Riebeckite is a sodium-rich member of the amphibole group of , chemical formula Na2(Fe2+3Fe3+2)Si8O22(OH)2. It forms a solid solution series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic showing a diamond-shaped cross section, but also in fibrous, bladed, acicular, columnar, and radiating forms. Its Mohs hardness is 5.0–6.0, and its specific gravity is 3.0–3.4. Cleavage is perfect, two directions in the shape of a diamond; fracture is uneven, splintery. It is often translucent to nearly opaque.
The mineral is also known as crocidolite.
Riebeckite granite was used for the facing stones of the Canton Viaduct from Moyles Quarry (a.k.a. Canton Viaduct Quarry) now part of Borderland State Park in Massachusetts, US. The commonwealth's name is even taken from an Algonquian word for the Great Blue Hill, which got its color from this form of granite.
Crocidolite asbestos was mined in South Africa, Bolivia, and Wittenoom, Western Australia. Bolivian crocidolite was used in approximately 13 billion Kent Micronite cigarette filters, manufactured from March 1952 until at least May 1956 by the Lorillard Tobacco Company (now part of the R. J. Reynolds Tobacco Company). Blue asbestos was also used to similar effect, and hazard, in early gas masks.