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
IMA symbol	Rbk[1]
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/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
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	[2][3][4][5]

Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, 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 crystals 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.

Name and discovery

Riebeckite was first described in 1888 for an occurrence on Socotra Island, Aden Governorate, Yemen, and named after German explorer Emil Riebeck (1853–1885).^[3]

The mineral is also known as crocidolite.^[6]: 50

Occurrence

Riebeckite typically forms dark-blue elongated to fibrous crystals in highly alkali granites, syenites, rarely in felsic volcanics, granite pegmatites and schist. It occurs in banded iron formations as the asbestiform variety crocidolite (blue asbestos). It occurs in association with aegirine, nepheline, albite, arfvedsonite in igneous rocks; with tremolite, ferro-actinolite in metamorphic rocks; and with grunerite, magnetite, hematite, stilpnomelane, ankerite, siderite, calcite, chalcedonic quartz in iron formations.^[2]

Riebeckite granite

The riebeckite granite known as ailsite, found on the island of Ailsa Craig in western Scotland, is prized for its use in the manufacture of curling stones.

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 (fibrous riebeckite)

The fibrous forms of riebeckite are known as crocidolite and are one of the six recognised types of asbestos. Often referred to as blue asbestos, it is considered the most hazardous. The association between blue asbestos and mesothelioma was established by J. C. Wagner, C. A. Sleggs, and P. Marchand by 1960.^[7]

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.