ED.15
Crystal system	Monoclinic: clinochrysotile (most common) Orthorhombic: orthochrysotile and parachrysotile (both rare)
Crystal class	Clinochrysotile: prismatic (2/m) Orthochrysotile and parachrysotile: pyramidal (mm2)
Space group	Clinochrysotile: C2/m Orthochrysotile and parachrysotile: Ccm21
Identification
Formula mass	277.11 g/mol (ideal)
Colour	Grey to green
Crystal habit	Acicular
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

Chrysotile or white asbestos is the most commonly encountered form of asbestos, accounting for approximately 95% of the asbestos in the United States and a similar proportion in other countries. 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. 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.

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

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. 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.