Riebeckite

Riebeckite
Riebeckite
	Black riebeckite in an alkaline pegmatite from Lindinosa near Évisa, Corsica, France
General
Category	Silicate mineral
Formula ; (repeating unit)	☐Na2(Fe2+3Fe3+2)Si8O22(OH)2
Crystal symmetry	Monoclinic 2/m
Unit cell	a = 9.76 Å, b = 18.04 Å, c = 5.33 Å; β = 103.59°; Z=2
Identification
Color	Black, dark blue; dark blue to yellow-green in thin section
Crystal habit	As prismatic crystals, commonly fibrous, asbestiform; earthy, massive
Crystal system	Monoclinic - Prismatic
Twinning	Simple or multiple twinning parallel to {100}
Cleavage	Perfect on {110}, intersecting at 56° and 124°; partings on {100} and {010}
Fracture	Conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	6
Luster	Vitreous to silky
Streak	Pale to bluish gray
Diaphaneity	Semitransparent
Specific gravity	3.28–3.44
Optical properties	Biaxial (-)
Refractive index	nα = 1.680–1.698 nβ = 1.683–1.700 nγ = 1.685–1.706
Birefringence	δ = 0.005–0.008
Pleochroism	X = blue, indigo; Y = yellowish green, yellow brown; Z = dark blue
2V angle	Measured: 68° to 85°, Calculated: 62° to 78°
Dispersion	Strong
References

Riebeckite is a silicate mineral. It is found in nature as both a fibrous form and a solid form. The fibrous form, called blue asbestos or crocidolite asbestos, is one of the six types of asbestos. It is considered by many experts to be the most dangerous type of asbestos. It can cause cancer and asbestosis. It was formerly used in Kent brand cigarettes as a filter material. It is rare in construction compared to white asbestos, but it is sometimes found in caulking, cement, wallboard, rope, and numerous other products. Due to its hazards, blue asbestos is no longer mined commercially.